Character of magnetic excitations in a quasi-one-dimensional   antiferromagnet near the quantum critical points: Impact on magneto-acoustic   properties
  We report results of magneto-acoustic studies in the quantum spin-chain magnet NiCl$_2$-4SC(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 3D bosonic character of the magnetic excitations in DTN in the vicinity of the QCPs. 
O. Chiatti, A. Sytcheva, J. Wosnitza, S. Zherlitsyn, A. A. Zvyagin, V.   S. Zapf, M. Jaime, and A. Paduan-Filho