Friday, February 15, 2019
In a crystal, the “twisting” of electronic wave functions in momentum space – as encoded in the Bloch band Berry curvature gives rise to a wealth of interesting “anomalous” behaviors that typify a wide new range of quantum materials. An emerging theme is how quantum geometry enables coupling between electric and magnetic degrees of freedom. I will discuss how coupled charge and magnetic degrees of freedom can conspire to produce a variety of unusual transport phenomena.
A particularly striking example occurs in gapped graphene (a nominally topologically trivial “vanilla” insulator). In vanilla/conventional insulators, carrier transport is expected to be exponentially activated at small but finite temperatures, leading to a severely muted current response when an electric field is applied. I will argue that this expectation fails in gapped graphene (with finite sample size) where bulk free carriers in valleys with non-vanishing Berry curvature give rise to low-dissipation edge currents, which are squeezed within a distance of the order of the valley diffusion length from the edge. This happens even in the absence of edge states [topological (gapless) or otherwise], and when the bulk equilibrium carrier concentration is thermally activated across the gap. Instead, this behavior arises from the unusual coupling between charge and magnetic degrees of freedom afforded by Berry curvature. If time permits, I will also discuss other recent examples of unexpected transport phenomena enabled by Bloch band quantum geometry, such as a geometric phase “proximity effect” in a trivial band (found in recent experiments on trilayer graphene)
Javier D. Sanchez-Yamagishi