Magnetic injection photocurrents in valley polarized states of twisted bilayer graphene

TL;DR

This paper investigates how magnetic injection photocurrents in twisted bilayer graphene can reveal valley polarization states, providing a new method to understand its complex phase diagram and broken symmetries.

Contribution

It introduces the use of magnetic injection photocurrents as a probe for valley polarization in twisted bilayer graphene, linking experimental observations with theoretical modeling.

Findings

Magnetic injection photocurrents depend on valley polarization states.

Theoretical calculations match experimental filling-dependent photocurrent behavior.

Proposes using photocurrent measurements to identify valley polarization scenarios.

Abstract

Magic-angle twisted bilayer graphene displays a complex phase diagram as a function of flat band filling, featuring compressibility cascade transitions and a variety of competing ground states with broken spin, valley and point group symmetries. Recent THz photocurrent spectroscopy experiments have shown a dependence on the filling which is not consistent with the simplest cascade picture of sequential filling of equivalent flat bands. In this work, we show that when time-reversal symmetry is broken due to valley polarization, a magnetic injection photocurrent develops which can be used to distinguish different spin-valley polarization scenarios. Using the topological heavy fermion model we compute both shift and injection currents as a function of filling and argue that current experiments can be used to determine the spontaneous valley polarization.