Cascade of transitions in twisted and non-twisted graphene layers within the van Hove scenario

TL;DR

This paper investigates the sequence of symmetry-breaking phase transitions in twisted and non-twisted graphene layers near van Hove singularities, explaining experimental observations through an SU(4) symmetry framework.

Contribution

It introduces a unified SU(4)-based theoretical model to explain the cascade of phase transitions observed in twisted and non-twisted graphene layers near van Hove points.

Findings

Symmetry-breaking patterns change with filling in graphene layers.

Each spin/isospin order splits the van Hove peak uniquely.

The model aligns with STM and compressibility experimental data.

Abstract

Motivated by measurements of compressibility and STM spectra in twisted bilayer graphene, we analyze the pattern of symmetry breaking for itinerant fermions near a van Hove singularity. Making use of an approximate SU(4) symmetry of the Landau functional, we show that the structure of the spin/isospin order parameter changes with increasing filling via a cascade of transitions. We compute the feedback from different spin/isospin orders on fermions and argue that each order splits the initially 4-fold degenerate van Hove peak in a particular fashion, consistent with the STM data and compressibility measurements, providing a unified interpretation of the cascade of transitions in twisted bilayer graphene. Our results follow from a generic analysis of an SU(4)-symmetric Landau functional and are valid beyond a specific underlying fermionic model. We argue that an analogous van Hove scenario explains the cascade of phase transitions in non-twisted Bernal bilayer and rhombohedral trilayer graphene.