Spectroscopy and Imaging
Quantum Materials
Anomalous electrons in a metallic kagome ferromagnet
Ordinary metals contain electron liquids within well-defined ‘Fermi’ surfaces at which the electrons behave as if they were non-interacting. In the absence of transitions to entirely new phases such as insulators or superconductors, interactions between electrons induce scattering that is quadratic in the deviation of the binding energy from the Fermi level. A long-standing puzzle is that certain materials do not fit this ‘Fermi liquid’ description. A common feature is strong intera...
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06 Mar 2024
Spectroscopy and Imaging
Quantum Materials
Anomalous electrons in a metallic kagome ferromagnet
Ordinary metals contain electron liquids within well-defined ‘Fermi’ surfaces at which the electrons behave as if they were non-interacting. In the absence of transitions to entirely new phases such as insulators or superconductors, interactions between electrons induce scattering that is quadratic in the deviation of the binding energy from the Fermi level. A long-standing puzzle is that certain materials do not fit this ‘Fermi liquid’ description. A common feature is strong interactions between electrons relative to their kinetic energies. One route to this regime is special lattices to reduce the electron kinetic energies. Twisted bilayer graphene is an example, and trihexagonal tiling lattices (triangular ‘kagome’), with all corner sites removed on a 2 × 2 superlattice, can also host narrow electron bands for which interaction effects would be enhanced. Here we describe spectroscopy revealing non-Fermi-liquid behaviour for the ferromagnetic kagome metal Fe3Sn2. We discover three C3-symmetric electron pockets at the Brillouin zone centre, two of which are expected from density functional theory. The third and most sharply defined band emerges at low temperatures and binding energies by means of fractionalization of one of the other two, most likely on the account of enhanced electron–electron interactions owing to a flat band predicted to lie just above the Fermi level. Our discovery opens the topic of how such many-body physics involving flat bands could differ depending on whether they arise from lattice geometry or from strongly localized atomic orbitals.
Read more at Nature:
https://www.nature.com/articles/s41586-024-07085-w
06 Mar 2024
All publications
Spectroscopy and Imaging
Covalent Au–C Contact Formation and C–C Homocoupling Reaction from Organotin Compounds in Single-Molecule Junctions
Formation of new chemical species has been achieved under an electric field by the use of the scanning tunneling microscope break junction technique, yet simultaneous implementation of catalytic reactions both at the organic/metal interface and in the bulk solution remains a challenging task. Herein, we show that n-butyl-substituted organotin-terminated benzene undergoes both an efficient cleavage of the terminal tributyltin group to form a covalent Au–C bond and a homocoupling reaction to yield biphenyl product when subjected to an electric field in the vicinity to Au electrodes. By using ex situ characterization of high-performance liquid chromatography with an UV–vis detector, we demonstrate that the homocoupling reaction can occur with high efficiency under an extremely low tip bias voltage of ∼5 mV. Additionally, we show that the efficiency of the homocoupling reaction varies significantly in different solvents; the choice of the solvent proves to be one of the methods for modulating this reaction. By synthesizing and testing varied molecular backbone structures, we show that an extended biphenyl backbone undergoes homocoupling to form a quarterphenylene backbone, and the C–C coupling reactions are prohibited when additional aurophilic or bulky chemical groups that exhibit a steric blockage are introduced
Read more at Journal of the American Chemical Society:
https://pubs.acs.org/doi/10.1021/jacs.4c03925
22 Sep 2024
Spectroscopy and Imaging
Antiferromagnetism and Phase Stability of CrMnFeCoNi High-Entropy Alloy
It has long been suspected that magnetism could play a vital role in the phase stability of multicomponent high-entropy alloys. However, the nature of the magnetic order, if any, has remained elusive. Here, by using elastic and inelastic neutron scattering, we demonstrate evidence of antiferromagnetic order below ∼80 K and strong spin fluctuations persisting to room temperature in a single-phase face-centered cubic (fcc) CrMnFeCoNi high-entropy alloy. Despite the chemical complexity, the magnetic structure in CrMnFeCoNi can be described as 𝛾-Mn-like, with the magnetic moments confined in alternating (001) planes and pointing toward the ⟨111⟩ direction. Combined with first-principles calculation results, it is shown that the antiferromagnetic order and spin fluctuations help stabilized the fcc phase in CrMnFeCoNi high-entropy alloy.
Read more at Physical Review Letters:
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.133.126701
17 Sep 2024
A quantitative comparison between velocity dependent SIDM cross-sections constrained by the gravothermal and isothermal models
One necessary step for probing the nature of self-interacting dark matter (SIDM) particles with astrophysical observations is to pin down any possible velocity dependence in the SIDM cross-section. Major challenges for achieving this goal include eliminating, or mitigating, the impact of the baryonic components and tidal effects within the dark matter halos of interest – the effects of these processes can be highly degenerate with those of dark matter self-interactions at small scales. In this work, we select 9 isolated galaxies and brightest cluster galaxies (BCGs) with baryonic components small enough such that the baryonic gravitational potentials do not significantly influence the halo gravothermal evolution processes. We then constrain the parameters of Rutherford and Møller scattering cross-section models with the measured rotation curves and stellar kinematics through the gravothermal fluid formalism and isothermal method. Cross-sections constrained by the two methods are consistent at 1σ confidence level, but the isothermal method prefers cross-sections greater than the gravothermal approach constraints by a factor of ∼3.
Read more at Monthly Notices of the Royal Astronomical Society:
https://academic.oup.com/mnras/article/533/4/4007/7745507
29 Aug 2024
Soft Matter and Biophysics
Chaperone solvent-assisted assembly of polymers at the interface of two immiscible liquids
The assembly of polymers at liquid-liquid interfaces offers a promising strategy for fabricating two-dimensional polymer films. However, a significant challenge arises when the polymers lack inherent interfacial traction. In response, we introduce an approach termed chaperone solvent-assisted assembly. This approach utilizes a target polymer, X, along with three solvents: α, β, and γ. α and β are poor solvents for X and immiscible with each other, while γ is a good solvent for X and miscible with both α and β, thus serving as the chaperone solvent. The cross-interface diffusion of γ induces the assembly of interfacially nonactive X at the α-β interface, and this mechanism is verified through systematic in situ and ex situ studies. We show that chaperone solvent-assisted assembly is versatile and reliable for the interfacial assembly of polymers, including those that are interfacially nonactive. Several practical applications based on chaperone solvent-assisted assembly are also demonstrated.
Read more Nature Communications at:
https://www.nature.com/articles/s41467-024-51657-3
28 Aug 2024
Theoretical and Computational Physics
Atomic, Molecular, and Optical Physics
Experimental Validation of Enhanced Information Capacity by Quantum Switch in Accordance with Thermodynamic Laws
This study examines how a quantum switch can affect thermodynamics. A quantum switch allows two processes to happen in different orders at the same time. When applied to heating and cooling processes, it showed surprising results that seemed to break a fundamental law of thermodynamics. The researchers found that this switching could improve how much information can be transferred. Using advanced experimental techniques, they confirmed the limits of this information increase and showed that, in some cases, those limits can be exceeded. Importantly, the quantum switch can change a heated state into a cooler one while using some energy from a control system. This research suggests new ways to explore quantum mechanics and thermodynamics together.
Read more at Physical Review Letters:
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.133.040401
26 Jul 2024
Theoretical and Computational Physics
Atomic, Molecular, and Optical Physics
Near-Field Spin Chern Number Quantized by Real-Space Topology of Optical Structures
The Chern number has been widely used to describe the topological properties of periodic structures in momentum space. Here, we introduce a real-space spin Chern number for the optical near fields of finite-sized structures. This new spin Chern number is intrinsically quantized and equal to the structure’s Euler characteristic. The relationship is robust against continuous deformation of the structure’s geometry and is irrelevant to the specific material constituents or external excitation. Our Letter enriches topological physics by extending the Chern number to real space, opening exciting possibilities for exploring the real-space topological properties of light.
Read more at Physical Review Letters:
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.132.233801
03 Jun 2024
Spectroscopy and Imaging
Applied Physics
Enabling High Stability of Co-Free LiNiO2 Cathode via a Sulfide-Enriched Cathode Electrolyte Interface
Cobalt-free lithium nickel oxide (LNO) has garnered significant interest as the end member of high-nickel layered oxide cathodes for next-generation batteries. However, its practical performance notably underperforms expectations because of the structural degradation and unstable interfacial chemistry with electrolytes during cycling. Here, we report that a durable cathode-electrolyte interface (CEI), enriched by in situ formed sulfides and borides, can inhibit LNO structural degradation and suppress Ni ion dissolution. With the CEI protection, the stability of LNO can be remarkably extended, and batteries demonstrate a capacity retention rate of 84% (30 °C) and 79% (50 °C) after 200 cycles at 1C, respectively. These results demonstrate that enriching CEI with sulfur-containing species can effectively stabilize the interfacial chemistry of LNO, particularly at an elevated temperature of 50 °C. This finding provides valuable perspectives on designing electrolytes for cobalt-free LNO and other high-Ni cathodes toward the development of next-generation high-energy-density lithium-ion batteries.
Read more at ACS Energy Letters:
https://pubs.acs.org/doi/10.1021/acsenergylett.4c00652
14 May 2024
Spectroscopy and Imaging
Applied Physics
Regulation of both bulk and surface structure by W/S co-doping for Li-rich layered cathodes with remarkable voltage and capacity stability
Lithium-rich layered oxides (LLOs) have gained significant attention due totheir high capacity of over 250 mAh g−1, which originates from the chargecompensation of oxygen anions activated under high voltage. However, thecharge compensation of oxygen anions is prone to over-oxidation, leading toserious irreversible oxygen release, surface-interface reactions, and structuralevolution. These detriments make LLOs undergo fast voltage decay andcapacity fading, which have hindered their practical applications for manyyears. Herein, this work develops a multifunctional co-doping strategy andconstructs W─O bonds with strong bonding interaction and covalence, lowbond energy Li─S bonds with non-binding electrons near the Fermi level, andcontinuous and homogeneous surface spinel-like layer induced by W/Sco-doping. Their synergistic effect significantly mitigates the irreversibleoxygen release and surface-interface reactions and improves structuralstability of Li-rich layered cathodes. Thus, the designed and prepared Co-freeLi-rich layered cathode (Li1.232Mn0.574Ni0.191W0.003O1.995S0.005) deliverssuperior voltage and capacity stability. Its capacity retention after 400 cycles isas large as 86%, and its voltage decay rate from the 10th to the 400th cycle isonly 0.626 mV cycle−1.
Read more at Advanced Functional Materials:
https://onlinelibrary.wiley.com/doi/full/10.1002/adfm.202404044
30 Apr 2024
Soft Matter and Biophysics
Reconfigurable aqueous 3D printing with adaptive dual locks
Using aqueous two-phase systems (ATPSs) for three-dimensional (3D) printed complex structures has attracted considerable attention in the field of biomedicine. In this study, we present an unusual approach to constructing reconfigurable 3D printed structures within an aqueous environment. Inspired by biological systems, we introduce both specific and nonspecific interactions to anchor functionalized nanoparticles to the water-water interface, thereby imparting adaptive dual locks of structural integrity and permeability to the 3D printed liquid structures. Using state-of-the-art in situ liquid-liquid interfacial atomic force microscopy imaging, we successfully demonstrate various morphologies of interfacial films formed at the ATPS interface. In addition, by incorporating d-glucose or sodium alginate into the systems, the dual locks can be easily manipulated. Our study paves a pathway for 3D printing multiresponsive all-aqueous systems with controllable structures and permeability, showing promising implications for the development of smart drug delivery systems and in vivo reactions.
Read more Science Advances at:
https://www.science.org/doi/10.1126/sciadv.adk4080
24 Apr 2024
Spectroscopy and Imaging
Applied Physics
Achieving Long‐Life Ni‐Rich Cathodes with Improved Mechanical‐Chemical Properties Via Concentration Gradient Structure
The irreversible deterioration of electrochemical performance in Ni-rich cathode materials, attributed to crack propagation and undesired side reactions, poses a critical barrier to the further development of high-energy power batteries for electrical vehicles (EVs). Herein, a concentration gradient strategy is proposed for synthesizing a Ni-rich cathode with enhanced mechanical and electrochemical stability to address the issues related to the irreversible structural deterioration. Notably, the concentration gradient structure contributes to superior mechanical strength in secondary particles due to the radially orientated primary particles resulted from Mn composition grading, which effectively alleviate the internal strain caused by structural changes and fatigue destruction during successive cycling. Moreover, the Mn-rich surface minimizes the parasitic side reactions at the electrode–electrolyte interface. Benefitting from the above, the concentration gradient sample can deliver ≈180.1 mA h g−1 at 1 C and retain 96.2% of its initial discharge capacity after 100 cycles. This work demonstrates that the concentration gradient structure can simultaneously improve the mechanical and chemical stabilities of Ni-rich cathode and offers a feasible way for designing stable lithium-ion batteries with high energy density
Read more at Advanced Functional Materials:
https://onlinelibrary.wiley.com/doi/full/10.1002/adfm.202400956
25 Mar 2024
Spectroscopy and Imaging
Quantum Materials
Anomalous electrons in a metallic kagome ferromagnet
Ordinary metals contain electron liquids within well-defined ‘Fermi’ surfaces at which the electrons behave as if they were non-interacting. In the absence of transitions to entirely new phases such as insulators or superconductors, interactions between electrons induce scattering that is quadratic in the deviation of the binding energy from the Fermi level. A long-standing puzzle is that certain materials do not fit this ‘Fermi liquid’ description. A common feature is strong interactions between electrons relative to their kinetic energies. One route to this regime is special lattices to reduce the electron kinetic energies. Twisted bilayer graphene is an example, and trihexagonal tiling lattices (triangular ‘kagome’), with all corner sites removed on a 2 × 2 superlattice, can also host narrow electron bands for which interaction effects would be enhanced. Here we describe spectroscopy revealing non-Fermi-liquid behaviour for the ferromagnetic kagome metal Fe3Sn2. We discover three C3-symmetric electron pockets at the Brillouin zone centre, two of which are expected from density functional theory. The third and most sharply defined band emerges at low temperatures and binding energies by means of fractionalization of one of the other two, most likely on the account of enhanced electron–electron interactions owing to a flat band predicted to lie just above the Fermi level. Our discovery opens the topic of how such many-body physics involving flat bands could differ depending on whether they arise from lattice geometry or from strongly localized atomic orbitals.
Read more at Nature:
https://www.nature.com/articles/s41586-024-07085-w
06 Mar 2024
Spectroscopy and Imaging
Quantum Materials
Phonon promoted charge density wave in topological kagome metal ScV6Sn6
Phonon promoted charge density wave in topological kagome metal ScV6Sn6
Charge density wave (CDW) orders in vanadium-based kagome metals have recently received tremendous attention, yet their origin remains a topic of debate. The discovery of ScV6Sn6, a bilayer kagome metal featuring an intriguing CDW order, offers a novel platform to explore the underlying mechanism behind the unconventional CDW. Here, we combine high-resolution angle-resolved photoemission spectroscopy, Raman scattering and density functional theory to investigate the electronic structure and phonon modes of ScV6Sn6. We identify topologically nontrivial surface states and multiple van Hove singularities (VHSs) in the vicinity of the Fermi level, with one VHS aligning with the in-plane component of the CDW vector near the point. Additionally, Raman measurements indicate a strong electron-phonon coupling, as evidenced by a two-phonon mode and new emergent modes. Our findings highlight the fundamental role of lattice degrees of freedom in promoting the CDW in ScV6Sn6.
Read more at Nature Communications:
https://www.nature.com/articles/s41467-024-45859-y
23 Feb 2024
Spectroscopy and Imaging
Quantum Materials
Non-trivial band topology and orbital-selective electronic nematicity in a titanium-based kagome superconductor
Electronic nematicity that spontaneously breaks rotational symmetry is a generic phenomenon in correlated quantum systems including high-temperature superconductors and the AV3Sb5 (A can be K, Rb or Cs) family of kagome superconductors. However, the underlying mechanism of nematicity in these systems is hard to identify because of its entanglement with other ordered phases. Recently, a family of titanium-based kagome superconductors ATi3Bi5 have been synthesized, where electronic nematicity occurs in the absence of charge order. It provides a platform to study nematicity in its pure form, as well as its interplay with orbital degrees of freedom. Here we reveal the band topology and orbital characters of the multiorbital RbTi3Bi5. We use polarization-dependent angle-resolved photoemission spectroscopy with density functional theory to identify the coexistence of flat bands, type-II Dirac nodal lines and non-trivial topology in this compound. Our study demonstrates the change in orbital character along the Fermi surface contributed by the kagome bands, implying a strong intrinsic interorbital coupling in the Ti-based kagome metals. Furthermore, doping-dependent measurements uncover the orbital-selective features in the kagome bands, which can be explained by d–p hybridization. Hence, interorbital coupling together with d–p hybridization is probably the origin of electronic nematicity in ATi3Bi5.
Read more at Nature Physics:
https://www.nature.com/articles/s41567-023-02215-z
21 Sep 2023
The impact of baryonic potentials on the gravothermal evolution of self-interacting dark matter haloes
The presence of a central baryonic potential can have a significant impact on the gravothermal evolution of self-interacting dark matter (SIDM) haloes. We extend a semi-analytical fluid model to incorporate the influence of a static baryonic potential and calibrate it using controlled N-body simulations. We construct benchmark scenarios with varying baryon concentrations and different SIDM models, including constant and velocity-dependent self-interacting cross-sections. The presence of the baryonic potential induces changes in SIDM halo properties, including central density, core size, and velocity dispersion, and it accelerates the halo’s evolution in both expansion and collapse phases. Furthermore, we observe a quasi-universality in the gravothermal evolution of SIDM haloes with the baryonic potential, resembling a previously known feature in the absence of the baryons. By appropriately rescaling the physical quantities that characterize the SIDM haloes, the evolution of all our benchmark cases exhibits remarkable similarity. Our findings offer a framework for testing SIDM predictions using observations of galactic systems where baryons play a significant dynamical role.
Read more at Monthly Notices of the Royal Astronomical Society:
https://doi.org/10.1093/mnras/stad2765
12 Sep 2023
Applied Physics
Critical breakthrough for enhancing battery capacity
Lithium-ion batteries (LiBs) are widely used in electronic devices, while lithium-(Li) and manganese-rich (LMR) layered oxides are a promising class of cathodes for LiBs due to their high capacity and low cost. However, the long-standing problem of voltage decay hinders their applications.
Professor Ren Yang, Head and Chair Professor of the Department of Physics (PHY), Professor Liu Qi, PHY, and their team have addressed the issue by unlocking the potential of LMR cathode materials. In their research, they stabilised the unique honeycomb-like structure within the cathode material, resulting in longer-lasting and more efficient batteries. Their insights are likely to transform the way we power our devices and are set to take the development of high-energy cathode materials to the next stage.
This research was recently published in Nature Energy titled “A Li-rich layered oxide cathode with negligible voltage decay”.
The team’s innovative approach focused on stabilising the honeycomb structure at the atomic level. By incorporating additional transition metal ions into the cathode material, the team reinforced the honeycomb structure, resulting in a negligible voltage decay of only 0.02 mV per cycle, the first time that LMR cathode material with such a low level of voltage decay has been reported.
Through advanced atomic-scale measurements and calculations, the team found that these interlayer transition metal ions act as a “cap” above or below the honeycomb structure, preventing cation migration and maintaining stability. The structure remained intact even at high cut-off voltages and throughout cycling, ensuring the batteries' structural integrity. “Our work has solved the voltage decay problem in the LMR cathode, with a capacity almost two times higher than the widely used cathode materials, ultimately paving the way for more powerful and sustainable energy storage solutions,” said Professor Liu.
These findings hold great potential for various applications, from powering electric vehicles to portable electronics. The next step involves scaling up the manufacturing process for large-scale battery production.
Read more at Nature Energy:
https://www.nature.com/articles/s41560-023-01289-6
Detailed news story can also be retrieved from CityU Research News (21 Sept 2023) and CityU Research Stories (27 SEP 2023).
