Publications Repository - Helmholtz-Zentrum Dresden-Rossendorf

We report results of sound-velocity and sound-attenuation measurements in the triangularlattice quasi-two-dimensional spin-1/2 antiferromagnet (AFM) Cs₂CuCl₄ (T_N = 0.6 K) in magnetic fields up to 18 T applied along the a axis. The possibility for a Bose-Einstein condensation of magnons in the AFM phase and a proximity to the spin liquid state at low temperatures beyond the AFM phase has been suggested for this compound. The longitudinal acoustic c₁₁ mode shows pronounced anomalies in sound velocity and sound attenuation at low temperatures and in applied magnetic field. Below 1.5 K, this mode demonstrates a softening with increasing field, followed by an increase of the sound velocity close to the saturation field, Bs approx 8.5 T. The ultrasonic results are analyzed with a theory based on exchange-striction coupling. There is good qualitative agreement between theoretical results and experiment.