Gregory Fiete, Northeastern: Nonlinear optical probing & control magnetic & electronic band topology

Gregory Fiete, Northeastern University, USA: Nonlinear optical probing and control of magnetic and electronic band topology
Illuminating a material with light can reveal both interesting aspects of electronic and lattice degrees of freedom, as well as drive phase and topological transitions in the material itself. In this talk, I will focus on two distinct responses of a material to light: (1) The non-linear photogalvanic response of Weyl semimetals with tilted cones and chiral charge up to 4 (the largest allowed in a lattice model), as well as the topological superconductor candidate 4Hb-TaS2, and (2) The coupling of phonons to electronic degrees of freedom to produce chiral phonons with large g-factors of order 1, which can be measured with Raman scattering. In the case of the non-linear current response of Weyl systems, I will review how the quantum geometry-and quantum metric in particular-produces a quantized response proportional to the chiral charge of the Weyl node. I will compare the predictions of low-energy theories with the full band structure for few and multi-band systems, and present new analytical results for chiral charge 4. For the case of a superconductor, I will show how the second order optical response can be used to distinguish topological phases of matter. In the case of chiral phonons, I will present a theory for the “giant” g-factors in insulating transition metal oxides based on atomic transitions of the transition metal ions. For both topics, I will review relevant experimental results before turning to the theory.