Media Coverage:
- 城大首創技術 大增電池儲存量 [東方日報] 2023-09-22 要聞港聞
- 城大研發近乎沒電壓衰減嶄新電池技術 [信報] 2023-09-21 時事脈搏 港聞
- Battery tech: CityU scientists achieve minimal voltage decay [Interesting Engineering] 2023-09-21 Innovation
- 新技術克服電壓衰減 可為電動車充電 城大研超級電池 容量增兩倍 [大公報] 2023-09-22 A8 港聞
- 創科路上/城大研超級電池 容量增兩倍 [大公報] 2023-09-22 創科路上
- 破鋰離子電池瓶頸 城大新招電壓衰減近零 [文匯報] 2023-09-22 港聞
- 香港首創 嶄新電池技術 [RTHK TV32] 2023-10-03 凝聚香港 第三百三十七集
06 Jul 2023
Spectroscopy and Imaging
Quantum Materials
Reconciling scaling of the optical conductivity of cuprate superconductors with Planckian resistivity and specific heat
Materials tuned to a quantum critical point display universal scaling properties as a function of temperature T and frequency ω. A long-standing puzzle regarding cuprate superconductors has been the observed power-law dependence of optical conductivity with an exponent smaller than one, in contrast to T-linear dependence of the resistivity and ω-linear dependence of the optical scattering rate. In this paper, we present and analyze resistivity and optical conductivity of La2−xSrxCuO4 with x = 0.24. We demonstrate ℏω/kBT scaling of the optical data over a wide range of frequency and temperature, T-linear resistivity, and optical effective mass proportional to ∼ ln𝑇 corroborating previous specific heat experiments. We show that a T, ω-linear scaling Ansatz for the inelastic scattering rate leads to a unified theoretical description of the experimental data, including the power-law of the optical conductivity. This theoretical framework provides new opportunities for describing the unique properties of quantum critical matter.
Read more at Nature Communications:
https://www.nature.com/articles/s41467-023-38762-5
26 May 2023
Soft Matter and Biophysics
Universality in RNA and DNA deformations induced by salt, temperature change, stretching force, and protein binding
Nucleic acid deformations play important roles in many biological processes. The physical understanding of nucleic acid deformation by environmental stimuli is limited due to the challenge in the precise measurement of RNA and DNA deformations and the complexity of interactions in RNA and DNA. Magnetic tweezers experiments provide an excellent opportunity to precisely measure DNA and RNA twist changes induced by environmental stimuli. In this work, we applied magnetic tweezers to measure double-stranded RNA twist changes induced by salt and temperature changes. We observed RNA unwinds when lowering salt concentration, or increasing temperature. Our molecular dynamics simulations revealed the mechanism: lowering salt concentration or increasing temperature enlarges RNA major groove width, which causes twist decrease through twist-groove coupling. Combining these results with previous results, we found some universality in RNA and DNA deformations induced by three different stimuli: salt change, temperature, and stretching force. For RNA, these stimuli first modify the major groove width, which is transduced into twist change through twist-groove coupling. For DNA, these stimuli first modify diameter, which is transduced into twist change through twist-diameter coupling. Twist-groove coupling and twist-diameter coupling appear to be utilized by protein binding to reduce DNA and RNA deformation energy cost upon protein binding.
Read more at Proceedings of the National Academy of Sciences USA:
https://www.pnas.org/doi/10.1073/pnas.2218425120
08 May 2023
Quantum Materials
Resolving the polar interface of infinite-layer nickelate thin films
Nickel-based superconductors provide a long-awaited experimental platform to explore possible cuprate-like superconductivity. Despite similar crystal structure and d electron filling, however, superconductivity in nickelates has thus far only been stabilized in thin-film geometry, raising questions about the polar interface between substrate and thin film. Here we conduct a detailed experimental and theoretical study of the prototypical interface between Nd1−xSrxNiO2 and SrTiO3. Atomic-resolution electron energy loss spectroscopy in the scanning transmission electron microscope reveals the formation of a single intermediate Nd(Ti,Ni)O3 layer. Density functional theory calculations with a Hubbard U term show how the observed structure alleviates the polar discontinuity. We explore the effects of oxygen occupancy, hole doping and cation structure to disentangle the contributions of each for reducing interface charge density. Resolving the non-trivial interface structure will be instructive for future synthesis of nickelate films on other substrates and in vertical heterostructures.
Read more at Nature Materials:
https://doi.org/10.1038/s41563-023-01510-7
27 Mar 2023
Atomic, Molecular, and Optical Physics
Stability of laser cavity-solitons for metrological applications
Laser cavity-solitons can appear in systems comprised of a nonlinear microcavity nested within an amplifying fiber loop. These states are robust and self-emergent and constitute an attractive class of solitons that are highly suitable for microcomb generation. Here, we present a detailed study of the free-running stability properties of the carrier frequency and repetition rate of single solitons, which are the most suitable states for developing robust ultrafast and high repetition rate comb sources. We achieve free-running fractional stability on both optical carrier and repetition rate (i.e., 48.9 GHz) frequencies on the order of 10−9 for a 1 s gate time. The repetition rate results compare well with the performance of state-of-the-art (externally driven) microcomb sources, and the carrier frequency stability is in the range of performance typical of modern free-running fiber lasers. Finally, we show that these quantities can be controlled by modulating the laser pump current and the cavity length, providing a path for active locking and long-term stabilization.
The work is featured on the front cover of the 20 March 2023 issue of Applied Physics Letters (Volume 122, Issue 12).
Read more at Applied Physics Letters:
https://doi.org/10.1063/5.0134147
20 Mar 2023
Theoretical and Computational Physics
Atomic, Molecular, and Optical Physics
Multidimensional Coherent Spectroscopy of Molecular Polaritons: Langevin Approach
We present a microscopic theory for nonlinear optical spectroscopy of N molecules in an optical cavity. Using the Heisenberg-Langevin equation, an analytical expression is derived for the time- and frequency-resolved signals accounting for arbitrary numbers of vibrational excitations. We identify clear signatures of the polariton-polaron interaction from multidimensional projections of the signal, e.g., pathways and timescales. Cooperative dynamics of cavity polaritons against intramolecular vibrations is revealed, along with a crosstalk between long-range coherence and vibronic coupling that may lead to localization effects. Our results further characterize the polaritonic coherence and the population transfer that is slower.
Read more at Physical Review Letters:
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.130.103001
10 Mar 2023
Atomic, Molecular, and Optical Physics
Stable optical lateral forces from inhomogeneities of the spin angular momentum
Transverse spin momentum related to the spin angular momentum (SAM) of light has been theoretically studied recently and predicted to generate an intriguing optical lateral force (OLF). Despite extensive studies, there is no direct experimental evidence of a stable OLF resulting from the dominant SAM rather than the ubiquitous spin-orbit interaction in a single light beam. Here, we theoretically unveil the nontrivial physics of SAM-correlated OLF, showing that the SAM is a dominant factor for the OLF on a nonabsorbing particle, while an additional force from the canonical (orbital) momentum is exhibited on an absorbing particle due to the spin-orbit interaction. Experimental results demonstrate the bidirectional movement of 5-μm-diameter particles on both sides of the beam with opposite spin momenta. The amplitude and sign of this force strongly depend on the polarization. Our optofluidic platform advances the exploitation of exotic forces in systems with a dominant SAM, facilitating the exploration of fascinating light-matter interactions.
Read more at Science Advances:
https://www.science.org/doi/full/10.1126/sciadv.abn2291
30 Nov 2022
Spectroscopy and Imaging
Quantum Materials
Unconventional superconductivity in topological Kramers nodal-line semimetals
The crystalline symmetry is a defining factor of the electronic band topology in solids, where many-body interactions often induce a spontaneous breaking of symmetry. Superconductors lacking an inversion center are among the best systems to study such effects or even to achieve topological superconductivity and realize Majorana zero modes. Here, we demonstrate that TRuSi materials (with T a transition metal) belong to this class. Their bulk normal states behave as three-dimensional Kramers nodal-line semimetals, characterized by large antisymmetric spin-orbit couplings and by hourglass-like dispersions protected by glide reflection. Our muon-spin spectroscopy measurements show that certain TRuSi compounds spontaneously break the time-reversal symmetry at TRuSi while unexpectedly showing a fully gapped superconductivity. Their unconventional behavior is consistent with a unitary (s + ip) pairing, reflecting a mixture of spin singlets and spin triplets. Hence, by combining an intrinsic time-reversal symmetry-breaking superconductivity with nontrivial electronic bands, TRuSi compounds provide an ideal platform for investigating the rich interplay between unconventional superconductivity and the exotic properties of Kramers nodal-line/hourglass fermions.
Read more at Science Advances:
https://www.science.org/doi/10.1126/sciadv.abq6589
28 Oct 2022
Theoretical and Computational Physics
Atomic, Molecular, and Optical Physics
Topological near fields generated by topological structures
The central idea of metamaterials and metaoptics is that, besides their base materials, the geometry of structures offers a broad extra dimension to explore for exotic functionalities. Here, we discover that the topology of structures fundamentally dictates the topological properties of optical fields and offers a new dimension to exploit for optical functionalities that are irrelevant to specific material constituents or structural geometries. We find that the nontrivial topology of metal structures ensures the birth of polarization singularities (PSs) in the near field with rich morphologies and intriguing spatial evolutions including merging, bifurcation, and topological transition. By mapping the PSs to non-Hermitian exceptional points and using homotopy theory, we extract the core invariant that governs the topological classification of the PSs and the conservation law that regulates their spatial evolutions. The results bridge singular optics, topological photonics, and non-Hermitian physics, with potential applications in chiral sensing, chiral quantum optics, and beyond photonics in other wave systems.
Read more at Science Advances:
https://www.science.org/doi/10.1126/sciadv.abq0910
14 Oct 2022
Atomic, Molecular, and Optical Physics
Deterministic Loading of Microwaves onto an Artificial Atom Using a Time-Reversed Waveform
Loading quantum information deterministically onto a quantum node is an important step toward a quantum network. Here, we demonstrate that coherent-state microwave photons with an optimal temporal waveform can be efficiently loaded onto a single superconducting artificial atom in a semi-infinite one-dimensional (1D) transmission-line waveguide. Using a weak coherent state (the number of photons (N) contained in the pulse ≪1) with an exponentially rising waveform, whose time constant matches the decoherence time of the artificial atom, we demonstrate a loading efficiency of 94.2% ± 0.7% from 1D semifree space to the artificial atom. The high loading efficiency is due to time-reversal symmetry: the overlap between the incoming wave and the time-reversed emitted wave is up to 97.1% ± 0.4%. Our results open up promising applications in realizing quantum networks based on waveguide quantum electrodynamics.
Read more at Nano Letters:
https://pubs.acs.org/doi/10.1021/acs.nanolett.2c02578
06 Oct 2022
Spectroscopy and Imaging
Quantum Materials
Tunable topological Dirac surface states and van Hove singularities in kagome metal GdV6Sn6
Tunable topological Dirac surface states and van Hove singularities in kagome metal GdV6Sn6
Transition-metal-based kagome materials at van Hove filling are a rich frontier for the investigation of novel topological electronic states and correlated phenomena. To date, in the idealized two-dimensional kagome lattice, topologically Dirac surface states (TDSSs) have not been unambiguously observed, and the manipulation of TDSSs and van Hove singularities (VHSs) remains largely unexplored. Here, we reveal TDSSs originating from a ℤ2 bulk topology and identify multiple VHSs near the Fermi level (EF) in magnetic kagome material GdV6Sn6. Using in situ surface potassium deposition, we successfully realize manipulation of the TDSSs and VHSs. The Dirac point of the TDSSs can be tuned from above to below EF, which reverses the chirality of the spin texture at the Fermi surface. These results establish GdV6Sn6 as a fascinating platform for studying the nontrivial topology, magnetism, and correlation effects native to kagome lattices. They also suggest potential application of spintronic devices based on kagome materials.
Read more at Science Advances:
https://www.science.org/doi/10.1126/sciadv.add2024
21 Sep 2022
Soft Matter and Biophysics
Resonant phonon modes induced by molecular rotations in alpha-pentaerythritol crystals
Solid-state cooling employing barocaloric materials, especially plastic crystals, paves a promising way for eco-friendly refrigeration. However, inadequate understanding of the thermal properties hinders their practical applications. Here, we take α-pentaerythritol crystals as an example and reveal that the thermal transport is anisotropic, and the lattice dynamics is highly anharmonic. More importantly, we find that the two lowest-energy resonant phonon modes, associated with two mutually perpendicular molecular rotations, play critical roles in thermal conductivity and phase stability. The two optical modes suppress the acoustic branches by the avoided-crossing effect and hybridize with and strongly scatter acoustic phonons, further hindering thermal transport in α-pentaerythritol crystals. Furthermore, pressure can largely tune the rotational dynamics, reduce the resonance, and increase the phonon transmission and by adjusting the pressure from 0 MPa to 3500 MPa, an increase of 108% and 103% in in-plane and out-of-plane thermal conductivity, respectively, can be achieved. Our work provides a microscopic understanding of the rotational dynamics in α-pentaerythritol crystals, which can facilitate designing molecules to achieve a better barocaloric effect with more excellent thermal properties.
Read more at Journal of Materials Chemistry C:
https://pubs.rsc.org/en/content/articlelanding/2022/tc/d2tc02878f
20 Sep 2022
Quantum Materials
Applied Physics
Electrostatic gating and intercalation in 2D materials
The doping or the alteration of crystals with guest species to obtain desired properties has long been a research frontier in materials science. However, the closely packed lattice structure in many crystals has limited the applicability of this strategy. The advent of 2D layered materials has led to revitalized interest in utilizing this approach through two important strategies, gating and intercalation, offering reversible modulation of the properties of the host material without breaking chemical bonds. In addition, these dynamically tunable techniques have enabled the synthesis of new hybrid materials. Here, we review how interactions between guest species and host 2D materials can tune the physics and chemistry of materials and discuss their remarkable potential for creating artificial materials and architectures beyond the reach of conventional methods.
Read more at Nature Review Materials :
https://doi.org/10.1038/s41578-022-00473-6
31 Aug 2022
Theoretical and Computational Physics
Robust entangling gate for capacitively coupled few-electron singlet-triplet qubits
The search for a sweet spot, the locus in qubit parameters where quantum control is first-order insensitive to noises, is key to achieve high-fidelity quantum gates. Efforts to search for such a sweet spot in conventional double-quantum-dot singlet-triplet qubits where each dot hosts one electron (“two-electron singlet-triplet qubit”), especially for two-qubit operations, have been unsuccessful. Here we consider singlet-triplet qubits allowing each dot to host more than one electron, with a total of four electrons in the double quantum dots (“four-electron singlet-triplet qubit”). We theoretically demonstrate, using configuration interaction calculations, that sweet spots appear in this coupled qubit system. We further demonstrate that, under realistic charge noise and hyperfine noise, a two-qubit operation at the proposed sweet spot could offer gate fidelities (∼99%) that are higher than the conventional two-electron singlet-triplet qubit system (∼90%). Our results should facilitate realization of high-fidelity two-qubit gates in singlet-triplet qubit systems.
Read more at Physical Review B:
https://doi.org/10.1103/PhysRevB.106.075417
19 Aug 2022
Atomic, Molecular, and Optical Physics
Self-emergence of robust solitons in a microcavity
In many disciplines, states that emerge in open systems far from equilibrium are determined by a few global parameters. These states can often mimic thermodynamic equilibrium, a classic example being the oscillation threshold of a laser that resembles a phase transition in condensed matter. However, many classes of states cannot form spontaneously in dissipative systems, and this is the case for cavity solitons that generally need to be induced by external perturbations, as in the case of optical memories.
Here we show that the slow non-linearities of a free-running microresonator-filtered fibre laser can transform temporal cavity solitons into the system’s dominant attractor. This phenomenon leads to reliable self-starting oscillation of microcavity solitons that are naturally robust to perturbations, recovering spontaneously even after complete disruption. These emerge repeatably and controllably into a large region of the global system parameter space in which specific states, highly stable over long timeframes, can be achieved.
Read more at Nature:
https://www.nature.com/articles/s41586-022-04957-x
10 Aug 2022
Quantum Materials
Applied Physics
Intrinsic magnetism in superconducting infinite-layer nickelates
We report muon spin rotation/relaxation studies of a series of superconducting infinite-layer nickelates. Regardless of the rare earth ion or doping, we observe an intrinsic magnetic ground state arising from local moments on the nickel sublattice. The coexistence of magnetism—which is likely to be antiferromagnetic and short-range ordered—with superconductivity is reminiscent of some iron pnictides and heavy fermion compounds, and qualitatively distinct from the doped cuprates.
Read more at Nature Physics:
https://doi.org/10.1038/s41567-022-01684-y
01 Aug 2022
Soft Matter and Biophysics
Shape-Reconfigurable Ferrofluids
Ferrofluids (FFs) can adapt their shape to a magnetic field. However, they cannot maintain their shape when the magnetic field is removed. Here, with a magneto-responsive and reconfigurable interfacial self-assembly (MRRIS) process, we show that FFs can be structured by a magnetic field and maintain their shape, like solids, after removing the magnetic field. The competing self-assembly of magnetic and nonmagnetic nanoparticles at the liquid interface endow FFs with both reconfigurability and structural stability. By manipulating the external magnetic field, we show that it is possible to “write” and “erase” the shape of the FFs remotely and repeatedly. To gain an in-depth understanding of the effect of MRRIS on the structure of FFs, we systematically study the shape variation of these liquids under both the static and dynamic magnetic fields. Our study provides a simple yet novel way of manipulating FFs and opens opportunities for the fabrication of all-liquid devices.
Read more at Nano Letters:
https://pubs.acs.org/doi/full/10.1021/acs.nanolett.2c01721
29 Jul 2022
Soft Matter and Biophysics
Forming double-helix phase of single polymer chains by the cooperation between local structure and nonlocal attraction
Double-helix structures, such as DNA, are formed in nature to realize many unique functions. Inspired by this, researchers are pursuing strategies to design such structures from polymers. A key question is whether the double helix can be formed from the self-folding of a single polymer chain without specific interactions. Here, using Langevin dynamics simulation and theoretical analysis, we find that a stable double-helix phase can be achieved by the self-folding of single semiflexible polymers as a result of the cooperation between local structure and nonlocal attraction. The critical temperature of double-helix formation approximately follows
Read more at Physical Review Letters:
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.128.197801
09 May 2022
Spectroscopy and Imaging
Quantum Materials
Rich nature of Van Hove singularities in Kagome superconductor CsV3Sb5
The recently discovered layered kagome metals AV3Sb5 (A = K, Rb, Cs) exhibit diverse correlated phenomena, which are intertwined with a topological electronic structure with multiple van Hove singularities (VHSs) in the vicinity of the Fermi level. As the VHSs with their large density of states enhance correlation effects, it is of crucial importance to determine their nature and properties. Here, we combine polarization-dependent angle-resolved photoemission spectroscopy with density functional theory to directly reveal the sublattice properties of 3d-orbital VHSs in CsV3Sb5. Four VHSs are identified around the M point and three of them are close to the Fermi level, with two having sublattice-pure and one sublattice-mixed nature. Remarkably, the VHS just below the Fermi level displays an extremely flat dispersion along MK, establishing the experimental discovery of higher-order VHS. The characteristic intensity modulation of Dirac cones around K further demonstrates the sublattice interference embedded in the kagome Fermiology. The crucial insights into the electronic structure, revealed by our work, provide a solid starting point for the understanding of the intriguing correlation phenomena in the kagome metals AV3Sb5.
Read more at Nature Communications:
https://www.nature.com/articles/s41467-022-29828-x
[Figure caption: Observation of high order Van Hove singularities in Kagome superconductor CsV3Sb5 with ARPES.]
25 Apr 2022
Atomic, Molecular, and Optical Physics
Measurement-Dependent Erasure of Distinguishability for the Observation of Interference in an Unbalanced SU(1,1) Interferometer
It is known that quantum interference can disappear with the mere possibility of distinguishability without actually performing the act but can be restored by erasing the path information. A common erasure method is by conditional projective measurement known as a “quantum eraser.” On the other hand, the essence of quantum interference is amplitude addition. So, direct amplitude measurement and subsequent addition can also lead to the recovery of interference. In this paper, we create temporal distinguishability in an unbalanced SU(1,1) interferometer and observe no interference in the direct photodetection of the outputs. Different from the quantum eraser scheme, we recover interference via amplitude measurement by homodyne detection even with distinguishability in photon generation but no conditional projective measurement. The new mechanism of recovering interference can be applied to other unbalanced interferometers and should have practical applications in quantum metrology and sensing.
Read more at Physical Review X Quantum:
https://journals.aps.org/prxquantum/abstract/10.1103/PRXQuantum.3.020313
19 Apr 2022
Theoretical and Computational Physics
Atomic, Molecular, and Optical Physics
Fermionic Many-Body Localization for Random and Quasiperiodic Systems in the Presence of Short- and Long-Range Interactions
We study many-body localization (MBL) for interacting one-dimensional lattice fermions in random (Anderson) and quasiperiodic (Aubry-Andre) models, focusing on the role of interaction range. We obtain the MBL quantum phase diagrams by calculating the experimentally relevant inverse participation ratio (IPR) at half-filling using exact diagonalization methods and extrapolating to the infinite system size. For short-range interactions, our results produce in the phase diagram a qualitative symmetry between weak and strong interaction limits. For long-range interactions, no such symmetry exists as the strongly interacting system is always many-body localized, independent of the effective disorder strength, and the system is analogous to a pinned Wigner crystal. We obtain various scaling exponents for the IPR, suggesting conditions for different MBL regimes arising from interaction effects.
