Terahertz Photogalvanics in Twisted Bilayer Graphene Close to the Second Magic Angle

Abstract

We report on the observation of photogalvanic effects in tBLG with a twist angle of 0.6 degrees. We show that excitation of the tBLG bulk causes a photocurrent, whose sign and magnitude are controlled by the 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 correlate with peaks in resistance measurements. These are well explained by interband transitions between a multitude of isolated bands in tBLG.