Detecting symmetry breaking in magic angle graphene using scanning tunneling microscopy

TL;DR

This paper demonstrates how atomically-resolved scanning tunneling microscopy can identify different symmetry-breaking states in magic angle graphene, especially the Kramers intervalley coherent state, by analyzing sublattice polarization and Kekulé distortions.

Contribution

It introduces a method to use STM as a fingerprint for symmetry-breaking order parameters in magic angle graphene, linking specific distortions to theoretical states.

Findings

Kekulé distortion appears only in magnetic fields for the Kramers intervalley coherent state.

STM patterns can distinguish between different symmetry-breaking states.

The method provides a way to experimentally identify the ground state in twisted bilayer graphene.

Abstract

A growing body of experimental work suggests that magic angle twisted bilayer graphene exhibits a "cascade" of spontaneous symmetry breaking transitions, sparking interest in the potential relationship between symmetry-breaking and superconductivity. However, it has proven difficult to find experimental probes which can unambiguously identify the nature of the symmetry breaking. Here we show how atomically-resolved scanning tunneling microscopy can be used as a fingerprint of symmetry breaking order. By analyzing the pattern of sublattice polarization and "Kekul\'{e}" distortions in small magnetic fields, order parameters for each of the most competitive symmetry-breaking states can be identified. In particular, we show that the "Kramers intervalley coherent state," which theoretical work predicts to be the ground state at even integer fillings, shows a Kekul\'{e} distortion which emerges only in a magnetic field.