Current Switching of Valley Polarization in Twisted Bilayer Graphene

TL;DR

This paper investigates how a DC current can switch valley polarization in twisted bilayer graphene aligned with h-BN, revealing a mechanism involving valley density differences and inter-valley scattering, especially near the magic angle.

Contribution

It introduces a microscopic model explaining current-induced valley polarization switching in TBG/h-BN, emphasizing the role of valley density differences and inter-valley scattering.

Findings

Current can induce valley density differences in TBG/h-BN.

Valley density differences can trigger first-order polarization transitions.

Effect is significantly enhanced near the magic angle.

Abstract

Twisted bilayer graphene (TBG) aligned with hexagonal boron nitride (h-BN) substrate can exhibit an anomalous Hall effect at 3/4 filling due to the spontaneous valley polarization in valley resolved moir\'e bands with opposite Chern number [Science 367, 900 (2020), Science 365, 605 (2019)]. It was observed that a small DC current is able to switch the valley polarization and reverse the sign of the Hall conductance [Science 367, 900 (2020), Science 365, 605 (2019)]. Here, we discuss the mechanism of the current switching of valley polarization near the transition temperature, where bulk dissipative transport dominates. We show that for a sample with rotational symmetry breaking, a DC current may generate an electron density difference between the two valleys (valley density difference). The current induced valley density difference in turn induces a first order transition in the valley polarization. We emphasize that the inter-valley scattering plays a central role since it is the channel for exchanging electrons between the two valleys. We further estimate the valley density difference in the TBG/h-BN system with a microscopic model, and find a significant enhancement of the effect in the magic angle regime.