Exotic S=1 spin liquid state with fermionic excitations on triangular lattice

TL;DR

This paper explores a novel spin-1 quantum antiferromagnet on a triangular lattice, revealing a unique spin-liquid state with coexisting gapless and gapped excitations, relevant to recent experimental findings.

Contribution

It introduces a new fermionic triplon mean field theory for spin-1 systems, uncovering a distinct spin-liquid phase with coexisting gapless and gapped modes, and characterizes its thermodynamic properties.

Findings

Discovery of a spin-liquid state with a Fermi surface of gapless triplons.

Identification of a state with coexisting gapless and gapped excitations.

Characterization of thermodynamic signatures like specific heat and susceptibility.

Abstract

Motivated by recent experiments on the material Ba_3NiSb_2O_9 we consider a spin-one quantum antiferromagnet on a triangular lattice with the Heisenberg bilinear and biquadratic exchange interactions and a single-ion anisotropy. Using a fermionic "triplon" representation for spins, we study the phase diagram within mean field theory. In addition to a fully gapped spin-liquid ground state, we find a state where one gapless triplon mode with Fermi surface coexists with d + id topological pairing of the other triplons. Despite the existence of a Fermi surface, this ground state has fully gapped bulk spin excitations. Such a state has linear in temperature specific heat and constant in plane spin susceptibility, with an unusually high Wilson ratio.