Read more at Physical Review Letters:
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.128.146601
06 Apr 2022
Soft Matter and Biophysics
Twist-diameter coupling drives DNA twist changes with salt and temperature
DNA deformations upon environmental changes, e.g., salt and temperature, play crucial roles in many biological processes and material applications. Here, our magnetic tweezers experiments observed that the increase in NaCl, KCl, or RbCl concentration leads to substantial DNA overwinding. Our simulations and theoretical calcula-tion quantitatively explain the salt-induced twist change through the mechanism: More salt enhances the screening of interstrand electrostatic repulsion and hence reduces DNA diameter, which is transduced to twist increase through twist-diameter coupling. We determined that the coupling constant is 4.5 ± 0.8 kBT/(degrees∙nm) for one base pair. The coupling comes from the restraint of the contour length of DNA backbone. On the basis of this coupling con-stant and diameter-dependent DNA conformational entropy, we predict the temperature dependence of DNA twist ≈ −0.01 degree/°C, which agrees with our and previous experimental results. Our analysis suggests that twist-diameter coupling is a common driving force for salt- and temperature-induced DNA twist changes.
Read more at Science Advances:
https://www.science.org/doi/epdf/10.1126/sciadv.abn1384
23 Mar 2022
Soft Matter and Biophysics
Multivalent Cations Reverse the Twist-Stretch Coupling of RNA
When stretched, both DNA and RNA duplexes change their twist angles through twist-stretch coupling. The coupling is negative for DNA but positive for RNA, which is not yet completely understood. Here, our magnetic tweezers experiments show that the coupling of RNA reverses from positive to negative by multivalent cations. Combining with the previously reported tension-induced negative-to-positive coupling reversal of DNA, we propose a unified mechanism of the couplings of both RNA and DNA based on molecular dynamics simulations. Two deformation pathways are competing when stretched: shrinking the radius causes positive couplings but widening the major groove causes negative couplings. For RNAwhose major groove is clamped by multivalent cations and canonical DNA, their radii shrink when stretched, thus exhibiting positive couplings. For elongated DNA whose radius already shrinks to the minimum and canonical RNA, their major grooves are widened when stretched, thus exhibiting negative couplings.
Read more at Physical Review Letters:
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.128.108103
The paper was featured in CityU Research Stories:
https://www.cityu.edu.hk/research/stories/2022/06/02/physical-mechanisms-explaining-dna-and-rna-twist-changes
11 Mar 2022
Theoretical and Computational Physics
Quantum Materials
Plasmonic Nanocavity Induced Coupling and Boost of Dark Excitons in Monolayer WSe2 at Room Temperature
Spin-forbidden excitons in monolayer transition metal dichalcogenides are optically inactive at room temperature. Probing and manipulating these dark excitons are essential for understanding exciton spin relaxation and valley coherence of these 2D materials. Here, we show that the coupling of dark excitons to a metal nanoparticle-on-mirror cavity leads to plasmon-induced resonant emission with the intensity comparable to that of the spin-allowed bright excitons. A three-state quantum model combined with full-wave electrodynamic calculations reveals that the radiative decay rate of the dark excitons can be enhanced by nearly six orders of magnitude through the Purcell effect, therefore compensating its intrinsic nature of weak radiation. Our nanocavity approach provides a useful paradigm for understanding the room-temperature dynamics of dark excitons, potentially paving the road for employing dark exciton in quantum computing and nanoscale optoelectronics.
Read more at Nano Letters:
https://doi.org/10.1021/acs.nanolett.1c04360
28 Feb 2022
Spectroscopy and Imaging
Ultralarge anti-Stokes lasing through tandem upconversion
Coherent ultraviolet light is important for applications in environmental and life sciences. However, direct ultraviolet lasing is constrained by the fabrication challenge and operation cost. Herein, we present a strategy for the indirect generation of deep-ultraviolet lasing through a tandem upconversion process. A core–shell–shell nanoparticle is developed to achieve deep-ultraviolet emission at 290 nm by excitation in the telecommunication wavelength range at 1550 nm. The ultralarge anti-Stokes shift of 1260 nm (~3.5 eV) stems from a tandem combination of distinct upconversion processes that are integrated into separate layers of the core–shell–shell structure. By incorporating the core–shell–shell nanoparticles as gain media into a toroid microcavity, single-mode lasing at 289.2 nm is realized by pumping at 1550 nm. As various optical components are readily available in the mature telecommunication industry, our findings provide a viable solution for constructing miniaturized short-wavelength lasers that are suitable for device applications.
Read more at Nature Communications:
https://www.nature.com/articles/s41467-022-28701-1
24 Feb 2022
Spectroscopy and Imaging
Quantum Materials
Multiple mobile excitons manifested as sidebands in quasi-one-dimensional metallic TaSe3
Exciton in solids is one bound state formed by electron and hole attracting each other via Coulomb interaction. Charge neutrality and an expected itinerant nature makes excitons potential transmitters of information. Excitons are well studied in insulators or semiconductors with negligible velocity that limits its transport efficiency and application, as it’s thought that screening effect from free charges usually suppresses the formation of excitons in metal. Until very recently, we discovered robust excitons with large velocity in quasi-one-dimensional metal Titanium Triselenide, taking advantage of its low dimensionality and carrier density.
Read more at Nature Materials:
https://www.nature.com/articles/s41563-022-01201-9
The paper was featured in CityU Research Stories:
https://www.cityu.edu.hk/research/stories/2022/03/18/fast-moving-excitons-observed-first-time-metal-unlocking-potential-speed-digital-communication
21 Feb 2022
Spectroscopy and Imaging
Applied Physics
A super-elastic high-energy Elinvar alloy
Metals become softer when they are heated up, as their stiffness decreases due to thermally induced expansion. More than a decade ago, Swiss physicist Charles Edourad Guillaume, who received the Nobel Prize in Physics in 1920, discovered a nickel based alloy with temperature insensitive elastic modules, which he named Elinvar. Such Elinvar alloys are very rare and the underlying mechnisms are still under debate. Recently, a research team led by Prof. Yang Yong at CityU together with international collaborators has discoveried a super-elastic high-entropy alloy, which exhibits an extraordinary Elinvar effect, retaining near-constant elastic modulus from room temperature to more than 600 degree Celsius. Furthermore, normal metals unusally have the elastic strain limit less than one percent, except those having pseuado-elasticity associated with phase transformation and large energy dissipation. This newly discovered alloy can attain a high elastic strain limit of two percent. We performed in situ high energy x-ray diffraction studies which demonstrated that the high elastic strain limit is not related to stress-induced phase transformation, but rather arises from the high lattice friction. Such a novel alloy with unprecendented mechanical properties has a great potential for aerospace engineering.
Read more at Nature:
https://www.nature.com/articles/s41586-021-04309-1
The paper was featured in CityU Research Stories:
https://www.cityu.edu.hk/research/stories/2022/02/10/super-elastic-high-entropy-elinvar-alloy-discovered-potential-aerospace-engineering
09 Feb 2022
Theoretical and Computational Physics
Atomic, Molecular, and Optical Physics
Exact mobility edges in the non-Hermitian t1-t2 model: Theory and possible experimental realizations
Quantum localization in 1D non-Hermitian systems, especially the search for exact single-particle mobility edges, has attracted considerable interest recently. While much progress has been made, the available methods to determine the ME in such models are still limited. In this work, we use a new method to find a new class of exact mobility edges in 1D non-Hermitian quasiperiodic models with parity-time (PT) symmetry. We illustrate our method by studying a specific model. We first use our method to determine the energy-dependent mobility edge as well as the spectrum for localized eigenstates in this model. We then demonstrate that the metal-insulator transition must occur simultaneously with the spontaneous PT-symmetry breaking transition in this model. Finally, we propose an experimental protocol based on a 1D photonic lattice to distinguish the extended and localized single-particle states in our model. The results in our work can be applied to studying other non-Hermitian quasiperiodic models.
Read more at Physical Review B:
https://journals.aps.org/prb/abstract/10.1103/PhysRevB.105.014207
31 Jan 2022
Theoretical and Computational Physics
SU(2)-in-SU(1,1) nested interferometer for highly sensitive, loss-tolerant quantum metrology
We present experimental and theoretical results on a new interferometer topology that nests a SU(2) interferometer, e.g., a Mach-Zehnder or Michelson interferometer, inside a SU(1,1) interferometer, i.e., a Mach-Zehnder interferometer with parametric amplifiers in place of beam splitters. This SU(2)-in-SU(1,1) nested interferometer (SISNI) simultaneously achieves a high signal-to-noise ratio (SNR), sensitivity beyond the standard quantum limit (SQL) and tolerance to photon losses external to the interferometer, e.g.,in detectors. We implement a SISNI using parametric amplification by four-wave mixing (FWM) in Rb vapor and a laser-fed Mach-Zehnder SU(2) interferometer. We observe path-length sensitivity with SNR 2.2 dB beyond the SQL at power levels (and thus SNR) 2 orders of magnitude beyond those of previous loss-tolerant interferometers. We find experimentally the optimal FWM gains and find agreement with a minimal quantum noise model for the FWM process. The results suggest ways to boost the in-practice sensitivity of high-power interferometers, e.g., gravitational wave interferometers, and may enable high sensitivity, quantum-enhanced interferometry at wavelengths for which efficient detectors are not available.
Read more at Physical Review Letters:
https://doi.org/10.1103/PhysRevLett.128.033601
20 Jan 2022
Atomic, Molecular, and Optical Physics
Programmable photon pair source
Photon pairs produced by the pulse-pumped nonlinear parametric processes have been a workhorse of quantum information science (QIS). Engineering the spectral property of quantum states is crucial for practical QIS applications. However, photon pairs with desirable spectral properties are currently achieved with specially engineered optical hardware but with severely limited flexibility in tuning the spectral properties of the sources. Here, we demonstrate a spectrally programmable photon pair source by exploiting a two-stage nonlinear interferometer scheme with a computer-controlled phase device. The phase-control device can introduce phase shifts for spectral engineering by a programmable phase function that can be arbitrarily defined. When the phase function is properly designed, the output spectrum of the source can be freely customized and changed without replacing any hardware component in the system. Our investigation provides a flexible and powerful new approach for engineering the mode profile of photon pairs and should find wide applications in QIS.
Read more at APL Photonics:
https://doi.org/10.1063/5.0069383
03 Jan 2022
Atomic, Molecular, and Optical Physics
Propagation of temporal mode multiplexed optical fields in fibers: influence of dispersion
The capacity and performance of communication system can be increased and improved by encoding information on the multiple degree of freedom of optical fields, such as wavelength, polarization and spatial structure. Temporal modes, which provide a convenient orthogonal basis for studying light pulses, have only come into play recently. However, the broadband nature due to ultrashort pulses makes it sensitive to dispersion of transmission media. In this work, we examine the effect of medium dispersion on different orders of temporal modes. We characterize how the spectral profiles of temporal modes change with the fiber induced dispersion with a fourth-order interference technique when the order number and bandwidth of temporal modes are varied. Moreover, we discuss how to recover the spectrally distorted temporal mode by measuring and compensating the transmission induced dispersion. Our investigation paves the way for further investigating the distribution of temporally multiplexed quantum states in fiber network.
This paper is selected as Editors' Pick.
Read more at Optics Express:
https://doi.org/10.1364/OE.448013
03 Jan 2022
Theoretical and Computational Physics
Atomic, Molecular, and Optical Physics
Stability of scar states in the two-dimensional PXP model against random disorder
Recently a class of quantum systems exhibiting weak ergodicity breaking has attracted much attention. These systems feature in the energy spectrum a special band of eigenstates called quantum many-body scar states. One prototype model that is known to host scar states is the so-called PXP model. In this work we study the fate of quantum many-body scar states in a 2D PXP model against random disorders. We show that in both the square lattice and the honeycomb lattice the scar states can persist up to a finite disorder strength, before eventually being erased by the disorder. We further study the localization properties of the system in the presence of even stronger disorders and show that whether a full localization transition occurs depends on the type of disorder we introduce. Our study thus reveals the fascinating interplay between disorder and quantum many-body scarring in a 2D system.
Read more at Physical Review B:
https://journals.aps.org/prb/abstract/10.1103/PhysRevB.104.214305
13 Dec 2021
Soft Matter and Biophysics
Crowding-induced polymer trapping in a channel
In this work, we report an intriguing phenomenon: crowding-induced polymer trapping in a channel. Using Langevin dynamics simulations and analytical calculations, we find that for a polymer confined in a channel, crowding particles can push a polymer into the channel corner through inducing an effective polymer-corner attraction due to the depletion effect. This phenomenon is referred to as polymer trapping. The occurrence of polymer trapping requires a minimum volume fraction of crowders,
Read more at Physical Review E:
https://journals.aps.org/pre/abstract/10.1103/PhysRevE.104.054502
08 Nov 2021
Theoretical and Computational Physics
Applied Physics
Generalized momentum conservation and Fedorov-Imbert linear shift of acoustic vortex beams at a metasurface
We study the transmission and reflection of a paraxial acoustic vortex beam (AVB) at a metasurface with a prescribed phase gradient. By using a plane-wave decomposition method, we obtain a closed-form expression for the transverse shift of the beam's gravity center. The transverse shift is proportional to the topological charge of the AVB and derives from momentum conservation and the reflection/transmission coefficient modulation. It corresponds to the acoustic Fedorov-Imbert linear shift and represents the orbital Hall effect for spinless longitudinal sound. We develop a generalized momentum conservation relation to understand the phenomena and find that the metasurface induces both linear momentum and angular momentum, which have different dependences on the incident angle. In particular, this mechanism can enable the Fedorov-Imbert linear shift even for normal incidence and total reflection. The results provide deeper insights into the physics underlying the interaction of AVBs with metasurfaces and may help the development of orbital angular momentum based acoustic devices.
Read more at Physical Review B:
https://journals.aps.org/prb/abstract/10.1103/PhysRevB.104.174301
01 Nov 2021
Theoretical and Computational Physics
Applied Physics
Spin-orbit interactions of transverse sound
Spin-orbit interactions (SOIs) endow light with intriguing properties and applications such as photonic spin-Hall effects and spin-dependent vortex generations. However, it is counterintuitive that SOIs can exist for sound, which is a longitudinal wave that carries no intrinsic spin. Here, we theoretically and experimentally demonstrate that airborne sound can possess artificial transversality in an acoustic micropolar metamaterial and thus carry both spin and orbital angular momentum. This enables the realization of acoustic SOIs with rich phenomena beyond those in conventional acoustic systems. We demonstrate that acoustic activity of the metamaterial can induce coupling between the spin and linear crystal momentum k, which leads to negative refraction of the transverse sound. In addition, we show that the scattering of the transverse sound by a dipole particle can generate spin-dependent acoustic vortices via the geometric phase effect. The acoustic SOIs can provide new perspectives and functionalities for sound manipulations beyond the conventional scalar degree of freedom and may open an avenue to the development of spin-orbit acoustics.
Read more at Nature Communications:
https://www.nature.com/articles/s41467-021-26375-9
The paper was featured in CityU Research Stories:
https://www.cityu.edu.hk/research/stories/2021/12/09/cityu-physicists-discovered-special-transverse-sound-wave
Media Coverage:
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Physicists discover special transverse sound wave [Phys.org] 2021-12-07 General Physics
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CityU physicists discovered special transverse sound wave [EurekAlert!] 2021-12-07 News Release
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CityU physicists discovered special transverse sound wave [ScienMag] 2021-12-07 Social & Behavioral Science
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Physicists discovered special transverse sound wave [Science Daily] 2021-12-07 Science News
21 Oct 2021
Soft Matter and Biophysics
Tube model for polymer knots with excluded volume interactions and its application
Knotting commonly occurs in long polymers and strongly affects polymer behavior, but the theoretical understanding of polymer knots is very limited. In this work, we apply the tube model to understand polymer knots and reveal many intriguing knot properties. The tube model assumes that the polymer segments in a knot core are confined within a virtual tube due to topological entanglements and presents a simplified view of knotted polymer conformations that appear irregular and disordered. To materialize the conceptual tube, we generate a large number of knotted polymer conformations by Monte Carlo simulations and superimpose them to obtain the tube. After comparing the tubes for polymer knots with and without excluded volume (EV) interactions, we find that EV interactions substantially reduce the accessible tube diameters but only weakly affect the tube axis. The tube model quantitatively explains the dramatic bending variation within knot cores, which can be applied to explain an intriguing phenomenon: knot positioning in a polymer with non-uniform bending stiffness. Overall, the tube model converts the complicated knotting problem to a tube-confinement problem, which is more tackleable by theory and can be used to calculate the shape, fluctuation, and free energy of polymer knots.
Read more at Macromolecules:
https://pubs.acs.org/doi/abs/10.1021/acs.macromol.1c01483
12 Oct 2021
Theoretical and Computational Physics
Atomic, Molecular, and Optical Physics
Robust exceptional point of arbitrary order in coupled spinning cylinders
Exceptional points (EPs), i.e., non-Hermitian degeneracies at which eigenvalues and eigenvectors coalesce, can be realized by tuning the gain/loss contrast of different modes in non-Hermitian systems or by engineering the asymmetric coupling of modes. Here we demonstrate a mechanism that can achieve EPs of arbitrary order by employing the non-reciprocal coupling of spinning cylinders sitting on a dielectric waveguide. The spinning motion breaks the time-reversal symmetry and removes the degeneracy of opposite chiral modes of the cylinders. Under the excitation of a linearly polarized plane wave, the chiral mode of one cylinder can unidirectionally couple to the same mode of the other cylinder via the spin-orbit interaction associated with the evanescent wave of the waveguide. The structure can give rise to arbitrary-order EPs that are robust against spin-flipping perturbations, in contrast to conventional systems relying on spin-selective excitations. In addition, we show that higher-order EPs in the proposed system are accompanied by enhanced optical isolation, which may find applications in designing novel optical isolators, nonreciprocal optical devices, and topological photonics.
Read more at Optics Express:
https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-29-19-29720&id=458332
31 Aug 2021
Theoretical and Computational Physics
Quantum fluctuation-dissipation theorem far from equilibrium
Fluctuations associated with relaxations in the far-from-equilibrium regime is of fundamental interest for a large variety of systems within broad scales. Recent advances in techniques such as spectroscopy have generated the possibility for measuring the fluctuations of the mesoscopic systems in connection to the relaxation processes when driving the underlying quantum systems far from equilibrium. We present a general nonequilibrium fluctuation-dissipation theorem (FDT) for quantum Markovian processes where the detailed-balance condition is violated. Apart from the fluctuations, the relaxation involves extra correlation that is governed by the quantum curl flux emerged in the far-from-equilibrium regime. Such a contribution vanishes for the thermal equilibrium, so that the conventional FDT is recovered. We finally apply the nonequilibrium FDT to the molecular junctions, elaborating the detailed-balance-breaking effects on the optical transmission spectrum. Our results have the advantage of and exceed the scope of the fluctuation-dissipation relation in the perturbative and near equilibrium regimes, and is of broad interest for the study of quantum thermodynamics.
Read more at Physical Review B:
https://doi.org/10.1103/PhysRevB.104.085439
30 Aug 2021
Spectroscopy and Imaging
Quantum Materials
Observation of a singular Weyl point surrounded by charged nodal walls in PtGa
Weyl points in condensed matter are analogous to magnetic monopoles in momentum space. Constrained by the Nielsen-Ninomiya no-go theorem, it is well believed that Weyl points should appear in pairs with opposite charges. As a consequence, Fermi arcs occur on surfaces which connect the projections of the WPs with opposite chiral charges. Paired Weyl points and related surface Fermi arcs are two essential characters to search for Weyl semimetal in all provious studies. However, this situation can be circumvented. Here, the authors report one unpaired Weyl point without surface Fermi arc for the first time, which is surrounded by charged Weyl nodal walls in PtGa. The attendance of closed Weyl nodal walls allow unpaired Weyl points to accour inside it.
Meanwhile, the geometry between unpaired WPs and Weyl nodal walls is quite different from that of all previous Weyl semimetals, so that the transport properties are expected to change significantly in this new kind of WSMs, which needs further study in the future. The observation will open a new avenue to the bulk topological properties of Weyl fermions in solids, which will promote the understanding of basic topological physics, and application of WSMs into spintronics.
