Shift-current response as a probe of quantum geometry and electron-electron interactions in twisted bilayer graphene

TL;DR

This paper demonstrates that the shift-current nonlinear optical response in twisted bilayer graphene reveals insights into its quantum geometry and electron-electron interactions, providing a new optical probe for moiré materials.

Contribution

It introduces the shift-current as a sensitive probe of quantum geometry and interactions in TBG, analyzing its dependence on twist angle, doping, and electron-electron interactions.

Findings

Electron-electron interactions modify the shift-current response.

Interactions enhance response at characteristic frequencies and create new resonances.

Optical response reflects the geometric and interaction-driven properties of TBG.

Abstract

Moir\'e materials, and in particular twisted bilayer graphene (TBG), exhibit a range of fascinating phenomena, that emerge from the interplay of band topology and interactions. We show that the non-linear second-order photoresponse is an appealing probe of this rich interplay. A dominant part of the photoresponse is the shift-current, which is determined by the geometry of the electronic wavefunctions and carrier properties, and thus becomes strongly modified by electron-electron interactions. We analyze its dependence on the twist angle and doping, and investigate the role of interactions. In the absence of interactions, the response of the system is dictated by two energy scales: the mean energy of direct transitions between the hole and electron flat bands, and the gap between flat and dispersive bands. Including electron-electron interactions, both enhance the response at the non-interacting characteristic frequencies as well as produce new resonances. We attribute these changes to the filling-dependent band renormalization in TBG. Our results highlight the connection between non-trivial geometric properties of TBG and its optical response, as well as demonstrate how optical probes can access the role of interactions in moi\'re materials.