Publications

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

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 $\sqrt{3}\times <!--\; \times \; -->\sqrt{3}\times <!--\; \times \; -->3$  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 $\stackrel{¯<!--\; ¯\; -->}{K}$ 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

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

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

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:

1. 城大首創技術 大增電池儲存量 [東方日報] 2023-09-22 要聞港聞
2. 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
3. Battery tech: CityU scientists achieve minimal voltage decay [Interesting Engineering] 2023-09-21 Innovation
4. 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
5. 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
6. 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
7. 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

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−_xSr_xCuO₄ with x = 0.24. We demonstrate ℏω/k_BT 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

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

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

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

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

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

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

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

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

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 (E_F) 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 E_F, 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

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

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

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

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

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

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

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 T^cri∼ln(k_θ) and T^cri∼ln(k_τ ), where k_θ and k_τ are the polymer bending and torsion stiffness, respectively. Furthermore, the double helix can exhibit major and minor grooves due to symmetric break for better packing. Our results provide a novel guide to the experimental design of the double helix.

Read more at Physical Review Letters:
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.128.197801

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.]

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

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

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

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 

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

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
 

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 

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  

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

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

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

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

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

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, , which scales as   for   and  for , where  is the crowder diameter,  is the monomer diameter,  is the polymer persistence length. For DNA,  is estimated to be around 0.25 for crowders with  = 2 nm. We find that also strongly depends on the shape of the channel cross-section, and  is much smaller for a triangle channel than a square channel. The polymer trapping leads to a nearly fully stretched polymer conformation along a channel corner, which may have practical applications, such as full stretching of DNA for the nanochannel-based genome mapping technology.

Read more at Physical Review E: 
https://journals.aps.org/pre/abstract/10.1103/PhysRevE.104.054502

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

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

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1. Physicists discover special transverse sound wave [Phys.org] 2021-12-07 General Physics

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3. CityU physicists discovered special transverse sound wave [ScienMag] 2021-12-07 Social & Behavioral Science

4. Physicists discovered special transverse sound wave [Science Daily] 2021-12-07 Science News

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

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

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

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

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

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

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: :

1. CityU scientists make a breakthrough towards solving the structural mystery of glass [EurekAlert!] 2021-06-08 Research News

2. CityU scientists make a breakthrough towards solving the structural mystery of glass [Bioengineer.org] 2021-06-08 Science News

3. CityU Scientists Make A Breakthrough Towards Solving The Structural Mystery Of Glass [ScienMag] 2021-06-08 Technology and Engineering

4. Breakthrough towards solving the structural mystery of glass [Phys.org] 2021-06-08 Materials Science

5. Solving the structural mystery of glass [Science Daily] 2021-06-08 Science News

6. Breakthrough Discovery Sheds Light on Glass Structure [Azom.com] 2021-06-09 Materials Analysis

7. 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 

8. CityU scientists make a breakthrough towards solving structural mystery of glass [Mirage News] 2021-06-08 Science

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
 

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
 

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

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

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

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

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

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 C_n, 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 C_2y 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

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

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

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

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

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

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

Knots can spontaneously form in polymers. How knotting affects polymer behavior depends on polymer knotting probability, p_knot. An intriguing result about p_knot in recent studies is that p_knot exhibits a non-monotonic dependence on the bending stiffness and is maximized at L_p ≈ 8a, where L_p 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 p_knot 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 p_knot. Based on this explanation, we derive an approximate analytic equation that captures the non-monotonic behavior of p_knot. Overall, this work provides new insights into p_knot beyond previous studies, in particular, unifying the effect of the knot size on p_knot and the effect of the polymer length on p_knot. The results can be applied to understand DNA knotting, considering that the effective L_p/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

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

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

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

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

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

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

The phase diagram of an additional family of nickelate superconductors (Pr_1-xSr_xNiO₂) 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

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 

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/

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

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

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

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 

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

Single crystals of the laminar Fe-based superconductor Ba(Fe_0.92-xCo_0.08V_x)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

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

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: 

1. Australia 'records fastest internet speed ever' [BBC News] 2020-05-22 Technology
2. '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
3. 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
4. World's fastest internet speed from a single optical chip [Science Daily] 2020-05-22 Science News
5. Un record de débit Internet atteint par des scientifiques australiens [Radio-Canada] 2020-05-23 Techno
6. 云爾錄 :城大研超快芯片 1秒下載千套戲  [Hong Kong Economic Journal] 2020-07-06 A16 獨眼香江 獨眼香江 紀曉風
7. 城大研高速晶片傳輸速度全球最快  [Sing Tao Daily] 2020-07-06 F01 教育
8. 城大研芯片 少於1秒下載千部高清電影  [Hong Kong Economic Times] 2020-07-06 A18 港聞
9. 城大研新型芯片 一秒內下載千套戲  [Oriental Daily News] 2020-07-06 A13 港聞
10. 1秒下載千套高清電影 城大研新芯片 網速升至史上最快  [Sky Post] 2020-07-06 P08 港聞
11. 城大研新型芯片 1 秒下載千部電影  [Hong Kong Commercial Daily] 2020-07-06 A09 香港新聞
12. 城大新芯片1 秒下載千部高清戲  [Lion Rock Daily] 2020-07-06 P07 港聞
13. 城大研究高速晶片 傳輸速度全球最快 [BASTILLE POST] 2020-07-06
14. 城大研高速晶片 傳輸速度全球最快 [Headline Daily online] 2020-07-06
15. 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
16. CityU scholars develops new data chips [OpenGov Asia] 2020-07-08 Digital Transformation

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

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

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

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

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

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

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 

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 

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.

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

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

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

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

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

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

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 

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 Ti₂Nb₁₀O_29−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 Ti₂Nb₁₀O_29−x@C composites. The optimized Ti₂Nb₁₀O_29−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

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

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

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.

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

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

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

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

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

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

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