Song Chien Wen, Justin
Date: Thursday, Apr. 4, 2019
Time: 14:00
Place: ETH Zurich, Hönggerberg, HPF G 6
Host: Ataç İmamoğlu
Squeezed edge currents without edge states
Justin Song Chien Wen
Nanyang Technological University, Singapore
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 Dirac systems (a nominally topologically trivial “vanilla” insulator), such as gapped graphene, gapped bilayer graphene and transition metal dichalcogenides (TMDs). 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 a new direction in our group in engineering the bandstructure of TMDs and other van der Waals heterostructures. These heterostrcutures possess a large range of tunability and can even realize relatively flatbands that serve as an unconventional (vdW) venue to realize interacting behavior.