Title: Phonon magnetic moment and chirality: electronic geometrical phase effect
Time： 10am Nov. 22, 2021 (Beijing time)
Zoom Meeting ID：949 559 7503
Speaker: Yafei Ren (University of Washington)
The Born-Oppenheimer approximation assumes an adiabatic evolution of electronic states following ions’ motion. During the evolution, the electronic state can collect nontrivial geometrical phase. The physical consequences of the phase will be discussed in this talk. On the one hand, the presence of a phonon can perturb the electronic system [PRL 127, 186403 (2021)]. In response to a circularly polarized phonon, the geometrical phase leads to a topological magnetic moment expressed in a second Chern form. This refines the Born effective charge to be k-resolved. When a Yang's monopole presents near the parameter space of interest, the second Chern form can diverge, resulting in a large phonon magnetic moment. Topological insulators/semimetals can also have phonons with large magnetic moment. On the other hand, the electronic response can have back actions on phonon [arXiv:2110.07102]. The geometrical phase leads to a non-local effective magnetic field for ions when the electronic ground state breaks time reversal symmetry. This can lift the degeneracy of optical phonons at the Gamma point and make them circularly polarized. As a result, zero-point lattice angular momentum and phonon bands with finite Chern number can arise.
If time allows, I will quickly introduce our previous work on orbital Chern insulators [PRB 94, 085411 (2016); PRL 126, 117602 (2021)] and higher-order topological insulators [PRL 124, 166804 (2020)], which are closely related to the orbital magnetization and electric polarization, separately.
About the speaker:
Yafei Ren, Ph.D., is a postdoctoral researcher at the University of Washington. He earned both his B.S. and Ph.D. degrees from the University of Science and Technology of China and worked as a postdoc at the University of Texas at Austin. His research focuses on the topological phases induced by spin-orbit coupling and interaction, including Chern insulator, higher-order topological insulator, and three-dimensional quantum Hall effect. He has recently turned his attention to the geometrical phase effect on the low-frequency elementary excitations, such as phonons. He has published 1 Nature, 1 review article in Rep. Prog. Phys., 4 PRL, and 7 PRB papers as a (co)-first author. He received the “Special Prize of President Scholarship for Postgraduate Students (中国科学院院长特别奖，2019)” and his Ph.D. thesis was named one of the “CAS Outstanding Ph.D. Theses (中国科学院优秀博士学位论文，2020)”.