Atomic motion with broken time-reversal symmetry

Abstract:
We group materials into five symmetry classes and determine in which of these classes atomic vibrations (phonons) carry angular momentum in the Brillouin zone, away from a high-symmetry point, line, or plane. In some materials, phonons acquire angular momentum via the forces induced by relative displacements of atoms out of their equilibrium positions. However, for other materials, such as ferromagnetic iron or CrI3, phonon angular momentum arises from the forces induced by relative velocities of atoms. These effects are driven by the spin-orbit interaction. I will present a first-principles approach to computing the effect of time-reversal symmetry breaking on phonon dynamics in a ferromagnet such as iron. While these effects are small in magnitude in comparison with the non-time-reversal effects included in the force-constant matrix, they are dominant terms for all physical properties where ionic motion requires time-reversal symmetry breaking (such as the Einstein-de Haas effect). Given these findings, I will also discuss how phonon angular momentum might give us new routes to the electric field control of magnetism.  Given time, at the end of my talk, I will place these findings in a broader context of physical properties that rely solely on the relative phase between electronic wavefunctions, as well as in the even broader context of novel approaches to the high-throughput calculations in computational materials science.

Bio:
Sinisa Coh received his Ph.D. in Physics from Rutgers University in 2011.  Upon completion of his Ph.D. Sinisa joined the University of California Berkeley and Lawrence Berkeley Lab as a postdoc in physics and materials science.  Since 2016 Sinisa has been a professor at the University of California, Riverside in Materials Science and Mechanical Engineering.  Sinisa's research group in computational materials science at the University of California, Riverside works on nanostructured materials, complex oxides, layered materials, magnetic materials, topological insulators, superconductors, and optical properties of real materials.  We design, discover, and characterize functional materials using state-of-the-art computational techniques, symmetry, nearsightedness principle, geometry, and topology.