Condensed Matter Seminar: Anomalous acceleration and quantum geometry in semimetals and flat bands

In many materials, transport properties are determined by the semiclassical velocity of the quasiparticle. In this seminar, I will talk about gapped and semimetallic materials which depart from this canonical picture and where it is necessary to account for additional components of the electronic motion which are most easily envisaged as quasiparticle accelerations. The first case is magnetotransport at the hydrodynamic-to-ballistic crossover in high-mobility graphene devices, which encompasses nonlocal correlations beyond the hydrodynamic description. There, shear- and Hall viscosity can be seen as competing accelerations, which are captured by the introduction of a viscous Hall angle. I will also present some results on the bulk photovoltaic effect, a second-order optical response in which incident light is converted into a dc-current. This current can be viewed as the result of acceleration terms which give access to many details of the band structure geometry. I discuss a new magnetogalvanic effect, a subgap photocurrent in which intraband and interband processes can be distinguished, and finally a large current response in the terahertz regime in twisted bilayer graphene.