General Physics Colloquium, Professor David Vanderbilt, Rutgers University

Title:  Theory of Adiabatic Phonon-Magnon Coupling in Magnetic Materials

Host:   Cyrus Umrigar

Abstract:   Conventional approaches for computing lattice dynamics do not fully account for the effects of time-reversal symmetry breaking in magnetic systems.  Recent approaches to rectify this involve incorporating the first-order force on each atom induced not only by the displacements, but also by the velocities, of other nearby atoms.  The latter “emergent Lorentz forces” arise as the leading-order terms in adiabatic perturbation theory, and turn out to be governed by nothing other than the Berry curvature of the electronic ground state with respect to atomic displacements. Because the spin degrees of freedom (“magnons”) evolve on a time scale that is similar to that of the lattice vibrations (“phonons”), a valid theory has to treat both on an equal footing. I will describe the theory of these effects, and present some representative results on CrI₃ and Cr₂O₃ as paradigmatic systems.