Global Phase Diagram of the Normal State of Twisted Bilayer Graphene

TL;DR

This paper maps the complex phase diagram of twisted bilayer graphene, revealing how various intertwined orders compete and coexist, and proposing a unified understanding of its normal state behavior based on comprehensive Hartree-Fock calculations.

Contribution

It introduces a detailed phase diagram of TBG's normal state, highlighting the prevalence of incommensurate Kekulé spiral order across doping levels and strain conditions, supported by theoretical calculations.

Findings

IKS order is ubiquitous at non-integer fillings.

The phase diagram explains experimental cascade and Landau fan phenomena.

Multiple intertwined orders coexist and compete in TBG's normal state.

Abstract

We investigate the full doping and strain-dependent phase diagram of the normal state of magic-angle twisted bilayer graphene (TBG). Using comprehensive Hartree-Fock calculations, we show that at temperatures where superconductivity is absent the global phase structure can be understood based on the competition and coexistence between three types of intertwined orders: a fully symmetric phase, spatially uniform flavor-symmetry-breaking states, and an incommensurate Kekul\'e spiral (IKS) order. For small strain, the IKS phase, recently proposed as a candidate order at all non-zero integer fillings of the moir\'e unit cell, is found to be ubiquitous for non-integer doping as well. We demonstrate that the corresponding electronic compressibility and Fermi surface structure are consistent with the 'cascade' physics and Landau fans observed experimentally. This suggests a unified picture of the phase diagram of TBG in terms of IKS order.