Moments and multiplets in moir\'e materials: A pseudo-fermion functional renormalization group for spin-valley models

TL;DR

This paper develops a pseudo-fermion functional renormalization group method for spin-valley models in moiré materials, enabling the study of complex magnetic states with higher SU(4) symmetry.

Contribution

It introduces a generalized RG approach for SU(4) spin-valley models, incorporating symmetry constraints for efficient numerical analysis of magnetic phases.

Findings

Identified a rich phase diagram with spin and valley ordered phases.

Demonstrated the method's capability on a triangular lattice model.

Enabled exploration of unconventional spin-valley liquid states.

Abstract

The observation of strongly-correlated states in moir\'e systems has renewed the conceptual interest in magnetic systems with higher SU(4) spin symmetry, e.g. to describe Mott insulators where the local moments are coupled spin-valley degrees of freedom. Here, we discuss a numerical renormalization group scheme to explore the formation of spin-valley ordered and unconventional spin-valley liquid states at zero temperature based on a pseudo-fermion representation. Our generalization of the conventional pseudo-fermion functional renormalization group approach for $\mathfrak{su}$(2) spins is capable of treating diagonal and off-diagonal couplings of generic spin-valley exchange Hamiltonians in the self-conjugate representation of the $\mathfrak{su}$(4) algebra. To achieve proper numerical efficiency, we derive a number of symmetry constraints on the flow equations that significantly limit the number of ordinary differential equations to be solved. As an example system, we investigate a diagonal SU(2)$_{\textrm{spin}}$ $\otimes$ U(1)$_{\textrm{valley}}$ model on the triangular lattice which exhibits a rich phase diagram of spin and valley ordered phases.