Qian Niu, Retired Professor, Department of Physics, UT-Austin

"Phonon magnetic moment and chirality: electronic geometrical phase effect"

Abstract: 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 [arXiv: 2103.05786]. 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 [To be submitted]. The geometrical phase leads to a non-local effective magnetic field for ionswhen 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.