Condensed Matter/Materials & Biological Physics Seminar with Yafei Ren on Geometric and nonlinear aspects of electron-phonon interactions

The interaction between electrons and phonons can significantly influence both the phononic and magnetic properties of materials. On the one hand, the time-reversal symmetry breaking of the electronic system generates nonlocal effective magnetic fields on lattice dynamics, manifested as molecular Berry curvatures. This leads to chiral optical phonons at the Brillouin zone center, topological phonon bands, and anomalous thermal Hall effects. We highlight the resulting nonreciprocity of acoustic phonons in PT-symmetric antiferromagnets that are tunable by external electric fields, captured by flexo-viscosity and torques in elastic theory. On the other hand, the presence of chiral phonons breaks time-reversal symmetry of electronic systems, thereby inducing magnetization in nonmagnetic systems, with both spin and orbital contributions. We highlight the topological orbital contribution, described by a second Chern form of electronic wavefunctions in the hybrid parameter space spanned by lattice displacement and quasi-momentum. Considering both processes as a whole, the mutual interactions form a feedback loop, showing inherent nonlinearity. When the system is driven out of equilibrium, the nonlinearity can lead to bistable behaviors and discrete time crystal phases, shading light on the on-demand control of magnetism and realizing intriguing nonequilibrium phases of matter.