ABSTRACT
The recent discovery of polar vortex in PbTiO3/SrTiO3 (PTO/STO) superlattices opens up exciting opportunities to explore the structure and dynamics of complex topological states of ferroelectric dipoles and potentially leads to novel electronic functionalities. To obtain deep insights into the vortex formation mechanisms, we applied machine learning analysis to the atomically resolved microscopy images of PTO/STO, revealing the ground-truth vortex fine structure which suggests the contribution of flexoelectricity. Phase-field modeling was performed to systematically survey the flexocoupling effect on the system. We matched the experiment and modeling results and thereby quantified the flexocoupling coefficients of PTO and STO. On the other hand, ultrafast electric field in the form of THz laser pulses can couple with the vortices via coherent dipole interaction, providing a well-defined, easy-to-model external control knob for probing the ultrafast structural dynamics of this system. We performed a THz pump, ultrafast X-ray scattering probe study of PTO/STO based on the hard X-ray free-electron laser at the Linac Coherent Light Source. We observed a set of low-energy collective excitations of the polar vortices stemming from the lattice dipoles, and further identified a unique tunable soft vortex mode. These observations were well corroborated by multiscale simulations combining the newly developed dynamical phase-field modeling and atomistic modeling.
References:
1. Q. Li et al, Nature 2021 (in press; arXiv:2102.05746)
2. Q. Li et al, Nature Commun., 8, 1468 (2017).
BIOGRAPHY
Prof Qian Li has been an Assistant Professor with School of Materials Science and Engineering, Tsinghua University since 2020, following the award of a Chinese Oversea Youth Talent grant. He obtained his Ph.D. in solid-state chemistry from the Australian National University in 2014. He then had postdoc positions at Oak Ridge National Laboratory and Argonne National Laboratory during 2014-2019. Prof. Qian Li has published more than 70 papers, including first-author papers in Nature, Nature Commun. Nano Lett., etc. His past research has been mainly centered on ferroelectric materials, with a strong emphasis on characterizations using X-ray/neutron scattering and scanning probe microscopy methods. More recently, his group at Tsinghua starts to delve into epitaxial thin-film growth and ultrafast optical spectroscopies.
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