Event

In the last few years, nonlinear responses have become increasingly important for the study and characterization of quantum materials, with the nonlinear anomalous Hall conductivity and the bulk photovoltaic effect being particularly successful examples.  At the same time, from a theory perspective nonlinear response is challenging for two reasons: Firstly, the corresponding expressions are often unwieldy, and secondly many of the physical intuitions acquired at linear order seem to fail beyond linear order. Hence, for decades several key aspects of nonlinear transport have remained controversial.

In this talk, I will present a new framework which connects the concepts of anomalous motion, coherent interband transitions, and wavepacket deformations. I show in which cases the Green-Kubo formalism can be continued to zero frequency to recover the Boltzmann kinetic approach and discuss two novel phenomena in the nonlinear conductivity: The appearance of a mixed axial-gravitational anomaly in magnetic metals and a nonlinear quantum anomalous Hall effect in certain magnetic insulators. [2208.00827,2209.09531]