Dynamics of anisotropic frustrated antiferromagnet Cs2CoBr4 in a spin-liquid regime

TL;DR

This study investigates the spin dynamics of the anisotropic frustrated antiferromagnet Cs2CoBr4 in a spin-liquid regime using ESR, revealing unique excitation modes that differ from ordered and paramagnetic phases.

Contribution

The paper provides the first ESR analysis of Cs2CoBr4 in a spin-liquid regime, identifying distinct excitation branches related to anisotropic pseudospin-1/2 chains.

Findings

Identification of two bright excitation branches in ESR spectra

Observation of weak satellite modes of unknown origin

Distinct spin dynamics compared to ordered and paramagnetic phases

Abstract

Cs2CoBr4 is a triangular-lattice antiferromagnet which can be viewed as weakly interacting spin chains due to spatially anisotropic frustrating exchange couplings. The spin-orbit interaction in Co(2+) spin-3/2 ions leads to a strong easy-plane single-ion anisotropy which allows to consider the low-energy spin dynamics of this system using an anisotropic pseudospin-1/2 model. By means of the electron spin resonance (ESR) technique, we study the spin dynamics of Cs2CoBr4 in magnetic field in a spin-liquid regime, i.e., above the N'eel temperature of 1.3 K but below the temperature of the crossover to in-chain correlations of pseudospins (6 K). Our experiments reveal two bright branches of excitations which strongly differ both from excitations in the low-temperature ordered phases and from high-temperature paramagnetic resonance of uncorrelated pseudospins and spins. These two branches are interpreted as excitations with zero momentum of an anisotropic spin-1/2 chain. Besides, we observe several weak modes of unknown origin which arise mostly as satellites of one of the bright modes.