Colloquium: Dr. Yafei Ren | University of Delaware

ABSTRACT: The interaction between phonons and magnetization can significantly influence both the phononic and magnetic properties of materials. Chiral phonons can induce magnetization by generating an effective magnetic field on electrons. Reciprocally, a nonzero magnetization can bestow chirality upon phonons via an effective magnetic field on phonons. The magnetization has orbital and spin contributions. The first half of this talk focuses on the interplay between phonons and orbital magnetization, closely tied to the electronic geometrical phases. We highlight chiral phonon-induced orbital magnetization, described by a topological second Chern form. We further explore its reciprocity, revealing how orbital magnetization endows optical phonons with chirality. Those geometrical phase effects are overlooked in the traditional Born-Oppenheimer approximation. 

This reciprocity extends to phonon-spin-coupled systems, which is the focus of the latter half. Here, we reveal the inherent nonlinearity of the reciprocal processes. Using a paramagnet as an illustration, we demonstrate the nonlinearity by unveiling spontaneous symmetry breaking under a periodic drive. The bistable behaviors and discrete time crystal phase shed light on the exploration of nonlinear phenomena in magnetic materials and present possibilities for on-demand control of magnetization.