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Dynamics of anisotropic frustrated antiferromagnet Cs2CoBr4 in a spin-liquid regime
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 Co2+ spin- 3
2 ions leads to a
strong easy-plane single-ion anisotropy which allows us 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 Néel 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. In addition, we
observe several weak modes of unknown origin which arise mostly as satellites of one of the bright modes.