Triangular-lattice anisotropic dimerized Heisenberg antiferromagnet: Stability and excitations of the quantum paramagnetic phase

TL;DR

This paper investigates the stability and excitations of the quantum paramagnetic phase in a dimerized, anisotropic triangular-lattice Heisenberg antiferromagnet, revealing a quantum Lifshitz point and potential spin-liquid states.

Contribution

It applies bond-operator theory to map the phase diagram and excitation spectrum, identifying a quantum Lifshitz point and analyzing triplon decay in a model relevant to organic Mott insulators.

Findings

Identification of a quantum Lifshitz point at the commensurate-incommensurate transition.

Determination of the triplon excitation spectrum and its wavevector dependence.

Discussion of high-energy triplon decay and relevance to organic Mott insulators.

Abstract

Motivated by experiments on non-magnetic triangular-lattice Mott insulators, we study one candidate paramagnetic phase, the columnar dimer (or valence-bond) phase. We apply variants of the bond-operator theory to a dimerized and spatially anisotropic spin-1/2 Heisenberg model and determine its zero-temperature phase diagram and the spectrum of elementary triplet excitations (triplons). Depending on model parameters, we find that the minimum of the triplon energy is located at either a commensurate or an incommensurate wavevector. Condensation of triplons at this commensurate-incommensurate transition defines a quantum Lifshitz point, with effective dimensional reduction which possibly leads to non-trivial paramagnetic (e.g. spin-liquid) states near the closing of the triplet gap. We also discuss the two-particle decay of high-energy triplons, and we comment on the relevance of our results for the organic Mott insulator EtMe3P[Pd(dmit)2]2.