Itinerant fermions on a triangular lattice: unconventional magnetism and other ordered states

TL;DR

This paper investigates the complex magnetic and ordered states of 2D fermions on a triangular lattice, revealing degenerate magnetic orders, their lifting beyond mean-field, and the interplay with superconductivity and charge order through renormalization group analysis.

Contribution

It introduces a detailed analysis of magnetic degeneracies, their lifting beyond mean-field, and the coupling of magnetic and bond orders, along with the RG flow of interactions affecting competing phases.

Findings

SDW order is degenerate at mean-field level.

Beyond mean-field, degeneracy is lifted, favoring specific magnetic orders.

System can develop superconductivity or unconventional charge order depending on parameters.

Abstract

We consider a system of 2D fermions on a triangular lattice with well separated electron and hole pockets of similar sizes, centered at certain high-symmetry-points in the Brillouin zone. We first analyze Stoner-type spin-density-wave (SDW) magnetism. We show that SDW order is degenerate at the mean-field level. Beyond mean-field, the degeneracy is lifted and is either $120^{\circ}$ "triangular" order (same as for localized spins), or a collinear order with antiferromagnetic spin arrangement on two-thirds of sites, and non-magnetic on the rest of sites. We also study a time-reversal symmetric directional spin bond order, which emerges when some interactions are repulsive and some are attractive. We show that this order is also degenerate at a mean-field level, but beyond mean-field the degeneracy is again lifted. We next consider the evolution of a magnetic order in a magnetic field starting from an SDW state in zero field. We show that a field gives rise to a canting of an SDW spin configuration. In addition, it necessarily triggers the directional bond order, which, we argue, is linearly coupled to the SDW order in a finite field. We derive the corresponding term in the Free energy. Finally, we consider the interplay between an SDW order and superconductivity and charge order. For this, we analyze the flow of the couplings within parquet renormalization group (pRG) scheme. We show that magnetism wins if all interactions are repulsive and there is little energy space for pRG to develop. However, if system parameters are such that pRG runs over a wide range of energies, the system may develop either superconductivity or an unconventional charge order, which breaks time-reversal symmetry.