Read more at Nature Communications:
https://www.nature.com/articles/s41467-021-24289-0
28 Jun 2021
Theoretical and Computational Physics
Quantum Materials
Implementation of geometric quantum gates on microwave-driven semiconductor charge qubits
Implementation of geometric quantum gates on microwave-driven semiconductor charge qubits
A semiconductor-based charge qubit, confined in double quantum dots, can be a platform to implement quantum computing. However, it suffers severely from charge noises. Here, a theoretical framework to implement universal geometric quantum gates in this system is provided. It is found that, while the detuning noise can be suppressed by operating near its corresponding sweet spot, the tunneling noise, on the other hand, is amplified and becomes the dominant source of error for single-qubit gates, a fact previously insufficiently appreciated. It is demonstrated, through numerical simulation, that the geometric gates outperform the dynamical gates across a wide range of tunneling noise levels, making them particularly suitable to be implemented in conjunction with microwave driving. To obtain a nontrivial two-qubit gate, a hybrid system is introduced with charge qubits coupled by a superconducting resonator. When each charge qubit is in resonance with the resonator, it is possible to construct an entangling geometric gate with fidelity higher than that of the dynamical gate for experimentally relevant noise levels. Therefore, our results suggest that geometric quantum gates are powerful tools to achieve high-fidelity manipulation for the charge qubit.
Read more at Advanced Quantum Technologies:
https://doi.org/10.1002/qute.202100011
12 Jun 2021
Atomic, Molecular, and Optical Physics
Synthesized soliton crystals
Synthesized soliton crystals
Dissipative Kerr soliton (DKS) featuring broadband coherent frequency comb with compact size and low power consumption, provides an unparalleled tool for nonlinear physics investigation and precise measurement applications. However, the complex nonlinear dynamics generally leads to stochastic soliton formation process and makes it highly challenging to manipulate soliton number and temporal distribution in the microcavity. Here, synthesized and reconfigurable soliton crystals (SCs) are demonstrated by constructing a periodic intra-cavity potential field, which allows deterministic SCs synthesis with soliton numbers from 1 to 32 in a monolithic integrated microcavity. The ordered temporal distribution coherently enhanced the soliton crystal comb lines power up to 3 orders of magnitude in comparison to the single-soliton state. The interaction between the traveling potential field and the soliton crystals creates periodic forces on soliton and results in forced soliton oscillation. Our work paves the way to effectively manipulate cavity solitons. The demonstrated synthesized SCs offer reconfigurable temporal and spectral profiles, which provide compelling advantages for practical applications such as photonic radar, satellite communication and radio-frequency filter.
Read more at Nature Communications:
https://doi.org/10.1038/s41467-021-23172-2
26 May 2021
Spectroscopy and Imaging
A medium-range structure motif linking amorphous and crystalline states
A medium-range structure motif linking amorphous and crystalline states
Amorphous materials have no long-range order, but there are ordered structures at short range (2–5 Å), medium range (5–20 Å) and even longer length scales1–5. While regular6,7 and semiregular polyhedra8–10 are often found as short-range ordering in amorphous materials, the nature of medium-range order has remained elusive11–14. Consequently, it is difficult to determine whether there exists any structural link at medium range or longer length scales between the amorphous material and its crystalline counterparts. Moreover, an amorphous material often crystallizes into a phase of different composition15, with very different underlying structural building blocks, further compounding the issue. Here, we capture an intermediate crystalline cubic phase in a Pd-Ni-P amorphous alloy and reveal the structure of the medium-range order, a six-membered tricapped trigonal prism cluster (6M-TTP) with a length scale of 12.5 Å. We find that the 6M-TTP can pack periodically to several tens of nanometres to form the cube phase. Our experimental observations provide evidence of a structural link between the amorphous and crystalline phases in a Pd-Ni-P alloy at the medium-range length scale and suggest that it is the connectivity of the 6M-TTP clusters that distinguishes the crystalline and amorphous phases. These findings will shed light on the structure of amorphous materials at extended length scales beyond that of short-range order.
Read more at Nature Materials:
https://doi.org/10.1038/s41563-021-01011-5
The paper was featured in CityU Research Stories:
https://www.cityu.edu.hk/research/stories/2021/06/07/cityu-scientists-make-breakthrough-towards-solving-structural-mystery-glass
Please click the following links for media coverage: :
- CityU scientists make a breakthrough towards solving the structural mystery of glass [EurekAlert!] 2021-06-08 Research News
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CityU scientists make a breakthrough towards solving the structural mystery of glass [Bioengineer.org] 2021-06-08 Science News
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CityU Scientists Make A Breakthrough Towards Solving The Structural Mystery Of Glass [ScienMag] 2021-06-08 Technology and Engineering
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Breakthrough towards solving the structural mystery of glass [Phys.org] 2021-06-08 Materials Science
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Solving the structural mystery of glass [Science Daily] 2021-06-08 Science News
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Breakthrough Discovery Sheds Light on Glass Structure [Azom.com] 2021-06-09 Materials Analysis
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我国学者与海外合作者在非晶态结构本质研究方面取得进展 [Ebiotrade] 2021-06-11
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CityU scientists make a breakthrough towards solving structural mystery of glass [Mirage News] 2021-06-08 Science
20 May 2021
Quantum Materials
Stabilization of Sr3Al2O6 Growth Templates for Ex Situ Synthesis of Freestanding Crystalline Oxide Membranes
Stabilization of Sr3Al2O6 Growth Templates for Ex Situ Synthesis of Freestanding Crystalline Oxide Membranes
A new synthetic approach has recently been developed for the fabrication of freestanding crystalline perovskite oxide nanomembranes, which involves the epitaxial growth of a water-soluble sacrificial layer. By utilizing an ultrathin capping layer of SrTiO3, here we show that this sacrificial layer, as grown by pulsed laser deposition, can be stabilized in air and therefore be used as transferrable templates for ex situ epitaxial growth using other techniques. We find that the stability of these templates depends on the thickness of the capping layer. On these templates, freestanding superconducting SrTiO3 membranes were synthesized ex situ using molecular beam epitaxy, enabled by the lower growth temperature which preserves the sacrificial layer. This study paves the way for the synthesis of an expanded selection of freestanding oxide membranes and heterostructures with a wide variety of ex situ growth techniques.
Read more at Nano Letters:
https://doi.org/10.1021/acs.nanolett.1c01194
14 May 2021
Theoretical and Computational Physics
Atomic, Molecular, and Optical Physics
Coherent control of the multiple wavelength lasing of N_2^+: coherence transfer and beyond
Coherent control of the multiple wavelength lasing of N_2^+: coherence transfer and beyond
Nitrogen molecules pumped by intense femtosecond laser pulses give rise to coherent emission in the forward direction at a series of wavelengths, coined “air lasing.” We demonstrate the coherent control of these emissions via a pair of seeding pulses at two different frequencies at low pressures, revealing a coherence transfer through vibrational motion. It is found that the injection of a 427.8 nm (391.4 nm) seeding pulse results in its amplification at the expense of the 391.4 nm (427.8 nm) signal, demonstrating a competition between the two spectral components of the emission from the upper level population. Moreover, when the delay between the seeding and pump pulses is finely tuned, the coherent control of both transitions is observed via the coherence transfer. A microscopic molecular relaxation model reproduces these observations, highlighting the crucial role of electronic and vibrational coherences, as well as their coupling, during the lasing action.
Read more at Optica:
https://www.osapublishing.org/optica/fulltext.cfm?uri=optica-8-5-668&id=450918
10 May 2021
Applied Physics
Hearing loss impacts gray and white matter across the lifespan: Systematic review, meta-analysis and meta-regression
Hearing loss is a heterogeneous disorder thought to affect brain reorganization across the lifespan. Here, structural alterations of the brain due to hearing loss are assessed by quantitatively analyzing publicly available brain magnetic resonance imaging data from patients and controls. Hearing loss was found to affect gray matter (GM) and underlying white matter (WM) in nearly every region of the brain, instead of being concentrated in regions typically associated with hearing. In congenital hearing loss, GM decreased most in the frontal lobe, not the temporal lobe where traditional auditory regions are located. Similarly, acquired hearing loss had a decrease in frontal lobe GM, albeit the insula was most decreased. In congenital, WM underlying the frontal lobe GM was most decreased. In congenital, the right hemisphere was more negatively impacted than the left hemisphere; however, in acquired, this was the opposite. The WM alterations most frequently underlined GM alterations in congenital hearing loss. Future studies should use brain imaging of hearing loss as a prognostic template for discerning clinical outcomes. This work was a collaborative effort between the City University of Hong Kong, McGill University, University of Pennsylvania, and John Hopkin's University. It has been published in the top neuroscience journal NeuroImage.
Read more at ScienceDirect:
https://doi.org/10.1016/j.neuroimage.2021.117826
01 May 2021
Theoretical and Computational Physics
Generalizable control for multiparameter quantum metrology
Generalizable control for multiparameter quantum metrology
One of the main goals of quantum metrology is to find ways to approach the highest possible precision for measuring unknown parameters. While optimal controls, typically found by gradient-based methods, provide a practical route to this goal, it is only optimized for a specific set of parameters and the entire algorithm must be rerun for a new set, making the procedure inefficient especially for an ensemble of systems with parameters varying in ranges. A "generalizable" method that can systematically update optimal controls with minimal cost is then desirable.
Based on a previous work of generalizable optimal control found by reinforcement learning for estimating a single parameter, we consider the situation involving multiple parameters in this paper. We have found that, in cases where controls are "complete", an analytical method which efficiently generates optimal controls for any parameter starting from an initial result found either by GRAPE or reinforcement learning can be applied. When the controls are restricted, the analytical scheme is invalid but reinforcement learning still retains a level of generalizability. Lastly, in cases where controls cannot compensate the shift in the Hamiltonian due to change in parameters, no generalizability is found. We argue that the generalization of reinforcement learning is through a mechanism similar to the analytical scheme.
Our results provide insights on when and how the optimal control in multi-parameter quantum metrology can be generalized, thereby facilitating efficient implementation of optimal quantum estimation of multiple parameters, particularly for an ensemble of systems with ranges of parameters.
Read more at Physical Review A:
https://doi.org/10.1103/PhysRevA.103.042615
29 Apr 2021
Theoretical and Computational Physics
Quantum Materials
Charge noise suppression in capacitively coupled singlet-triplet spin qubits under magnetic field
Charge noise suppression in capacitively coupled singlet-triplet spin qubits under magnetic field
Charge noises are the main obstacle to realize scalable quantum computation, and working around sweet spots is effective to reduce charge noise. However, existence of sweet spots for two singlet-triplet qubits is unclear. We demonstrate, through full configuration interaction calculations, that a range of nearly sweet spots appear in the coupled singlet-triplet qubits under a large enough magnetic field. We further demonstrate that ramping to and from such nearly sweet spots using pulses from shortcut to adiabaticity offers maximal protection of the qubit against decoherence. Our results should facilitate realization of high-fidelity two-qubit gates in coupled singlet-triplet qubit systems.
Read more at Physical Review B:
https://doi.org/10.1103/PhysRevB.103.L161409
27 Apr 2021
Theoretical and Computational Physics
Generic detection-based error mitigation using quantum autoencoders
Generic detection-based error mitigation using quantum autoencoders
One of the major challenges in the Noisy intermediate-scale quantum (NISQ) era is the lack of a general error mitigation technique demanding minimal additional resources. The protocol developed in this work is particularly useful in the NISQ era, as it does not require extra qubits, can be applied to different types of noise models and quantum data, and is more powerful than existing techniques in many situations. This generic protocol can be directly applied to a variety of algorithms running on near-term quantum devices.
Read more at Physical Review A:
https://doi.org/10.1103/PhysRevA.103.L040403
22 Apr 2021
Quantum Materials
Applied Physics
Large Tuning of Electroresistance in an Electromagnetic Device Structure Based on the Ferromagnetic–Piezoelectric Interface
Large Tuning of Electroresistance in an Electromagnetic Device Structure Based on the Ferromagnetic–Piezoelectric Interface
The electrical control of the conducting state through phase transition and/or resistivity switching in heterostructures of strongly correlated oxides is at the core of the large on-going research activity of fundamental and applied interest. In an electromechanical device made of a ferromagnetic–piezoelectric heterostructure, we observe an anomalous negative electroresistance of ∼−282% and a significant tuning of the metal-to-insulator transition temperature when an electric field is applied across the piezoelectric. We have identified the electric field applied along the conducting bridge of the device as the plausible origin: stretching the underlying piezoelectric substrate gives rise to a lattice distortion of the ferromagnetic manganite overlayer through epitaxial strain. Large modulations of the resistance are also observed by applying static dc voltages across the thickness of the piezoelectric substrate. These results indicate that the emergent electronic phase separation in the manganites can be selectively manipulated when interfacing with a piezoelectric material, which offers great opportunities in designing oxide-based electromechanical devices.
Read more at ACS Publications:
https://pubs.acs.org/doi/10.1021/acsami.1c00085
14 Apr 2021
Spectroscopy and Imaging
Quantum Materials
Discovery of 𝐶2 rotation anomaly in topological crystalline insulator SrPb
Discovery of 𝐶2 rotation anomaly in topological crystalline insulator SrPb
Topological crystalline insulators (TCIs) are insulating electronic states with nontrivial topology protected by crystalline symmetries. Recently, theory has proposed new classes of TCIs protected by rotation symmetries Cn, which have surface rotation anomaly evading the fermion doubling theorem, i.e., n instead of 2n Dirac cones on the surface preserving the rotation symmetry. Here, we report the first realization of the C2 rotation anomaly in a binary compound SrPb. Our first-principles calculations reveal two massless Dirac fermions protected by the combination of time-reversal symmetry T and C2y on the (010) surface. Using angle-resolved photoemission spectroscopy, we identify two Dirac surface states inside the bulk band gap of SrPb, confirming the C2 rotation anomaly in the new classes of TCIs. The findings enrich the classification of topological phases, which pave the way for exploring exotic behavior of the new classes of TCIs.
Read more at Nature Communications:
https://www.nature.com/articles/s41467-021-22350-6
06 Apr 2021
Theoretical and Computational Physics
Atomic, Molecular, and Optical Physics
Optical isolation induced by subwavelength spinning particle via spin-orbit interaction
Optical isolation induced by subwavelength spinning particle via spin-orbit interaction
Optical isolation enables nonreciprocal manipulations of light with broad applications in optical communications. Optical isolation by rotating structures has drawn considerable attention due to its magnetic-free nature and unprecedented performance. Conventional rotation-based optical isolation relies on the use of bulky cavities hindering applications in subwavelength photonics. Here, we propose a mechanism of optical isolation by integrating the unique dispersion of a hyperbolic metamaterial with the transverse spin-orbit interaction of evanescent waves. We show that rotation of a subwavelength hyperbolic nanoparticle breaks the time-reversal symmetry and yields two resonant chiral modes that selectively couple to the transverse spin of waveguide modes. Remarkably, the transverse spin-orbit interaction can give rise to unidirectional coupling and more than 95% isolation of infrared light at an experimentally feasible rotation speed. Our work fuses the two important fields of optical isolation and photonic spin-orbit interactions, leading to magnetic-free yet compact nonreciprocal devices for applications in optical communications, chiral quantum optics, and topological photonics.
Read more at Physical Review B:
https://journals.aps.org/prb/abstract/10.1103/PhysRevB.103.094105
10 Mar 2021
Applied Physics
CHO cell dysfunction due to radiation-induced bystander signals observed by real-time electrical impedance measurement
CHO cell dysfunction due to radiation-induced bystander signals observed by real-time electrical impedance measurement
Radiation-induced bystander effects (RIBE) have raised many concerns about radiation safety and protection. In RIBE, unirradiated cells receive signals from irradiated cells and exhibit irradiation effects. Until now, most RIBE studies have been based on morphological and biochemical characterization. However, research on the impact of RIBE on biophysical properties of cells has been lagging. Non-invasive indium tin oxide (ITO)-based impedance systems have been used as bioimpedance sensors for monitoring cell behaviors. This powerful technique has not been applied to RIBE research. In this work, we employed an electrical cell-ITO substrate impedance system (ECIIS) to study the RIBE on Chinese hamster ovary (CHO) cells. The bioimpedance of bystander CHO cells (BCHO), alpha(α)-particle (Am-241) irradiated CHO (ICHO), and untreated/unirradiated CHO (UCHO) cells were monitored with a sampling interval of 8 seconds over a period of 24 hours. Media from ICHO cells exposed to different radiation doses (0.3 nGy, 0.5 nGy, and 0.7 nGy) were used to investigate the radiation dose dependence of BCHO cells' impedance. In parallel, we imaged the cells at times where impedance changes were observed. By analyzing the changes in absolute impedance and cell size/cell number with time, we observed that BCHO cells mimicked ICHO cells in terms of modification in cell morphology and proliferation rate. Furthermore, these effects appeared to be time-dependent and inversely proportional to the radiation dose. Hence, this approach allows a label-free study of cellular responses to RIBE with high sensitivity and temporal resolution and can provide crucial insights into the RIBE mechanism.
Read more at Biosensors and Bioelectronics: https://www.sciencedirect.com/science/article/pii/S0956566321001792
05 Mar 2021
Soft Matter and Biophysics
DNA Knot Malleability in Single-Digit Nanopores
DNA Knot Malleability in Single-Digit Nanopores
Knots in long DNA molecules are prevalent in biological systems and serve as a model system for investigating static and dynamic properties of biopolymers. We explore the dynamics of knots in double-stranded DNA in a new regime of nanometer-scale confinement, large forces, and short time scales, using solid-state nanopores. We show that DNA knots undergo isomorphic translocation through a nanopore, retaining their equilibrium morphology by swiftly compressing in a lateral direction to fit the constriction. We observe no evidence of knot tightening or jamming, even for single-digit nanopores. We explain the observations as the malleability of DNA, characterized by sharp buckling of the DNA in nanopores, driven by the transient disruption of base pairing. Our molecular dynamics simulations support the model. These results are relevant not only for the understanding of DNA packing and manipulation in living cells but also for the polymer physics of DNA and the development of nanopore-based sequencing technologies.
Read more at Nano Letters: https://pubs.acs.org/doi/10.1021/acs.nanolett.0c05142
04 Mar 2021
Soft Matter and Biophysics
Ferromagnetic liquid droplets with adjustable magnetic properties
Ferromagnetic liquid droplets with adjustable magnetic properties
Structured functional liquids combine mechanical versatility of fluids with solid-state properties, such as ferromagnetism, and offer a route to synthesize and control magnetic liquids for adaptive liquid robotics. Studies on these intriguing materials are only in their nascent state, and a profound understanding of the physical state is still lacking. We use hydrodynamics experiments to probe how the magnetization of ferromagnetic liquid droplets, governed by the assembly and jamming of magnetic nanoparticles at liquid–liquid interfaces, and their response to external stimuli can be tuned by chemical, structural, and magnetic means. Our results highlight the leading role of structural short-range order on magnetic properties, which provide a path toward nano-patterning structured liquids.
Read more at PNAS: https://www.pnas.org/content/118/8/e2017355118.short
23 Feb 2021
Spectroscopy and Imaging
Quantum Materials
Unconventional Transverse Transport above and below the Magnetic Transition Temperature in Weyl Semimetal EuCd2As2
Unconventional Transverse Transport above and below the Magnetic Transition Temperature in Weyl Semimetal EuCd2As2
As exemplified by the growing interest in the quantum anomalous Hall effect, the research on topology as an organizing principle of quantum matter is greatly enriched from the interplay with magnetism. In this vein, we present a combined electrical and thermoelectrical transport study on the magnetic Weyl semimetal EuCd2As2. Unconventional contribution to the anomalous Hall and anomalous Nernst effects were observed both above and below the magnetic transition temperature of EuCd2As2, indicating the existence of significant Berry curvature. EuCd2As2 represents a rare case in which this unconventional transverse transport emerges both above and below the magnetic transition temperature in the same material. The transport properties evolve with temperature and field in the antiferromagnetic phase in a different manner than in the paramagnetic phase, suggesting different mechanisms to their origin. Our results indicate EuCd2As2 is a fertile playground for investigating the interplay between magnetism and topology, and potentially a plethora of topologically nontrivial phases rooted in this interplay.
Read more at Physical Review Letter:
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.126.076602
18 Feb 2021
Theoretical and Computational Physics
Atomic, Molecular, and Optical Physics
Polarization singularities in light scattering by small particles
Polarization singularities in light scattering by small particles
Using full-wave numerical simulations and analytical multipole expansions, we investigated the properties of real-space polarization singularities that emerge in light scattering by subwavelength particles. We considered spherical and torus particles under the excitation of a linearly polarized plane wave. We determined the topological indices and the trajectories of electric-field polarization singularities in both the near-field and far-field regions. In the far-field region, a total of four singularities are identified, and the sum of their polarization topological indices is two, independent of the particle's geometric shape. In the near-field region, the polarization singularities strongly depend on the particle's shape and the polarization of incident light, and their index sum is not governed by the Poincaré-Hopf theorem anymore due to the nontransverse nature of the fields. From near field to far field, a flipping of sign can happen to the polarization topological indices of the C lines. The far-field properties of the singularities can be well explained by the interference of the excited multipoles, but their near-field properties can be strongly affected by the evanescent fields that are not captured by the multipole expansions. Our work uncovers the important relationship between particles' geometric properties and the polarization singularities of their scattering field. The results can be applied to manipulate polarization singularities in nanophotonic systems, and they could generate novel applications in optical sensing.
