Character of magnetic excitations in a quasi-one-dimensional antiferromagnet near the quantum critical points: Impact on magnetoacoustic properties

Abstract

We report results of magnetoacoustic studies in the quantum spin-chain magnet ${\text{NiCl}}_{2}\text{\ensuremath{-}}4\text{SC}{({\text{NH}}_{2})}_{2}$ (DTN) having a field-induced ordered antiferromagnetic (AF) phase. In the vicinity of the quantum critical points (QCPs) the acoustic ${c}_{33}$ mode manifests a pronounced softening accompanied by energy dissipation of the sound wave. The acoustic anomalies are traced up to $T>{T}_{N}$, where the thermodynamic properties are determined by fermionic magnetic excitations, the ``hallmark'' of one-dimensional (1D) spin chains. On the other hand, as established in earlier studies, the AF phase in DTN is governed by bosonic magnetic excitations. Our results suggest the presence of a crossover from a 1D fermionic to a three-dimensional bosonic character of the magnetic excitations in DTN in the vicinity of the QCPs.