Terahertz photocurrent probe of quantum geometry and interactions in magic-angle twisted bilayer graphene

TL;DR

This paper uses terahertz photocurrent measurements to explore the quantum geometry and electron interactions in magic-angle twisted bilayer graphene, revealing new states and behaviors undetectable by traditional methods.

Contribution

It introduces polarization-resolved terahertz photocurrent as a novel probe for quantum geometry and interactions in moiré materials, uncovering hidden states and interaction effects.

Findings

Detection of inversion-breaking gapped states

Observation of polarization axis changes due to band renormalization

Recurring photocurrent patterns tracking quantum geometry evolution

Abstract

Moir\'e materials represent strongly interacting electron systems bridging topological and correlated physics. Despite significant advances, decoding wavefunction properties underlying the quantum geometry remains challenging. Here, we utilize polarization-resolved photocurrent measurements to probe magic-angle twisted bilayer graphene, leveraging its sensitivity to the Berry connection that encompasses quantum "textures" of electron wavefunctions. Using terahertz light resonant with optical transitions of its flat bands, we observe bulk photocurrents driven by broken symmetries and reveal the interplay between electron interactions and quantum geometry. We observe inversion-breaking gapped states undetectable through quantum transport, sharp changes in the polarization axes caused by interaction-induced band renormalization, and recurring photocurrent patterns at integer fillings of the moir\'e unit cell that track the evolution of quantum geometry through the cascade of phase transitions. The large and tunable terahertz response intrinsic to flat-band systems offers direct insights into the quantum geometry of interacting electrons and paves the way for innovative terahertz quantum technologies.