Read more at Physical Review A:
https://journals.aps.org/pra/abstract/10.1103/PhysRevA.103.023520
16 Feb 2021
Soft Matter and Biophysics
Revisiting the Non-monotonic Dependence of Polymer Knotting Probability on the Bending Stiffness
Revisiting the Non-monotonic Dependence of Polymer Knotting Probability on the Bending Stiffness
Knots can spontaneously form in polymers. How knotting affects polymer behavior depends on polymer knotting probability, pknot. An intriguing result about pknot in recent studies is that pknot exhibits a non-monotonic dependence on the bending stiffness and is maximized at Lp ≈ 8a, where Lp is the persistence length and a is the hardcore diameter of the monomer. In this work, we propose a new explanation for the non-monotonic behavior of pknot based on the fact that polymer knots are typically localized. We find that the non-monotonic behavior results from the competition of a special entropic effect arising from the variation in the sizes of localized knots and an effect arising from the variation in the free-energy cost of forming a localized knot on a fragment of a polymer. The first effect refers to the situation that shrinking the knot size for a polymer with a fixed length essentially increases the number of “slots” for knot formation and enhances pknot. Based on this explanation, we derive an approximate analytic equation that captures the non-monotonic behavior of pknot. Overall, this work provides new insights into pknot beyond previous studies, in particular, unifying the effect of the knot size on pknot and the effect of the polymer length on pknot. The results can be applied to understand DNA knotting, considering that the effective Lp/a for DNA can be widely varied by the ionic strength.
Read more at Macromolecules: https://pubs.acs.org/doi/10.1021/acs.macromol.0c02640
09 Feb 2021
Applied Physics
Lithium from breast‐milk inhibits thyroid iodine uptake and hormone production, which are remedied by maternal iodine supplementation
Lithium from breast‐milk inhibits thyroid iodine uptake and hormone production, which are remedied by maternal iodine supplementation
Lithium is the primary medication for Bipolar Disorder (BD). In women with BD, lithium is effective postpartum, but breast-feeding for medicated mothers is controversial because of harmful effects for her child. At present, the biological mechanisms of lithium are not well understood, affecting its usage and overall health implications. We showed that breast-fed infants (rats) exposed to lithium via breastmilk, even with the mother on a sub-therapeutic dose, experienced weight gain and reduced blood thyroid hormone levels. These are symptoms of hypothyroidism, which is frequently seen in BD patients. We further showed that lithium inhibited iodine uptake by the thyroid, initiating a molecular mechanism leading to reduced thyroid hormone production. Importantly, infant thyroid function can be significantly improved by administering supplementary iodine to the medicated mother’s diet during breast-feeding. This study has elucidated the mechanisms of lithium in thyroid function, provided valuable information on use postpartum, and found a clinically applicable remedy to side-effects. The results are particularly important for patients (and their infants) who respond well to lithium and need, or choose, to breast-feed. This multi-national study led by the City University of Hong Kong will soon appear in top psychiatry journal Bipolar Disorders.
Read more at Bipolar Disorders: https://onlinelibrary.wiley.com/doi/10.1111/bdi.13047
28 Jan 2021
Applied Physics
Hearing loss impacts gray and white matter across the lifespan: systematic review, meta-analysis and meta-regression
Hearing loss impacts gray and white matter across the lifespan: systematic review, meta-analysis and meta-regression
Hearing loss affects brain reorganization across the lifespan. Here, we quantitatively analyze brain structural alterations using all brain magnetic resonance imaging studies of hearing loss to date. Hearing loss was found to affect gray matter (GM) and underlying white matter (WM) in nearly every brain region. In particular, GM and WM in the frontal lobe were significantly affected. The hemispheric asymmetry of frontal lobe changes was opposite in congenital and acquired hearing loss subjects. These results open new research avenues and possibly a paradigm shift in the treatment of hearing loss. This work will appear in the journal NeuroImage.
Read more at NeuroImage:
https://www.sciencedirect.com/science/article/pii/S1053811921001038
27 Jan 2021
Theoretical and Computational Physics
Atomic, Molecular, and Optical Physics
Excitation-energy transfer under strong laser drive
Excitation-energy transfer under strong laser drive
Strong molecule-light interaction enables the control of molecular structures and dynamical processes. A model with a strong laser drive is proposed to greatly enhance the intermolecular distance of resonant energy transfer, where the molecules are strongly driven by an optical cavity. The optimal Rabi frequency and quantum yield of energy transfer are observed, resulting from the trade-off between dipole-dipole interaction and molecule-cavity coupling. When the strong drive at a certain Rabi frequency is applied, a larger spatial range of effective energy transfer and a slower decay rate with the distance compared to the Förster mechanism of resonant energy transfer are observed in our model. Our work sheds light on the spectroscopic study of cooperative energy transfer in molecular polaritons.
Read more at Physical Review A: https://doi.org/10.1103/PhysRevA.103.013516
20 Jan 2021
Spectroscopy and Imaging
Applied Physics
Stacking faults driven phase transformation in CrCoNi medium entropy alloy
Stacking faults driven phase transformation in CrCoNi medium entropy alloy
Phase transformation is an effective means to increase the ductility of a material. However, even for a commonly observed face-centered-cubic to hexagonal-close-packed (fcc-to-hcp) phase transformation, the underlying mechanisms are far from being settled. In fact, different transformation pathways have been proposed, especially with regard to nucleation of the hcp phase at the nanoscale. In CrCoNi, a so-called medium-entropy alloy, an fcc-to-hcp phase transformation has long been anticipated. Here, we report an in situ loading study with neutron diffraction, which revealed a bulk fcc-to-hcp phase transformation in CrCoNi at 15 K under tensile loading. By correlating deformation characteristics of the fcc phase with the development of the hcp phase, it is shown that the nucleation of the hcpphase was triggered by intrinsic stacking faults. The confirmation of a bulk phase transformation adds to the myriads of deformation mechanisms available in CrCoNi, which together underpin the unusually large ductility at low temperatures.
Read more at Nano Letters: https://doi.org/10.1021/acs.nanolett.0c04244
19 Jan 2021
Spectroscopy and Imaging
Quantum Materials
Extreme Suppression of Antiferromagnetic Order and Critical Scaling in a Two-Dimensional Random Quantum Magnet
Extreme Suppression of Antiferromagnetic Order and Critical Scaling in a Two-Dimensional Random Quantum Magnet
Sr2CuTeO6 is a square-lattice Néel antiferromagnet with superexchange between first-neighbor S=1/2 Cu spins mediated by plaquette centered Te ions. Substituting Te by W, the affected impurity plaquettes have predominantly second-neighbor interactions, thus causing local magnetic frustration. Our newly constructed cold neutron triple-axis spectrometer Xingzhi was used to investigate the extremely rapid suppression of the Néel order by the W induced magnetic frustration.
Read more at Physical Review Letters: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.126.037201
19 Jan 2021
Atomic, Molecular, and Optical Physics
Applied Physics
11 TOPS photonic convolutional accelerator for optical neural networks
11 TOPS photonic convolutional accelerator for optical neural networks
Convolutional neural networks, inspired by biological visual cortex systems, are a powerful category of artificial neural networks that can extract the hierarchical features of raw data to provide greatly reduced parametric complexity and to enhance the accuracy of prediction. They are of great interest for machine learning tasks such as computer vision, speech recognition, playing board games and medical diagnosis. Optical neural networks offer the promise of dramatically accelerating computing speed using the broad optical bandwidths available. Here we demonstrate a universal optical vector convolutional accelerator operating at more than ten TOPS (trillions (1012) of operations per second, or tera-ops per second), generating convolutions of images with 250,000 pixels—sufficiently large for facial image recognition. We use the same hardware to sequentially form an optical convolutional neural network with ten output neurons, achieving successful recognition of handwritten digit images at 88 per cent accuracy. Our results are based on simultaneously interleaving temporal, wavelength and spatial dimensions enabled by an integrated microcomb source. This approach is scalable and trainable to much more complex networks for demanding applications such as autonomous vehicles and real-time video recognition.
Read more at Nature: https://www.nature.com/articles/s41586-020-03063-0
06 Jan 2021
Quantum Materials
Applied Physics
Phase diagram of infinite layer praseodymium nickelate Pr1−xSrxNiO2 thin films
Phase diagram of infinite layer praseodymium nickelate Pr1−xSrxNiO2 thin films
The phase diagram of an additional family of nickelate superconductors (Pr1-xSrxNiO2) has been discovered, which marks the first step towards establishing a more generic superconducting phase diagram of the nickelates enriched with many exotic properties and intriguing physics. The findings indicate an infinite layer nickelate phase diagram that is relatively insensitive to the rare-earth element but suggest that disorder arising from the variations of the ionic radii on the rare-earth site affects the superconducting dome.
Read more at Physical Review Materials: https://doi.org/10.1103/PhysRevMaterials.4.121801
15 Dec 2020
Applied Physics
LiMnO2 cathode stabilized by interfacial orbital ordering for sustainable lithium-ion batteries
LiMnO2 cathode stabilized by interfacial orbital ordering for sustainable lithium-ion batteries
Global lithium-ion battery deployments stand poised to grow substantially in the coming years, but it will be necessary to include sustainability considerations in the design of electrode materials. The current cathode chemistry relies heavily on cobalt, which, due to its scarcity and the environmental abuse and violation of human rights during its mining, must be replaced by abundant and environmentally friendly elements such as redox-active manganese. LiMnO2 is a strong contender for sustainable cathodes but cycles poorly because the Jahn–Teller distorted Mn3+ ions destabilize the lattice framework. Here, we report a LiMnO2 cathode design with interwoven spinel and layered domains. At the interface between these two domains, the Mn dz2 orbitals are oriented perpendicular to each other, giving rise to interfacial orbital ordering, which suppresses the otherwise cooperative Jahn–Teller distortion and Mn dissolution. As a result, the heterostructured cathode delivers enhanced structural and electrochemical cycling stability. This work provides a new strategy for interface engineering, possibly stimulating more research on Mn-rich cathode materials for sustainable lithium-ion batteries.
Read more at Nature Sustainability: https://www.nature.com/articles/s41893-020-00660-9
14 Dec 2020
Soft Matter and Biophysics
Direct observation of nanoparticle-surfactant assembly and jamming at the water-oil interface
Direct observation of nanoparticle-surfactant assembly and jamming at the water-oil interface
Electrostatic interactions between nanoparticles (NPs) and functionalized ligands lead to the formation of NP surfactants (NPSs) that assemble at the water-oil interface and form jammed structures. To understand the interfacial behavior of NPSs, it is necessary to understand the mechanism by which the NPSs attach to the interface and how this attachment depends on the areal coverage of the interface. Through direct observation with high spatial and temporal resolution, using laser scanning confocal microscopy and in situ atomic force microscopy (AFM), we observe that early-stage attachment of NPs to the interface is diffusion limited and with increasing areal density of the NPSs, further attachment requires cooperative displacement of the previously assembled NPSs both laterally and vertically. The unprecedented detail provided by in situ AFM reveals the complex mechanism of attachment and the deeply nonequilibrium nature of the assembly.
The work is featured on the cover of the Nov 2020 volume of Science Advances (Volume 6, Issue 48).
Read more at Science Advances: https://advances.sciencemag.org/content/6/48/eabb8675
The paper was also featured in ScienceDaily:
https://www.sciencedaily.com/releases/2021/02/210225082555.htm
Moreover, the paper was highlighted in a press release by Lawrence Berkeley National Laboratory:
https://newscenter.lbl.gov/2021/02/25/nanoparticles-get-in-shape/
25 Nov 2020
Spectroscopy and Imaging
Soft Matter and Biophysics
Stability improvement for dried droplet pretreatment by suppression of coffee ring effect using electrochemical anodized nanoporous tin dioxide substrate
Stability improvement for dried droplet pretreatment by suppression of coffee ring effect using electrochemical anodized nanoporous tin dioxide substrate
In analytical chemistry, accurately measuring the quantity of low concentration analytes in small samples is challenging. One approach for such analysis is the dried-droplet pretreatment method, used with a range of laser and x-ray based analytical methods. However, dried-droplet formation often exhibits the coffee-ring effect, which adversely affects accuracy and detection limit. CityU, in collaboration with Huazhong University of Science and Technology, has developed a novel nanoporous tin substrate that suppresses the coffee-ring effect. This led to marked improvement in measurement accuracy and detection limit. Dried-droplet pretreatment combined with the nanoporous substrate has potential to quantitatively analyze nanoliter liquid and microgram solid samples. This significant advancement will be published in the top journal Microchimica Acta.
Read more at Microchimica Acta: https://link.springer.com/article/10.1007/s00604-020-04640-w
17 Nov 2020
Theoretical and Computational Physics
Atomic, Molecular, and Optical Physics
Optical forces in coupled chiral particles
Optical forces in coupled chiral particles
Structural chirality can induce counterintuitive optical forces due to inherent symmetry properties. While optical forces on a single chiral particle in the Rayleigh regime have been well studied, optical forces in coupled chiral particles remain less explored. By using full-wave numerical simulations and an analytical method of source representation, we investigated the optical forces induced by a plane wave on two chiral particles coupling with each other via the evanescent near fields. We found that the induced electric and magnetic dipoles of the chiral particles have complicated couplings that give rise to dark and bright modes. The interaction force between the particles can be either attractive or repulsive, and its magnitude can be significantly enhanced by the resonance modes. The attractive force is much stronger if two particles are of opposite handedness compared with the case of the same handedness. The electric dipole force and the magnetic dipole force have the same sign for two particles with the same handedness, while they are of different signs for two particles with opposite handedness. The results can lead to a better understanding of chirality-induced optical forces with potential applications in optical manipulations and chiral light-matter interactions.
The 1st and 3rd authors of this publication are our undergraduate students.
Read more at Physical Review A:
https://journals.aps.org/pra/abstract/10.1103/PhysRevA.102.043526
28 Oct 2020
Soft Matter and Biophysics
Application of the tube model to explain the unexpected decrease in polymer bending energy induced by knot formation
Application of the tube model to explain the unexpected decrease in polymer bending energy induced by knot formation
Knotting is common in long polymers and significantly affects the polymer behavior. It is generally believed that knotting should increase polymer bending energy. However, many recent studies found that knotting can surprisingly decrease polymer bending energy, while the reason remains unclear. In this work, we quantitatively explained this surprising phenomenon using the tube model. In this model, polymer segments in a knot core are confined in a virtual tube. The tube affects the bending of knots in two opposite ways. First, the tube is curved to assume a knotted shape, which increases the bending. Second, the tube confinement suppresses the bending. We built a phase diagram to quantify the competition of these two effects. The second effect overwhelms under certain conditions because of two crucial yet often overlooked factors: (i) excluded volume interactions reduce the accessible diameter of the tube and (ii) polymer segments tend to escape the tube and thus produce an entropic force, which straightens the conformation. Overall, this work clarifies the special role of bending in polymer knots and demonstrates the usefulness of the tube model for polymer knots.
Read more at Macromolecules: https://pubs.acs.org/doi/10.1021/acs.macromol.0c01436
26 Oct 2020
Quantum Materials
Unconventional valley-dependent optical selection rules and landau level mixing in bilayer graphene
Unconventional valley-dependent optical selection rules and landau level mixing in bilayer graphene
When two-dimensional materials are placed in a strong perpendicular magnetic field, the energy levels of the electrons are quantized into the so-called Landau levels (LL). Optical transitions between these discrete energy levels can often reveal crucial information regarding the electronic properties of the parent material, such as the band structure and electron correlations. The usual optical transitions in pristine bilayer graphene have been well understood and summarized in a set of selection rules that are associated with the LL index. In this work, the authors perform photocurrent spectroscopy measurements on high-quality bilayer graphene samples and study the optical transitions between different Landau levels. For the first time, they observed optical transitions that go beyond the usual transition rules, which can be attributed to the presence of strong electron correlations.
Read more at Nature Communications:
https://doi.org/10.1038/s41467-020-16844-y
10 Jun 2020
Applied Physics
Spectroscopy and Imaging
Observation of High-Frequency Transverse Phonons in Metallic Glasses
Observation of High-Frequency Transverse Phonons in Metallic Glasses
Using inelastic neutron scattering and molecular dynamics simulations on a model Zr-Cu-Al metallic glass, we show that transverse phonons persist well into the high-frequency regime, and can be detected at large momentum transfer. Furthermore, the apparent peak width of the transverse phonons was found to follow the static structure factor. The one-to-one correspondence, which was demonstrated for both Zr-Cu-Al metallic glass and a three-dimensional Lennard-Jones model glass, suggests a universal correlation between the phonon dynamics and the underlying disordered structure. This remarkable correlation, not found for longitudinal phonons, underscores the key role that transverse phonons hold for understanding the structure-dynamics relationship in disordered materials.
Read more at Physical Review Letters:
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.124.225902
The paper was featured in CityU Research Stories:
https://www.cityu.edu.hk/research/stories/2020/08/06/high-frequency-transverse-phonons-amorphous-materials-observed-first-time
05 Jun 2020
Spectroscopy and Imaging
Quantum Materials
Evolution of Superconductivity and Antiferromagnetic Order in Ba (Fe0.92-xCo0.08Vx)2 As2
Evolution of Superconductivity and Antiferromagnetic Order in Ba (Fe0.92-xCo0.08Vx)2 As2
Single crystals of the laminar Fe-based superconductor Ba(Fe0.92-xCo0.08Vx)2As2 were grown by the self-flux method for investigation on the interplay between superconductivity and antiferromagnetic correlations. The superconducting state emerges at a critical V concentration where the lattice structural transition with an associated spin-glass state reaches a maximum transition temperature (see the Figure). The result from the transport, magnetization and neutron diffraction experiments suggects a Griffiths quantum critical point with a lattice twist at the onset of the unconventional superconductivity in the BaFe2As2 system.
This is the first completed scientific output of our newly constructed cold neutron triple-axis spectrometer at China Advanced Research Reactor outside Beijing, as a National Major Research Instrumentation Project.
Read more at Physical Review B:
http://doi.org/10.1103/PhysRevB.101.174516
28 May 2020
Atomic, Molecular, and Optical Physics
Applied Physics
Optical multi-stability in a nonlinear high-order microring resonator filter
Optical multi-stability in a nonlinear high-order microring resonator filter
Optical bistability is a phenomenon which the transmission of light in a resonance structure varies discontinuously and exhibits a hysteresis cycle. For a higher order resonance structures it is possible to create multiple discontinuities and a more complex hysteresis cycle. In this work, we theoretically analyze and experimentally demonstrate optical bi-stability and multi-stability in an integrated nonlinear high-order microring resonator filter based on high-index contrast doped silica glass. To understand the role of the intracavity power distribution, we investigate the detuning of the individual rings of the filter from the optical response with a pump–probe experiment. The work provides a comprehensive understanding of the relationship between the nonlinear behavior and the intracavity power distribution for the high-order microring resonator filter will help the design and implementation of future all-optical switching systems using this type of filter.
This paper was selected as an Editor’s Pick.
Read more at APL Photonics:
https://aip.scitation.org/doi/10.1063/5.0002941
22 May 2020
Atomic, Molecular, and Optical Physics
Applied Physics
Ultra-dense optical data transmission over standard fibre with a single chip source
Ultra-dense optical data transmission over standard fibre with a single chip source
Optical micro-combs are tiny optical structures that convert a single laser into many different wavelengths of light. These devices have been used in a wide range of exciting demonstrations, including optical communications. The key to micro-combs are optical resonator structures, tiny rings (see picture) that when hit with enough light convert the incoming single wavelength into a precise rainbow of wavelengths. This work uses a novel kind of micro-comb called a “solition crystal”, that produces 80 separate wavelengths of light that can carry different signals at the same time. To prove the micro-comb could be used in a real-world environment, we transmitted the data through installed optical fibres in Melbourne (provided by AARNet) between RMIT’s City campus and Monash’s Clayton campus and back, for a round trip of 75 kilometres and demonstrated data transmission at > 40 Tbps. A world record in terms of spectral efficiency on a standard optical fiber.
