Terahertz photogalvanics in twisted bilayer graphene close to the second magic angle

TL;DR

This study demonstrates photogalvanic effects in twisted bilayer graphene near the second magic angle, revealing symmetry reduction, gate-dependent oscillations, and edge currents driven by asymmetric scattering and inter-band transitions.

Contribution

It provides the first observation of photogalvanic effects in low-angle twisted bilayer graphene and develops a theory explaining the observed photocurrents.

Findings

Photocurrent sign and magnitude depend on radiation polarization and helicity.

Gate voltage induces oscillations in photocurrent, correlating with resistance peaks.

Edge and bulk photocurrents are explained by asymmetric scattering and inter-band transitions.

Abstract

We report on the observation of photogalvanic effects in twisted bilayer graphene (tBLG) with a twist angle of 0.6{\deg}. We show that excitation of tBLG bulk causes a photocurrent, whose sign and magnitude are controlled by orientation of the radiation electric field and the photon helicity. The observed photocurrent provides evidence for the reduction of the point group symmetry in low twist-angle tBLG to the lowest possible one. The developed theory shows that the current is formed by asymmetric scattering in gyrotropic tBLG. We also detected the photogalvanic current formed in the vicinity of the edges. For both, bulk and edge photocurrents, we demonstrate the emergence of pronounced oscillations upon variation of the gate voltage. The gate voltages associated with the oscillations coincide well with peaks in resistance measurements. These are well explained by inter-band transitions between a multitude of isolated bands in tBLG.