Dimer description of the SU(4) antiferromagnet on the triangular lattice

TL;DR

This paper investigates the SU(4) antiferromagnetic model on a triangular lattice, revealing a valence bond solid ground state and exploring the effects of symmetry-breaking interactions, relevant to twisted multilayer graphene.

Contribution

It introduces a dimer expansion and numerical analysis of the SU(4) model, uncovering a VBS ground state and potential phase transitions due to Hund's coupling.

Findings

Ground state is a valence bond solid with 12-site unit cell.

Numerical DMRG suggests a non-magnetic ground state.

Effective dimer model indicates translation symmetry breaking.

Abstract

In systems with many local degrees of freedom, high-symmetry points in the phase diagram can provide an important starting point for the investigation of their properties throughout the phase diagram. In systems with both spin and orbital (or valley) degrees of freedom such a starting point gives rise to SU(4)-symmetric models. Here we consider SU(4)-symmetric "spin" models, corresponding to Mott phases at half-filling, i.e. the six-dimensional representation of SU(4). This may be relevant to twisted multilayer graphene. In particular, we study the SU(4) antiferromagnetic "Heisenberg" model on the triangular lattice, both in the classical limit and in the quantum regime. Carrying out a numerical study using the density matrix renormalization group (DMRG), we argue that the ground state is non-magnetic. We then derive a dimer expansion of the SU(4) spin model. An exact diagonalization (ED) study of the effective dimer model suggests that the ground state breaks translation invariance, forming a valence bond solid (VBS) with a 12-site unit cell. Finally, we consider the effect of SU(4)-symmetry breaking interactions due to Hund's coupling, and argue for a possible phase transition between a VBS and a magnetically ordered state.