Read more at Nature Communications:
https://www.nature.com/articles/s41467-020-16265-x
The paper was featured in CityU Research News:
https://www.cityu.edu.hk/media/news/2020/07/06/new-chips-accelerate-data-transmission
The paper was also featured in CityU Research Stories:
https://www.cityu.edu.hk/research/stories/2020/07/02/new-chips-developed-cityu-physicist-help-break-spectral-efficiency-record-optical-data-transmission
Please click the following links for media coverage:
- Australia 'records fastest internet speed ever' [BBC News] 2020-05-22 Technology
- 'Fastest ever' internet speed capable of downloading 1,000 HD movies in under a second is recorded in Australia - and it is four million times faster than the country's average connection [Daily Mail] 2020-05-22 Science & Tech
- Internet traffic is growing 25% each year. We created a fingernail-sized chip that can help the NBN keep up [The Conversation] 2020-05-22 Science & Technology
- World's fastest internet speed from a single optical chip [Science Daily] 2020-05-22 Science News
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- 1秒下載千套高清電影 城大研新芯片 網速升至史上最快 [HKET online] 2020-07-06
- CityU scholars develops new data chips [OpenGov Asia] 2020-07-08 Digital Transformation
22 May 2020
Atomic, Molecular, and Optical Physics
Optofluidic Micro-Engine in A Dynamic Flow Environment via Self-Induced Back-Action
Optofluidic Micro-Engine in A Dynamic Flow Environment via Self-Induced Back-Action
Most existing optofluidic particle engines only operate in a static environment. Here, we present a four-energy-state optofluidic microengine that operates stably in a dynamic flow environment, a function unattainable by existing systems due to the disturbance of the fluidic drag force. This microengine is powered synergistically by both the optical force and fluidic drag force, and it exploits the intriguing behavior of the particle in an asymmetric two-dimensional light interference pattern under the self-induced back-action (SIBA) effect. The mechanism of the microengine is studied in detail, and a microengine comprising a single cell and a cell–particle complex has been demonstrated. Our optofluidic microengine is the first of its kind to operate in the dynamic flow environment, and it provides a new platform to study single cell dynamics and cell–particle or cell–cell interactions in the dynamic fluidic environment.
The work is featured on the cover of ACS Photonics, Volume 7, Issue 6.
Read more at ACS Photonics:
https://pubs.acs.org/doi/abs/10.1021/acsphotonics.0c00295
08 May 2020
Applied Physics
Synergy of Ion Doping and Spiral Array Architecture on Ti2Nb10O29: A New Way to Achieve High‐Power Electrodes
Synergy of Ion Doping and Spiral Array Architecture on Ti2Nb10O29: A New Way to Achieve High‐Power Electrodes
Ameliorating electronic/ionic transport and structural stability of electrode materials is important to the development of power‐intensive lithium ion batteries. Despite its great potential as a high‐power anode, titanium niobium oxide (Ti2Nb10O29, TNO) still underperforms due to its unsatisfactory electronic/ionic conductivity. In this work, a powerful synergistic strategy by combining ion doping and spiral array architecture to boost high‐rate performance of TNO is reported. Cr3+ doped TNO nanoparticles (Cr‐TNO) of 5–10 nm intimately grow on a conductive vertical graphene @TiC‐C (VGTC) skeleton, forming novel Cr‐TNO @VGTC spiral arrays. The unique spiral growth of TNO is achieved due to the confinement effect of VGTC skeleton. Meanwhile, a more open TNO crystal structure with faster ion transfer paths and enhanced structural stability is realized by Cr3+ doping, demonstrated via density functional theory calculation and in situ synchrotron X‐ray diffraction technique. Benefiting from the superior conductive network, enhanced intrinsic electronic/ionic conductivity of Cr‐TNO and reinforced structural stability, the Cr‐TNO @VTC arrays show prominent high‐power performance with a large capacity of 220 mAh g−1 at 40 C (power density of ≈11 kW kg−1) and superior durability (91% retention after 500 cycles). This work provides a new path for the construction of widespread high‐power electrodes for fast energy storage.
Read more at Advanced Functional Materials :
https://onlinelibrary.wiley.com/doi/full/10.1002/adfm.202002665?af=R
29 Apr 2020
Soft Matter and Biophysics
Cytosine methylation enhances DNA condensation revealed by equilibrium measurements using magnetic tweezers
Cytosine methylation enhances DNA condensation revealed by equilibrium measurements using magnetic tweezers
CpG methylation of DNA is common in mammalian cells. In sperms, the DNA has the highest level of CpG methylation and is condensed into toroidal structures. How CpG methylation affects DNA structures and interactions is important to understand its biological roles but is largely unknown. Using an RNA-DNARNA structure, we observed the equilibrium hopping dynamics between the condensed and extended states of DNA in the presence of polyamines or polylysine peptide as a reduced model of histone tails. Combing with the measured DNA elasticities, we report that CpG methylation of each cytosine nucleotide substantially increases DNA-DNA attraction by up to 0.2 kBT. For the DNA with 57% GC content, the relative increase caused by CpG methylation is up to 32% for the spermine-induced DNA-DNA attraction and up to 9% for the polylysine-induced DNA-DNA attraction. These findings help us to evaluate the energetic contributions of CpGmethylation in sperm development and chromatin regulation.CpG methylation of DNA is common in mammalian cells. In sperms, the DNA has the highest level of CpG methylation and is condensed into toroidal structures. How CpG methylation affects DNA structures and interactions is important to understand its biological roles but is largely unknown. Using an RNA-DNARNA structure, we observed the equilibrium hopping dynamics between the condensed and extended states of DNA in the presence of polyamines or polylysine peptide as a reduced model of histone tails. Combing with the measured DNA elasticities, we report that CpG methylation of each cytosine nucleotide substantially increases DNA-DNA attraction by up to 0.2 kBT. For the DNA with 57% GC content, the relative increase caused by CpG methylation is up to 32% for the spermine-induced DNA-DNA attraction and up to 9% for the polylysine-induced DNA-DNA attraction. These findings help us to evaluate the energetic contributions of CpGmethylation in sperm development and chromatin regulation.
Read more at Journal of the American Chemical Society:
https://pubs.acs.org/doi/10.1021/jacs.9b11957
24 Apr 2020
Atomic, Molecular, and Optical Physics
Applied Physics
2D Layered Graphene Oxide Films Integrated with Micro‐Ring Resonators for Enhanced Nonlinear Optics
2D Layered Graphene Oxide Films Integrated with Micro‐Ring Resonators for Enhanced Nonlinear Optics
In this work, we demonstrate significantly enhanced nonlinear four‐wave mixing (FWM) in complementary‐metal‐oxide‐semiconductor compatible micro‐ring resonators integrated with 2D layered graphene oxide (GO) films. Four‐wave‐mixing (FWM) measurements for different pump powers and resonant wavelengths show a significant improvement in efficiency of ≈7.6 dB for a uniformly coated device with 1 GO layer and ≈10.3 dB for a patterned device with 50 GO layers. The measurements agree well with theory, with the enhancement in FWM efficiency resulting from the high Kerr nonlinearity and low loss of the GO films combined with the strong light–matter interaction within the MRRs. The dependence of GO's third‐order nonlinearity on layer number and pump power is also extracted from the FWM measurements, revealing interesting physical insights about the evolution of the GO films from 2D monolayers to quasi bulk‐like behavior. These results confirm the high nonlinear optical performance of integrated photonic resonators incorporated with 2D layered GO films.
Read more at Small:
https://onlinelibrary.wiley.com/doi/full/10.1002/smll.201906563
The work is featured on the cover of the Volume 16 of Small:
https://onlinelibrary.wiley.com/doi/abs/10.1002/smll.202070085
23 Apr 2020
Soft Matter and Biophysics
Developing the tube theory for polymer knots
Developing the tube theory for polymer knots
Entanglements make polymers fundamentally different from other molecules and thus are a major theme in polymer physics research. Interchain entanglements have been extensively investigated in past decades, while intrachain entanglements, often appearing as polymer knots, are much less understood. In this work, we apply the tube theory for polymer knots based on the tube model: the polymer segments in a knot core are confined within a tube due to topological entanglements. We use an approach of visualizing and quantifying the “tubes.” First, we perform Monte Carlo simulations to generate a large number of polymer knots. Then, we superimpose knot cores to obtain average knot conformations. The fluctuations of individual knot conformations around average knot conformations produce tubes, which materialize the conceptual tubes. Analyzing the tubes validates many scaling relationships and determines the relevant parameters. Furthermore, we reveal a heart shape for polymer trefoil knots, which results from the competition of entropy and bending energy. Overall, this work builds the foundation of the tube theory for polymer knots.
Read more at Physical Review Research:
https://journals.aps.org/prresearch/abstract/10.1103/PhysRevResearch.2.022014
16 Apr 2020
Applied Physics
Unveiling the solid-solution charge storage mechanism in 1T vanadium disulfide nanoarray cathodes
Unveiling the solid-solution charge storage mechanism in 1T vanadium disulfide nanoarray cathodes
Transition metal disulfides (TMDs) have achieved intensive attention in the field of energy storage materials owing to their natural layered structures. However, most TMDs are in the semiconductor phase with inferior electrical conductivity, which has prevented their widespread applications. The 1T-phase vanadium disulfide, in contrast, exhibits some metal characteristic qualities, which enable it to be a superb intercalation host material for Li-ion batteries. We herein demonstrate high rate capability, and the capacity retention can achieve 84.4% after 1000 cycles. In situ synchrotron X-ray diffraction (XRD) results reveal that the VS2 electrode exhibits a typical solid solution reaction, which is of great help in inhibiting the structural transition. Density functional theory (DFT) calculations further indicate that the monolayer 1T-phase VS2 can withstand the high lithiation rate without structural changes and possesses a lower energy barrier with unrestricted diffusion pathways. This work is quite significant and reliable for the subsequent study on two-dimensional materials.
Read more at Journal of Materials Chemistry A:
https://pubs.rsc.org/en/content/articlehtml/2020/ta/d0ta02922j
07 Apr 2020
Theoretical and Computational Physics
Charge Transfer Boosting Moisture Resistance of Seminude Perovskite Nanocrystals via Hierarchical Alumina Modulation
Charge Transfer Boosting Moisture Resistance of Seminude Perovskite Nanocrystals via Hierarchical Alumina Modulation
Boosting the stability improvement of cesium lead halide (CsPbX3) perovskite nanocrystals (NCs) remains a serious challenge. In this work, CsPbX3 NCs are effectively anchored on a hierarchical (h-) alumina (Al2O3) substrate to form seminude CsPbX3@h-Al2O3 composites, which can emit strong green light even after being stored in water for 30 days, in sharp contrast to the pure CsPbBr3 NCs. Other oxides, such as TiO2, ZnO, and SiO2, have no boosting effect on the moisture resistance of perovskite NCs. Subsequent density functional theory calculations (DFT) reveal a significant charge transfer and strong Coulomb attraction between CsPbBr3 and Al2O3. The substantial charge transfer via alumina substrate modulation not only can enhance the internal stability of CsPbBr3 but also can cause CsPbBr3 to be insensitive to water adsorption. These findings are expected to deepen our understanding of improving the stability of CsPbBr3 NCs and shed light on the design of novel perovskite composites for longterm stable optoelectronic devices.
Read more at The Journal of Physical Chemistry Letters:
https://pubs.acs.org/doi/10.1021/acs.jpclett.0c00811
03 Apr 2020
Spectroscopy and Imaging
Functional magnetic resonance imaging of enhanced central auditory gain and electrophysiological correlates in a behavioral model of hyperacusis
Functional magnetic resonance imaging of enhanced central auditory gain and electrophysiological correlates in a behavioral model of hyperacusis
Hyperacusis is a debilitating hearing condition in which normal everyday sounds are perceived as exceedingly loud, annoying, aversive or even painful. The prevalence of hyperacusis approaches 10%, making it an important, but understudied medical condition. In this work, we develop functional magnetic resonance imaging (fMRI) for studying hyperacusis in a behaviorally verified rat model. Pairing animal disease models with novel methodology is extremely valuable for understanding the basic mechanisms of the disease, aiding in future prevention and treatment. We observed significant neural hyperactivity in response to sound in the brain cortex, but less in lower regions such as the brainstem. This work will also help bridge basic understanding acquired from animal models with clinical observations in human patients as fMRI is performed on both.
The work is featured on the cover of the Apr 2020 volume of Hearing Research.
Read more at Hearing Research:
https://www.sciencedirect.com/science/article/pii/S037859551930365X
01 Apr 2020
Applied Physics
Cooperative deformation in high-entropy alloys at ultralow temperatures
Cooperative deformation in high-entropy alloys at ultralow temperatures
High-entropy alloys exhibit exceptional mechanical properties at cryogenic temperatures, due to the activation of twinning in addition to dislocation slip. The co-existence of multiple deformation pathways raises an important question regarding how individual deformation mechanisms compete or synergize during plastic deformation. Using in-situ neutron diffraction, we demonstrate the interaction of a rich variety of deformation mechanisms in high-entropy alloys at 15 K, which began with dislocation slip, followed by stacking faults and twining, before transitioning to inhomogeneous deformation by serrations. Quantitative analysis showed that the cooperation of these different deformation mechanisms led to extreme work hardening. The low stacking fault energy plus the stable face-centered-cubic structure at ultralow temperatures, enabled by the high-entropy alloying, played a pivotal role bridging dislocation slip and serration. Insights from the in-situ experiments point to the role of entropy in the design of structural materials with superior properties.
Read more at Science Advances:
https://advances.sciencemag.org/content/6/13/eaax4002
The paper was featured in Phys.org:
https://phys.org/news/2020-03-multi-stage-deformation-high-entropy-alloys-ultra-low.html
The paper was also featured in CityU Research:
https://www.cityu.edu.hk/research/stories/2020/03/28/multi-stage-deformation-process-high-entropy-alloys-ultra-low-temperatures-revealed
Press release from Japan Atomic Energy Agency (Japanese only):
https://www.jaea.go.jp/02/press2019/p20032801/
[Photo caption] Muhammad Naeem prepares the experiment at TAKUMI, an engineering materials diffractometer at Japan Proton Accelerator Research Complex (J-PARC) used to perform in-situ neutron diffraction measurements multiple HEA samples, which all showed a multi-stage deformation process.
27 Mar 2020
Quantum Materials
Observation of the Kondo screening cloud
Observation of the Kondo screening cloud
In research published in Nature, an international research group has ended a fifty-year quest by directly observing a quantum phenomenon known as a Kondo screening cloud. This phenomenon, which is important for many physical phenomena such as high-temperature superconductivity, is essentially a cloud that masks magnetic impurities in a material. It was known to exist but its spatial extension had never been experimentally observed, creating controversy over whether such an extension actually existed.
Read more at Nature:
https://www.nature.com/articles/s41586-020-2058-6
The paper was also featured in CityU Research:
https://www.cityu.edu.hk/research/stories/2020/03/12/worlds-first-experimental-observation-kondo-cloud
11 Mar 2020
Theoretical and Computational Physics
Detection of Hole Pockets in the Candidate Type-II Weyl Semimetal MoTe2 from Shubnikov–de Haas Quantum Oscillations
Detection of Hole Pockets in the Candidate Type-II Weyl Semimetal MoTe2 from Shubnikov–de Haas Quantum Oscillations
Transition metal dichalcogenide MoTe2 has been extensively studied in the field of condensed matter physics owing to its intriguing non-trivial band topological and magnetotransport properties. Despite many years of efforts, disagreement between the Fermi surface topology as derived from quantum oscillation experiments and that predicted from density function theory (DFT) remains. Specifically, the hole pockets at the Brillouin zone center as predicted by DFT calculation had not been conclusively detected in quantum oscillation experiments so far, raising doubt about the realizability of Majorana states in the material. In this work, we reported the detection of these hole pockets in quantum oscillation and established a possible explanation for the absence of the hole pocket signals in previous quantum oscillation experiments. Combining systematic DFT calculations and high-pressure studies, we showed that the surface curvature of hole pockets decreases under pressure. At ambient pressure, the large effective mass associated with the large Fermi surface curvature makes the detection of the hole pockets challenging. Our finding resolves the long-standing doubt in the community and reestablishes MoTe2 at ambient pressure as a strong candidate of Weyl semimetal.
Read more at Physical Review Letters:
https://doi.org/10.1103/PhysRevLett.124.076402
19 Feb 2020
Applied Physics
Achieving Ultrahigh‐Rate and High‐Safety Li+ Storage Based on Interconnected Tunnel Structure in Micro‐Size Niobium Tungsten Oxides
Achieving Ultrahigh‐Rate and High‐Safety Li+ Storage Based on Interconnected Tunnel Structure in Micro‐Size Niobium Tungsten Oxides
Developing advanced high-rate electrode materials has been a crucial aspect for next-generation lithium ion batteries (LIBs). A conventional nanoarchitec-turing strategy is suggested to improve the rate performance of materials but inevitably brings about compromise in volumetric energy density, cost, safety, and so on. Here, micro-size Nb14W3O44 is synthesized as a durable high-rate anode material based on a facile and scalable solution combustion method. Aberration-corrected scanning transmission electron microscopy reveals the existence of open and interconnected tunnels in the highly crystalline Nb14W3O44, which ensures facile Li+ diffusion even within micro-size particles. In situ high-energy synchrotron XRD and XANES combined with Raman spec-troscopy and computational simulations clearly reveal a single-phase solid-solution reaction with reversible cationic redox process occurring in the NWO framework due to the low-barrier Li+ intercalation. Therefore, the micro-size Nb14W3O44 exhibits durable and ultrahigh rate capability, i.e., ≈130 mAh g−1at 10 C, after 4000 cycles. Most importantly, the micro-size Nb14W3O44 anode proves its highest practical applicability by the fabrication of a full cell incor-porating with a high-safety LiFePO4 cathode. Such a battery shows a long calendar life of over 1000 cycles and an enhanced thermal stability, which is superior than the current commercial anodes such as Li4Ti5O12.
Read more at Advanced Materials:
https://onlinelibrary.wiley.com/doi/full/10.1002/adma.201905295
19 Feb 2020
Spectroscopy and Imaging
Ultralow thermal conductivity from transverse acoustic phonon suppression in distorted crystalline α-MgAgSb
Ultralow thermal conductivity from transverse acoustic phonon suppression in distorted crystalline α-MgAgSb
Low thermal conductivity is favorable for preserving the temperature gradient between the two ends of a thermoelectric material, in order to ensure continuous electron current generation. In high-performance thermoelectric materials, there are two main low thermal conductivity mechanisms: the phonon anharmonic in PbTe and SnSe, and phonon scattering resulting from the dynamic disorder in AgCrSe2 and CuCrSe2, which have been successfully revealed by inelastic neutron scattering. Using neutron scattering and ab initio calculations, we report here a mechanism of static local structure distortion combined with phonon-anharmonic-induced ultralow lattice thermal conductivity in α-MgAgSb. Since the transverse acoustic phonons are almost fully scattered by the compound’s intrinsic distorted rocksalt sublattice, the heat is mainly transported by the longitudinal acoustic phonons. The ultralow thermal conductivity in α-MgAgSb is attributed to its atomic dynamics being altered by the structure distortion, which presents a possible microscopic route to enhance the performance of similar thermoelectric materials.
Read more at Nature Communications:
https://www.nature.com/articles/s41467-020-14772-5
18 Feb 2020
Soft Matter and Biophysics
Identifying knot types of polymer conformations by machine learning
Identifying knot types of polymer conformations by machine learning
We investigate the use of artificial neural networks (NNs) as an alternative tool to current analytical methods for recognizing knots in a given polymer conformation. The motivation is two-fold. First, it is of interest to examine whether NNs are effective at learning the global and sequential properties that uniquely define a knot. Second, knot classification is an important and unsolved problem in mathematical and physical sciences, and NNs may provide new insights into this problem. Motivated by these points, we generate millions of polymer conformations for five knot types, and design various NN models for classification. Our best model achieves a five-class classification accuracy of above 99% on a polymer of 100 monomers. We find that the sequential modeling ability of recurrent NNs is crucial for this result, as it outperforms feed-forward NNs, and successfully generalizes to differently-sized conformations as well. We present our methods and suggest that deep learning may be used in specific applications of knot detection where some error is permissible. Hopefully, with further development, NNs can offer an alternative computational method for knot identification and facilitate knot research in mathematical and physical sciences.
Read more at Physical Review E:
https://journals.aps.org/pre/abstract/10.1103/PhysRevE.101.022502
The paper was featured in Physics magazine:
https://physics.aps.org/synopsis-for/10.1103/PhysRevE.101.022502
Moreover, the paper was highlighted in Nature as well:
https://www.nature.com/articles/d41586-020-00483-w
The paper was also featured in CityU Research:
https://www.cityu.edu.hk/research/stories/2020/04/16/cityu-scientists-classify-knots-efficiently-artificial-intelligence
11 Feb 2020
Soft Matter and Biophysics
Opposite effects of high-valent cations on the elasticities of DNA and RNA duplexes revealed by magnetic tweezers
Opposite effects of high-valent cations on the elasticities of DNA and RNA duplexes revealed by magnetic tweezers
We report that multivalent cations decrease the persistence length, stretching modulus, helical density and size of plectonemes formed under torque of DNA but increase those of RNA. Multivalent cations affect these physical parameters synchronously, which may imply they are intrinsically correlated. The helical densities of DNA and RNA duplexes are significantly affected by multivalent cations, which may affect the structures and the interactions with proteins in vivo.
Read more at Physical Review Letters:
https://journals.aps.org/prl/accepted/f307eY30R971d878009c28f8568064cdd887f4e57
07 Feb 2020
Applied Physics
Ultralow-strain Zn-substituted Layered Oxide Cathode with Suppressed P2-O2 Transition for Stable Sodium Ion Storage
Ultralow-strain Zn-substituted Layered Oxide Cathode with Suppressed P2-O2 Transition for Stable Sodium Ion Storage
Layered transition metal oxides have drawn much attention as a promising candidate cathode material for sodium‐ion batteries. However, their performance degradation originating from strains and lattice phase transitions remains a critical challenge. Herein, a high‐concentration Zn‐substituted NaxMnO2 cathode with strongly suppressed P2–O2 transition is investigated, which exhibits a volume change as low as 1.0% in the charge/discharge process. Such ultralow strain characteristics ensure a stable host for sodium ion storage, which significantly improves the cycling stability and rate capability of the cathode material. Also, the strong coupling between the highly reversible capacity and the doping content of Zn in NaxMnO2 is investigated. It is suggested that a reversible anionic redox reaction can be effectively triggered by Zn ions and is also highly dependent on the Zn content. Such an ion doping strategy could shed light on the design and construction of stable and high‐capacity sodium ion host.
Read more at Advanced Functional Materials:
https://onlinelibrary.wiley.com/doi/10.1002/adfm.201910327
02 Feb 2020
Applied Physics
Boosting fast energy storage by synergistic engineering of carbon and deficiency
Boosting fast energy storage by synergistic engineering of carbon and deficiency
Exploring advanced battery materials with fast charging/discharging capability is of great significance to the development of modern electric transportation. Herein it reports a powerful synergistic engineering of carbon and deficiency to construct high-quality three/two-dimensional cross-linked Ti2Nb10O29−x@C composites at primary grain level with conformal and thickness-adjustable boundary carbon. Such exquisite boundary architecture is demonstrated to be capable of regulating the mechanical stress and concentration of oxygen deficiency for desired performance. Consequently, significantly improved electronic conductivity and enlarged lithium ion diffusion path, shortened activation process and better structural stability are realized in the designed Ti2Nb10O29−x@C composites. The optimized Ti2Nb10O29−x@C composite electrode shows fast charging/discharging capability with a high capacity of 197 mA h g−1 at 20 C (∼3 min) and excellent long-term durability with 98.7% electron and Li capacity retention over 500 cycles. Most importantly, the greatest applicability of our approach has been demonstrated by various other metal oxides, with tunable morphology, structure and composition.
Read more at Nature Communications:
https://www.nature.com/articles/s41467-019-13945-1
09 Jan 2020
Spectroscopy and Imaging
Facet-dependent active sites of a single Cu2O particle photocatalyst for CO2 reduction to methanol
Facet-dependent active sites of a single Cu2O particle photocatalyst for CO2 reduction to methanol
Atomic-level understanding of the active sites and transformation mechanisms under realistic working conditions is a prerequisite for rational design of high-performance photocatalysts. Here, by using correlated scanning fluorescence X-ray microscopy and environmental transmission electron microscopy at atmospheric pressure, in operando, we directly observe that the (110) facet of a single Cu2O photocatalyst particle is photocatalytically active for CO2 reduction to methanol while the (100) facet is inert. The oxidation state of the active sites changes from Cu(i) towards Cu(ii) due to CO2 and H2O co-adsorption and changes back to Cu(i) after CO2 conversion under visible light illumination. The Cu2O photocatalyst oxidizes water as it reduces CO2. Concomitantly, the crystal lattice expands due to CO2 adsorption then reverts after CO2 conversion. The internal quantum yield for unassisted wireless photocatalytic reduction of CO2 to methanol using Cu2O crystals is ~72%.
Read more at Nature Energy:
https://www.nature.com/articles/s41560-019-0490-3
22 Nov 2019
Spectroscopy and Imaging
Aqueous Li-ion battery enabled by halogen conversion–intercalation chemistry in graphite
Aqueous Li-ion battery enabled by halogen conversion–intercalation chemistry in graphite
More recently, defect-engineering has been demonstrated as a facile but effective route to boost electrocatalytic performance. In this work, an activity-enhanced MnO2 catalyst with abundant oxygen vacancies and edges is developed by Ar-plasma strategy to enhance the electrochemical performance of Al-air batteries. Based on the detailed morphology and structure analysis, rich defects are successfully induced on the surface of MnO2 nanowires, and the defect-engineered MnO2 catalyst displays higher activities (more positive reduction potential and larger reduction current density) towards oxygen reduction reaction (ORR) compared with pristine MnO2. However, excessively high defects can work against the ORR catalytic enhancement due to the structure distortion. The resultant Al-air battery displays higher voltage, larger power density and better durability. The remarkable ORR activity is due to the formation of defective active sites, which is beneficial for the oxygen species adsorption and activation of O-O bond, confirmed by DFT calculation. This strategy represents a new route for the development of non-noble Electrocatalyst for Al-air batteries.
Read more at Nature: https://www.nature.com/articles/s41586-019-1175-6
09 May 2019
Theoretical and Computational Physics
Observation of Many-Body Localization in a One-Dimensional System with a Single-Particle Mobility Edge
Observation of Many-Body Localization in a One-Dimensional System with a Single-Particle Mobility Edge
In this work we experimentally study many-body localization (MBL) with ultracold atoms in a weak one-dimensional quasiperiodic potential [namely, the generalized Aubry-Andre (GAA) model], which in the noninteracting limit exhibits an intermediate phase that is characterized by a coexistence of localized and extended single-particle orbitals. We measure the time evolution of an initial charge density wave after a quench and analyze the corresponding relaxation exponents. We find clear signatures of MBL when the corresponding noninteracting model is deep in the localized phase. We also critically compare and contrast our results with those from a tight-binding Aubry-Andre model, which does not exhibit a single-particle intermediate phase, in order to identify signatures of a potential many-body intermediate phase.
Read more at Physical Review Letters: https://doi.org/10.1103/PhysRevLett.122.170403
[Caption] Left: Cartoon picture of the initial charge density wave state in the experiment. The background curve represents the potential landscape and the black dots represent the initial positions of the atoms. Right: Heuristic phase diagram of the GAA model we study in this work. In the noninteracting limit the GAA model exhibits three phases [single-particle extended, single-particle intermediate (SPIP), and single-particle localized], with the phase boundary denoted by A and B. Here Δ is the strength of the detuning lattice, while U is the strength of the Hubbard on-site interactions. The situation with finite interactions is unknown in theory, although a full MBL phase is believed to exist in the regime where the corresponding noninteracting system is single-particle localized. Below the single-particle localization transition point A, interactions will lead to a thermal phase, where the eigenstate thermalization hypothesis (ETH) holds. The existence of a many-body intermediate phase (MBIP, marked in gray) is highly debated, which is one of the main motivations of our work.
03 May 2019
Atomic, Molecular, and Optical Physics
Integrating temporal and spatial control of electronic transitions for bright multiphoton upconversion
Integrating temporal and spatial control of electronic transitions for bright multiphoton upconversion
The applications of lanthanide-doped upconversion nanomaterials are limited by unsatisfactory brightness currently. Herein, a general strategy is proposed for boosting the upconversion efficiency in Er3+ions, based on combined use of a core−shell nanostructured host and an integrated optical waveguide circuit excitation platform. A NaErF4@NaYF4core−shell nanoparticle is constructed to host the upconversion process for minimizing non-radiative dissipation of excitation energy by surface quenchers. Furthermore, an integrated optical microring resonator is designed to promote absorption of excitation light by the nanoparticles, which alleviates quenching of excited states due to cross-relaxation andphonon-assisted energy transfer. As a result, multiphoton upconversion emission with a large anti-Stokes shift (greater than 1150 nm) and a high energy conversion efficiency (over 5.0%) is achieved under excitation at 1550 nm. These advances in controlling photon upconversion offer exciting opportunities for important photonics applications.
Read more at Nature Communications: https://rdcu.be/bxDP2
19 Apr 2019
Soft Matter and Biophysics
Early Stage Alterations in White Matter and Decreased Functional Interhemispheric Hippocampal Connectivity in the 3xTg Mouse Model of Alzheimer’s Disease
Early Stage Alterations in White Matter and Decreased Functional Interhemispheric Hippocampal Connectivity in the 3xTg Mouse Model of Alzheimer’s Disease
Alzheimer's disease (AD), also referred to simply as Alzheimer's, is a chronic neurodegenerative disease that usually starts slowly and gradually worsens over time. It is the cause of 60–70% of cases of dementia. In this study, Dr. Lau and coworkers discovered a translational biomarker in preclinical stages of the AD.
Read more at Frontiers in Aging Neuroscience:
https://www.frontiersin.org/articles/10.3389/fnagi.2019.00039/full
22 Mar 2019
Atomic, Molecular, and Optical Physics
Laser cavity-soliton microcombs
Laser cavity-soliton microcombs
Microcavity-based frequency combs, or ‘microcombs’ have enabled many fundamental breakthroughs through the discovery of temporal cavity-solitons. These self-localized waves, described by the Lugiato-Lefever equation, are sustained by a background of radiation usually containing 95% of the total power. Simple methods for their efficient generation and control are currently being investigated to finally establish microcombs as out-of-the-lab tools. Here, we demonstrate microcomb laser cavity-solitons.
Laser cavity solitons are intrinsically background-free and have underpinned key breakthroughs in semiconductor lasers. By merging their properties with the physics of multimode systems, we provide a new paradigm for soliton generation and control in microcavities. We demonstrate 50-nm-wide bright soliton combs induced at average powers more than one order of magnitude lower than the Lugiato–Lefever soliton power threshold, measuring a mode efficiency of 75% versus the theoretical limit of 5% for bright Lugiato–Lefever solitons. Finally, we can tune the repetition rate by well over a megahertz without any active feedback.
Read more at Nature Photonics: https://www.nature.com/articles/s41566-019-0379-5
11 Mar 2019
Spectroscopy and Imaging
Nitrogen-Doped Sponge Ni Fibers as Highly Efficient Electrocatalysts for Oxygen Evolution Reaction
Nitrogen-Doped Sponge Ni Fibers as Highly Efficient Electrocatalysts for Oxygen Evolution Reaction
Controllable synthesis of highly active micro/nanostructured metal electrocatalysts for oxygen evolution reaction (OER) is a particularly significant and challenging target. Herein, we report a 3D porous sponge-like Ni material, prepared by a facile hydrothermal method and consisting of cross-linked micro/nanofibers, as an integrated binder-free OER electrocatalyst. To further enhance the electrocatalytic performance, an N-doping strategy is applied to obtain N-doped sponge Ni (N-SN) for the first time, via NH3 annealing. Due to the combination of the unique conductive sponge structure and N doping, the as-obtained N-SN material shows improved conductivity and a higher number of active sites, resulting in enhanced OER performance and excellent stability. Remarkably, N-SN exhibits a low overpotential of 365 mV at 100 mA cm−2 and an extremely small Tafel slope of 33 mV dec−1, as well as superior long-term stability, outperforming unmodified sponge Ni. Importantly, the combination of X-ray photoelectron spectroscopy and near-edge X-ray adsorption fine structure analyses shows that γ-NiOOH is the surface-active phase for OER. Therefore, the combination of conductive sponge structure and N-doping modification opens a new avenue for fabricating new types of high-performance electrodes with application in electrochemical energy conversion devices.
Read more at Nano-Micro Letters:
https://link.springer.com/article/10.1007%2Fs40820-019-0253-5
09 Mar 2019
Theoretical and Computational Physics
Arbitrary order exceptional point induced by photonic spin–orbit interaction in coupled resonators
Arbitrary order exceptional point induced by photonic spin–orbit interaction in coupled resonators
Non-Hermitian physical systems (e.g. systems with loss) can have a special type of degeneracy called “exceptional points” which are branch points of complex energy surfaces at which eigenvalues and eigenvectors coalesce. These points of degeneracy can result in optical structures that are ultrasensitive to external perturbations, and therefore can enhance the performance of optical sensors. The sensitivity enhancement is proportional to the order of exceptional points. High order exceptional points have been hotly sought in recent years. This, however, is rather difficult to achieve since it usually involves tuning of multiple system parameters. In this paper, we theoretically and experimentally demonstrate a novel mechanism that can give rise to arbitrary order exceptional points without tuning any parameters. This is achieved by employing a chain of spherical resonators unidirectionally coupled with each other via photonic spin-orbit interaction. Our results lay the foundation for future exploration of topological and non-Hermitian properties of high order exceptional points.
Read more at Nature Communications: https://www.nature.com/articles/s41467-019-08826-6
19 Feb 2019
Spectroscopy and Imaging
Multiscale Graphene‐Based Materials for Applications in Sodium Ion Batteries
Multiscale Graphene‐Based Materials for Applications in Sodium Ion Batteries
Scrupulous design and smart hybridization of bespoke electrode materials are of great importance for the advancement of sodium ion batteries (SIBs). Graphene‐based nanocomposites are regarded as one of the most promising electrode materials for SIBs due to the outstanding physicochemical properties of graphene and positive synergetic effects between graphene and the introduced active phase. In this review, the recent progress in graphene‐based electrode materials for SIBs with an emphasis on the electrode design principle, different preparation methods, and mechanism, characterization, synergistic effects, and their detailed electrochemical performance is summarized. General design rules for fabrication of advanced SIB materials are also proposed. Additionally, the merits and drawbacks of different fabrication methods for graphene‐based materials are briefly discussed and summarized. Furthermore, multiscale forms of graphene are evaluated to optimize electrochemical performance of SIBs, ranging from 0D graphene quantum dots, 2D vertical graphene and reduced graphene oxide sheets, to 3D graphene aerogel and graphene foam networks. To conclude, the challenges and future perspectives on the development of graphene‐based materials for SIBs are also presented.
Read more at Advanced Energy Materials:
https://onlinelibrary.wiley.com/doi/full/10.1002/aenm.201803342
21 Jan 2019
Applied Physics
Stoichiometry Controlled Bipolar Conductivity in Nanocrystalline NixCd1−xO1+δ Thin Films
Stoichiometry Controlled Bipolar Conductivity in Nanocrystalline NixCd1−xO1+δ Thin Films
The availability of wide gap oxides with bipolar conductivity is critical to the development of emerging transparent optoelectronic technologies. However, most metal oxides show a strong propensity for n-type conductivity while p-type doping is extremely challenging due to their unique band structures. In this work, we demonstrated that bipolar conductivity can be achieved by alloying two transition metal oxides with type III band offset, CdO and NiO and manipulating the alloy stoichiometry through the growth parameters. The results strongly suggest that these alloys have great technological potentials for transparent optoelectronic applications.
Read more at Physical Review Applied:
https://doi.org/10.1103/PhysRevApplied.11.014019
10 Jan 2019
Applied Physics
Advanced adaptive photonic RF filters with 80 taps based on an integrated optical micro-comb source
We demonstrate a photonic radio frequency (RF) transversal filter based on an integrated optical micro-comb source featuring a record low free spectral range of 49 GHz, yielding 80 micro-comb lines across the C -band. This record high number of taps, or wavelengths for the transversal filter results in significantly increased performance including a Q RF factor more than four times higher than previous results.
Furthermore, by employing both positive and negative taps, an improved out-of-band rejection of up to 48.9 dB is demonstrated using a Gaussian apodization, together with a tunable center frequency covering the RF spectra range, with a widely tunable 3-dB bandwidth and versatile dynamically adjustable filter shapes. Our experimental results match well with theory, showing that our transversal filter is a competitive solution to implement advanced adaptive RF filters with broad operational bandwidth, high frequency selectivity, high reconfigurability, and potentially reduced cost and footprint. This approach is promising for applications in modern radar and communications systems.
Read more at IEEE Journal of Lightwave Technology:
https://ieeexplore.ieee.org/document/8606970
This paper has received the Best Paper Award for 2019 by the Journal of Lightwave Technology’s Steering and Coordinating Committee. Learn more:
https://www.ieee-jlt.org/JLT-Awards
10 Jan 2019
Atomic, Molecular, and Optical Physics
High-dimensional one-way quantum processing implemented on d-level cluster states
High-dimensional one-way quantum processing implemented on d-level cluster states
Cluster states are a particularly important class of multi-partite states, formed by multiple parties, such as multiple atoms or photons, characterized by two unique properties: maximal connectedness and highest persistency of entanglement. These properties make cluster states equivalent to universal one-way quantum computers, where different algorithms can be implemented by performing measurements on the individual parties of the cluster states. This approach greatly simplifies quantum processing. In this work, we present a general approach to prepare and coherently manipulate discrete d-level multi-partite quantum systems based on the simultaneous entanglement, or hyper-entanglement, of two photons in time and frequency, by exploiting integrated photonics circuits. This is the first experimental realization and characterization of qudit cluster states as well as the first hyper-entangled state employing only a single degree of freedom.
Read more at Nature Physics: https://www.nature.com/articles/s41567-018-0347-x
03 Dec 2018
Spectroscopy and Imaging
A monoclinic polymorph of sodium birnessite for ultrafast and ultrastable sodium ion storage
A monoclinic polymorph of sodium birnessite for ultrafast and ultrastable sodium ion storage
Sodium transition metal oxides with layered structures are attractive cathode materials for sodium-ion batteries due to their large theoretical specific capacities. However, these layered oxides suffer from poor cyclability and low rate performance because of structural instability and sluggish electrode kinetics. In the present work, we show the sodiation reaction of Mn3O4 to yield crystal water free NaMnO2−y−δ(OH)2y, a monoclinic polymorph of sodium birnessite bearing Na/Mn(OH)8 hexahedra and Na/MnO6 octahedra. With the new polymorph, NaMnO2−y−δ(OH)2y exhibits an enlarged interlayer distance of about 7 Å, which is in favor of fast sodium ion migration and good structural stability. In combination of the favorable nanosheet morphology, NaMn2−y−δ(OH)2y cathode delivers large specific capacity up to 211.9 mAh g–1, excellent cycle performance (94.6% capacity retention after 1000 cycles), and outstanding rate capability (156.0 mAh g–1 at 50 C). This study demonstrates an effective approach in tailoring the structural and electrochemical properties of birnessite towards superior cathode performance in sodium-ion batteries.
Read more at Nature Communications: https://www.nature.com/articles/s41467-018-07595-y
30 Nov 2018
Spectroscopy and Imaging
Modifying High-Voltage Olivine-Type LiMnPO4 Cathode via Mg Substitution in High-Orientation Crystal
Modifying High-Voltage Olivine-Type LiMnPO4 Cathode via Mg Substitution in High-Orientation Crystal
The LiMnPO4 material with Mg2+ ion substitution in transition metal site is successfully obtained by a facile solvothermal method combining with the subsequent spray drying and pyrolysis. The TEM image shows the controlled crystalline orientation of the primary Mg-doped LiMnPO4 particle, and the three-dimensional (3D) hierarchical micro-nano structure and the electronic structure of the Mg-doped material are investigated by the reconstructed nano-tomography and X-ray absorption spectroscopy, respectively. The uniformity of Mg2+ ions substitution at the secondary particle level is confirmed by the transmission X-ray microscopy combined with X-ray absorption near edge structure. The prepared material exhibits an excellent electrochemical performance with the capacity of 156 mAh/g at the rate of 0.1 C, and good capacity retention of 100 % after 400 cycles at the rate of 1 C. The in operando synchrotron X-ray diffraction data indicates that the performance improvement of the Mg-doped materials is attributed to the changed electrochemical reaction mechanism from the typical two-phase reaction for pristine LiMnPO4 material to the two-phase reaction combined with the single-phase (solid-solution) reaction process. These findings provide a new approach and understanding on developing high-energy cathode materials for lithium-ion batteries.
Read more at ACS Applied Energy Materials:
https://pubs.acs.org/doi/abs/10.1021/acsaem.8b00923
01 Oct 2018
Spectroscopy and Imaging
Defect-engineered MnO2 enhancing oxygen reduction reaction for high performance Al-air batteries
Defect-engineered MnO2 enhancing oxygen reduction reaction for high performance Al-air batteries
More recently, defect-engineering has been demonstrated as a facile but effective route to boost electrocatalytic performance. In this work, an activity-enhanced MnO2 catalyst with abundant oxygen vacancies and edges is developed by Ar-plasma strategy to enhance the electrochemical performance of Al-air batteries. Based on the detailed morphology and structure analysis, rich defects are successfully induced on the surface of MnO2 nanowires, and the defect-engineered MnO2 catalyst displays higher activities (more positive reduction potential and larger reduction current density) towards oxygen reduction reaction (ORR) compared with pristine MnO2. However, excessively high defects can work against the ORR catalytic enhancement due to the structure distortion. The resultant Al-air battery displays higher voltage, larger power density and better durability. The remarkable ORR activity is due to the formation of defective active sites, which is beneficial for the oxygen species adsorption and activation of O-O bond, confirmed by DFT calculation. This strategy represents a new route for the development of non-noble Electrocatalyst for Al-air batteries.
Read more at Energy Storage Materials: https://www.sciencedirect.com/science/article/pii/S2405829718307724
01 Oct 2018
Soft Matter and Biophysics
Microscopic origin of the logarithmic relaxation in molecular glass-forming liquids
Microscopic origin of the logarithmic relaxation in molecular glass-forming liquids
Logarithmic relaxation is a unique relaxation process exhibited by a few molecular liquids and biomolecules. However, the microscopic origin of logarithmic relaxation is still unclear. To understand the origin of this process, we studied two liquids that exhibit logarithmic relaxation in a dissolved state using quasielastic neutron scattering (QENS) and depolarized dynamic light scattering (DDLS). Although the intermolecular potential of the liquids is drastically different in the dissolved state from the bulk liquids, we observed that the logarithmic relaxation still persists. Our results indicate that the intermolecular potential does not play a role in determining the logarithmic relaxation process. The coupling of rotational and translational relaxation processes could be the origin of the logarithmic relaxation process exhibited by the molecular liquids.
Read more at Physical Review B:
https://journals.aps.org/prb/abstract/10.1103/PhysRevB.98.094203
27 Sep 2018
Spectroscopy and Imaging
Empowering multicomponent cathode materials for sodium ion batteries by exploring three-dimensional compositional heterogeneities
Empowering multicomponent cathode materials for sodium ion batteries by exploring three-dimensional compositional heterogeneities
Affordable sodium ion batteries hold great promise for revolutionizing stationary energy storage technologies. Sodium layered cathode materials are usually multicomponent transition metal (TM) oxides and each TM plays a unique role in the operating cathode chemistry, e.g., redox activity, structural stabilization. Engineering the three-dimensional (3D) distribution of TM cations in individual cathode particles can take advantage of a depth-dependent charging mechanism and enable a path towards tuning local TM–O chemical environments and building resilience against cathode–electrolyte interfacial reactions that are responsible for capacity fading, voltage decay and safety hazards. In this study, we create 3D compositional heterogeneity in a ternary and biphasic (O3–P3) sodium layered cathode material (Na0.9Cu0.2Fe0.28Mn0.52O2). The cells containing this material deliver stable voltage profiles, and discharge capacities of 125 mA h g−1 at C/10 with almost no capacity fading after 100 cycles and 75 mA h g−1 at 1C with negligible capacity fading after 200 cycles. The direct performance comparison shows that this material outperforms other materials with similar global compositions but different mesoscale chemical distributions. Synchrotron X-ray spectroscopy/imaging and density functional theory studies reveal depth-dependent chemical environments due to changes to factors such as charge compensation and strength of orbital hybridization. Finally, 3D spectroscopic tomography illuminates the path towards optimizing multicomponent sodium layered cathode materials, to prevent the migration of TMs upon prolonged cycling. The study reports an inaugural effort of multifaceted and counterintuitive investigation of sodium layered cathode materials and strongly implies that there is plenty of room at the bottom by tuning nano/meso scale chemical distributions for stable cathode chemistry.
Read more at Energy & Environmental Science: https://pubs.rsc.org/en/content/articlehtml/2018/ee/c8ee00309b
25 Jun 2018
Applied Physics
An antibacterial platform based on capacitive carbon-doped TiO2 nanotubes after direct or alternating current charging
An antibacterial platform based on capacitive carbon-doped TiO2 nanotubes after direct or alternating current charging
Electrical interactions between bacteria and the environment are delicate and essential. In this study, an external electrical current is applied to capacitive titania nanotubes doped with carbon (TNT-C) to evaluate the effects on bacteria killing and the underlying mechanism is investigated. When TNT-C is charged, post-charging antibacterial effects proportional to the capacitance are observed. This capacitance-based antibacterial system works well with both direct and alternating current (DC, AC) and the higher discharging capacity in the positive DC (DC+) group leads to better antibacterial performance. Extracellular electron transfer observed during early contact contributes to the surface-dependent post-charging antibacterial process. Physiologically, the electrical interaction deforms the bacteria morphology and elevates the intracellular reactive oxygen species level without impairing the growth of osteoblasts. Our finding spurs the design of light-independent antibacterial materials and provides insights into the use of electricity to modify biomaterials to complement other bacteria killing measures such as light irradiation.
Read More at Nature Communications: https://www.nature.com/articles/s41467-018-04317-2
Related article: New Technology Prevents Post-surgery Bacterial Infection Developed by Professor Paul Chu
31 May 2018
Theoretical and Computational Physics
Spin-redirection Phase of Sound Wave Demonstrated for the First Time
Spin-redirection Phase of Sound Wave Demonstrated for the First Time
The concept of geometric phase has revolutionized our understanding of state evolution in both classical and quantum physics and has led to the discovery of topological insulators. Geometric phase arising from the SO(3) group rotation of states in real space is called spin-redirection phase and can be observed in vector waves, such as light. It is commonly believed that such a phase should be absent in scalar waves, such as airborne sound. Dr. Shubo WANG and his collaborators theoretically and experimentally demonstrated, for the first time, spin-redirection phase effects in airborne sound. This is achieved by exploiting sound vortex traveling through a path that bestows a nontrivial winding of the wave vector in momentum space. It provides the important insight that the topological properties of different parameter spaces, e.g. real space and momentum space, are correlated, and one can serve as the origin of the other. The work lays the foundation for future exploitation of spin-redirection phase as a new degree of freedom for the manipulation of scalar waves.
Read more at Science Advances: http://advances.sciencemag.org/content/4/2/eaaq1475
Related article: http://www.cityu.edu.hk/vprt/news/2018/spin-redirection-phase-of-sound-wave-demonstrated-for-the-first-time/
23 Feb 2018
Quantum Materials
Investigation of Supercurrent in the Quantum Hall Regime in Graphene Josephson Junctions
Investigation of Supercurrent in the Quantum Hall Regime in Graphene Josephson Junctions
In this study, we examine multiple encapsulated graphene Josephson junctions to determine which mechanisms may be responsible for the supercurrent observed in the quantum Hall (QH) regime. Rectangular junctions with various widths and lengths were studied to identify which parameters affect the occurrence of QH supercurrent. We also studied additional samples where the graphene region is extended beyond the contacts on one side, making that edge of the mesa significantly longer than the opposite edge. This is done in order to distinguish two potential mechanisms: (a) supercurrents independently flowing along both non-contacted edges of graphene mesa, and (b) opposite sides of the mesa being coupled by hybrid electron–hole modes flowing along the superconductor/graphene boundary. The supercurrent appears suppressed in extended junctions, suggesting the latter mechanism.
Read More at Journal of Low Temperature Physics:
https://link.springer.com/article/10.1007%2Fs10909-018-1872-9
15 Feb 2018
Applied Physics
Room Temperature Synthesized High Mobility Transparent Amorphous CdO-Ga2O3 Alloys with Widely Tunable Electronic Bands
Room Temperature Synthesized High Mobility Transparent Amorphous CdO-Ga2O3 Alloys with Widely Tunable Electronic Bands
Currently, transparent conducting oxides (TCOs) are widely used as transparent conductors for thin film solar cells. Because of the potential of roll-to-roll, large-area processing on flexible low-cost substrates, flexible solar cells are a very attractive technology for the low-cost photovoltaic market. For flexible solar cells, an additional requirement for the transparent electrode is that the deposition must be carried out at temperatures lower than the deformation temperature of flexible polymer substrates. In this work we demonstrated that amorphous Cd1-xGaxO1+d alloy films with 0.3<x<0.4 syntehsize at room temperature have favorable optoelectronic properties as transparent conductors on flexible and/or organic substrates while their band edges and electrical conductivity can also be manipulated for transparent thin film transistors as well as electron transport layers.
Read more at ACS Applied Materials & Interfaces:
https://doi.org/10.1021/acsami.7b18254
01 Feb 2018
Quantum Materials
Magic angle for barrier-controlled double quantum dots
Magic angle for barrier-controlled double quantum dots
We show that the exchange interaction of a singlet-triplet spin qubit confined in double quantum dots, when being controlled by the barrier method, is insensitive to a charged impurity lying along certain directions away from the center of the double-dot system. These directions differ from the polar axis of the double dots by the magic angle, equaling arccos(1/√3)≈54.7∘, a value previously found in atomic physics and nuclear magnetic resonance. This phenomenon can be understood from an expansion of the additional Coulomb interaction created by the impurity, but also relies on the fact that the exchange interaction solely depends on the tunnel coupling in the barrier-control scheme. Our results suggest that for a scaled-up qubit array, when all pairs of double dots rotate their respective polar axes from the same reference line by the magic angle, crosstalk between qubits can be eliminated, allowing clean single-qubit operations. While our model is a rather simplified version of actual experiments, our results suggest that it is possible to minimize unwanted couplings by judiciously designing the layout of the qubits.
Read more at Physical Review A - Atomic, Molecular, and Optical Physics:
https://journals.aps.org/pra/abstract/10.1103/PhysRevA.97.012304
09 Jan 2018
Applied Physics
Room-Temperature Red–Green–Blue Whispering-Gallery Mode Lasing and White-Light Emission from Cesium Lead Halide Perovskite (CsPbX3, X = Cl, Br, I) Microstructures
Room-Temperature Red–Green–Blue Whispering-Gallery Mode Lasing and White-Light Emission from Cesium Lead Halide Perovskite (CsPbX3, X = Cl, Br, I) Microstructures
Based on composition‐tunable cesium lead halide perovskite (CsPbX3, X = Cl, Br, I) microcrystal structures grown by chemical vapor deposition, broad‐band light emission devices can be achieved. Room‐temperature optically pumped red–green–blue whispering‐gallery mode lasers as well as white‐light emission are successfully realized. This work evidently suggests a feasible route to the design of red–green–blue lasers and white-light emitters for potential applications in full-color displays as well as photonic devices.
Read more at Advanced Optical Materials:
https://doi.org/10.1002/adom.201700993
27 Dec 2017
Applied Physics
Breakthrough Technologies for Cancer Treatment and Energy Saving Developed by Prof Paul Chu
Breakthrough Technologies for Cancer Treatment and Energy Saving Developed by Prof Paul Chu
Professor Paul Chu Kim-ho, Chair Professor in the Department of Physics and the Department of Materials Science and Engineering, and his research team recently developed breakthrough technologies for effective cancer treatment and energy saving.
Both applications were developed based on the study of photoluminescence and light scattering mechanisms, as well as the plasmonic properties of micro-nanostructures.
This research findings have earned the team, comprising researchers from CityU and Nanjing University in mainland China, the highly prestigious First Class Award (Natural Science) in the 2017 Higher Education Outstanding Scientific Research Output Awards (Science and Technology) of the Ministry of Education, China.
One of the applications involves using intense heat triggered by a photothermal process to kill cancer cells. Professor Chu described the strategy as a “Trojan horse” carrying “bombs”. He said “when the ‘bombs’ interact with near-infrared light, the temperature rapidly increases, killing the tumours.” His team has discovered two effective types of carriers with photothermal agents: Bi2Se3-laden-macrophages and Nile blue dye with black phosphorus. In trials, the tumours in mice were completely destroyed and removed, and there was no recurrence until the end of the experiment. The other application is thermochromic smart coating, which can control the transmission of solar radiation dynamically and automatically in accordance with the ambient temperature and illumination intensity.
Detailed story can be retrieved from CityU News (21 December 2017).
Please click the following links for media coverage:
- 城大南大研新光熱技術殺癌 [Wen Wei Po] 2017-12-22 A8 港聞
- 城大研發光熱治癌技術 [Sing Tao Daily] 2017-12-22 教育
- 城大研究 如木馬屠城 光熱高溫殺癌細胞 [am730] 2017-12-22
- 城大研透明智能塗層 可調節冷暖 [SkyPost] 2017-12-22 港聞
- 城大「光熱治療」高溫殺癌細胞 [Sing Pao Daily News] 2017-12-22 港聞
- 城大研光熱治療法殺癌細胞 [Ta Kung Pao] 2017-12-22
- 城大研光熱治療法 高溫殺死癌細胞 [on.cc] 2017-12-21港聞
- 城大研發「光熱治癌技術」 70度熱力殺死癌細胞 [Headline Daily] 2017-12-22港聞
21 Dec 2017
Soft Matter and Biophysics
The multi-level impact of chronic intermittent hypoxia on central auditory processing
The multi-level impact of chronic intermittent hypoxia on central auditory processing
Sleep breathing disorders (SBDs) affect a significant fraction of the population and has adverse health effects, such as cardiovascular disease and memory deficits. Until recently, little is known about the impact of SBDs on hearing. In this study entitled “The multi-level impact of chronic intermittent hypoxia on central auditory processing”, Dr. Condon LAU examined the hearing of a rodent SBD model with brain imaging. The researchers observe that SBD reduces the responses of two important centers of the brain to sound, the auditory cortex and midbrain. These changes may impact the ability of SBD patients to verbally communicate with others. In young patients, these changes may impact their ability to succeed at school.
Read more at NeuroImage: http://www.sciencedirect.com/science/article/pii/S1053811917304287
01 Oct 2017
Theoretical and Computational Physics
Randomized Benchmarking of Barrier versus Tilt Control of a Singlet-Triplet Qubit
Randomized Benchmarking of Barrier versus Tilt Control of a Singlet-Triplet Qubit
Dr. WANG Xin and his collaborators have recently published an article entitled “Randomized Benchmarking of Barrier versus Tilt Control of a Singlet-Triplet Qubit” in Physical Review Letters, with CityU research student Mr. Zhang Chengxian being the first author and Dr. Wang the corresponding author.
Read more at Physical Review Letters: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.118.216802
01 Sep 2017
40-year Mystery Solved by Scientist Group Led by Prof Xun-li Wang
40-year Mystery Solved by Scientist Group Led by Prof Xun-li Wang
Spectroscopy and Imaging
40-year Mystery Solved by Scientist Group Led by Prof Xun-li Wang
40-year Mystery Solved by Scientist Group Led by Prof Xun-li Wang
Professor Xun-li WANG, our Head of Department and Chair Professor of Physics, together with his research collaborators from Australia, Japan and United States, recently uncovered a hidden amorphous phase in the formation of metallic glass. This new breakthrough in the discovery of metallic glass formation has been published in the internationally acclaimed journal Nature Communications.
Owing to its high resilience which can sustain larger elastic deformation, metallic glass has been used in a variety of applications including but not limited to sports equipment, medical devices and electricity transformers. Building upon the discovery of the hidden amorphous phase, where atoms show a different kind of packing, researchers hope to utilize the simple processing methods such as heat treatment to develop novel materials. The discovery solved a 40-year scientific mystery and will undoubtedly signify an important milestone in the development of new and better metallic alloys.
Read more at: http://www.nature.com/articles/ncomms14679.
Detailed news story can also be retrieved from CityU News (20 March 2017).
20 Mar 2017
Applied Physics
Highly mismatched GaN1−xSbx alloys: synthesis, structure and electronic properties
Highly mismatched GaN1−xSbx alloys: synthesis, structure and electronic properties
Prof. Kin Man YU published an Invited Topical Review article entitled “Highly mismatched GaN1−xSbx alloys: synthesis, structure and electronic properties” in Semiconductor Science and Technology. Highly mismatched alloys (HMAs) are a class of semiconductor alloys whose constituents are distinctly different in terms of size, ionicity and/or electronegativity. Electronic properties of the alloys deviate significantly from an interpolation scheme based on small deviations from the virtual crystal approximation. Most of the HMAs were only studied in a dilute composition limit. Recent advances in understanding of the semiconductor synthesis processes allowed growth of thin films group III-N-V HMAs over almost the entire composition under non-equilibrium. This paper reviewed recent work of the authors on the synthesis, structural and optical characterization of GaN1−xSbx HMAs which has been suggested as a potential material for solar water dissociation devices. This is an international collaboration with researchers from the US, UK and HK.
Read More at Semiconductor Science and Technology: http://iopscience.iop.org/article/10.1088/0268-1242/31/8/083001
28 Jun 2016
Applied Physics
Surface Coordination of Black Phosphorus for Robust Air and Water Stability
Surface Coordination of Black Phosphorus for Robust Air and Water Stability
Black phosphorus (BP) has attracted increasing interest owing to its unique electronic and optical properties, yet its application is hampered by its poor air and water stability. In their paper entitled “Surface Coordination of Black Phosphorus for Robust Air and Water Stability”, Prof. Paul K. Chu and co-workers designed a titanium sulfonate ligand to coordinate with BP to prevent oxidation. The coordinated BP surface exhibited robust stability in air and water, thereby extending the lifetime and spurring broader applications of BP.
This paper is featured on Cover of Angew. Chem. Int. Ed.
Read more at http://onlinelibrary.wiley.com/doi/10.1002/anie.201512038/abstract
01 Jun 2016
Spectroscopy and Imaging
China's first pulsed neutron source
China's first pulsed neutron source
Professor Xun-Li Wang and Professor Hesheng Chen of Chinese Academy of Sciences authored an article in Nature Materials on China Spallation Neutron Source (CSNS), which is under construction in Dongguan, China. The article is one of the six in a series on Materials Science in China. It provides an overview of CSNS as a user facility and what it means for science in China and elsewhere.
Read more at Nature Materials: http://www.nature.com/nmat/journal/v15/n7/full/nmat4655.html
01 May 2016
Applied Physics
Pursuing high-efficiency photovoltaics with novel alloys
Pursuing high-efficiency photovoltaics with novel alloys
Prof. Kin Man YU published a feature article in Compound semiconductor entitled “Pursuing high-efficiency photovoltaics with novel alloys”. Compound Semiconductor is a trade magazine widely read by semiconductor researchers and technologists. The article was written in “not very technical” language for a more general audience, describing efforts of the authors to realize the novel idea of high efficiency intermediate band solar cells (IBSC) using non-conventional compound semiconductor alloys- the highly mismatched alloys. Experimental demonstration on the working principles of the using dilute GaAsN alloys was presented.
Read more at Compound semiconductor: http://www.compoundsemiconductor.net/magazine/
01 Apr 2016
Atomic, Molecular, and Optical Physics
Generation of multi photon entangled quantum states by means of integrated frequency comb
Generation of multi photon entangled quantum states by means of integrated frequency comb
The March 11 issue of Science features an article co-authored by Dr. ST Chu. The paper, by Reimer et al., is entitled “Generation of multi photon entangled quantum states by means of integrated frequency comb”. Here is the Editor’s summary: The ability to generate optical frequency combs in which the output light is made up of millions of sharp lines precisely spaced apart has been important for optical applications and for fundamental science. Reimer et al. now show that frequency combs can be taken into the quantum regime. They took individual teeth of the combs and quantum-mechanically entangled them to form complex optical states. Because the method is compatible with existing finer and semiconductor technology, the results demonstrate a possible scalable and practical platform for quantum technologies.
Read more at Science: http://science.sciencemag.org/content/351/6278/1176.
11 Mar 2016
Applied Physics
Multicolor Electroluminescence from Intermediate Band Solar Cell Structures
Multicolor Electroluminescence from Intermediate Band Solar Cell Structures
Intermediate band solar cells are a new generation of photovoltaics that allow for better utilization of the solar spectrum. The key and most challenging requirement for these cells is an efficient optical coupling between the intermediate band and the charge conducting bands. In this work two‐color electroluminescence associated with transitions from and to the intermediate band is observed in GaNAs based intermediate band solar cell structures. These results provide the first direct evidence for the optical coupling required for the cell operation and offer the potential of using similar structures for multicolor light emitting devices operating under forward and reverse bias conditions.
Read more at Advanced Energy Materials:
https://onlinelibrary.wiley.com/doi/full/10.1002/aenm.201501820
12 Dec 2015
Atomic, Molecular, and Optical Physics
Cross-polarized photon-pair generation and bi-chromatically pumped optical parametric oscillation on a chip
Cross-polarized photon-pair generation and bi-chromatically pumped optical parametric oscillation on a chip
This paper, entitled "Cross-polarized photon-pair generation and bi-chromatically pumped optical parametric oscillation on a chip" reports frequency mixing between orthogonal polarization modes in a compact integrated microring resonator and demonstrate a bi-chromatically pumped optical parametric oscillator. Operating the device above and below threshold, orthogonally polarized beams and photon pairs are respectively generated. The study can find applications, for example, in optical communication and quantum optics.
Read more at Nature Communications: http://www.nature.com/ncomms/2015/150914/ncomms9236/full/ncomms9236.html
14 Sep 2015
Theoretical and Computational Physics
Dynamic crystallography reveals early signaling events in ultraviolet photoreceptor UVR8
Dynamic crystallography reveals early signaling events in ultraviolet photoreceptor UVR8
This paper, entitled "Dynamic crystallography reveals early signaling events in ultraviolet photoreceptor UVR8" reported how plants absorb UV light by a photoreceptor UVR8. Prof. RQ Zhang and Dr. Fan’s DFT computation together with experimental data reveal the mechanism of photoreaction and dimer dissociation of UVR8.
Read more at Nature Plants: http://www.nature.com/articles/nplants20146
08 Jan 2015