Acoustic study of the low-dimensional quantum antiferromagnet Cs₂CuCl₄

We report on results of acoustic measurements in the triangular-lattice low-dimensional spin-1/2 antiferromagnet (AFM) Cs2CuCl4 (TN = 0.6 K) at high magnetic fields. At low temperature, the magnetic ordering in this compound has been the subject of intensive experimental and theoretical studies. This is caused by the possible occurrence of a Bose-Einstein condensation of magnons in the AFM phase and the proximity to a spin-liquid state beyond it. The Cu2+ in Cs2CuCl4 are arranged in planar triangular lattices within the bc plane. The magnetic inter-plane interaction along the a axis is much weaker than the in-plane interaction. Therefore, the material can be regarded as magnetically low dimensional. We measured the sound velocity and attenuation of the longitudinal acoustic c22 mode, i.e., the sound wave propagating along the b axis, in fields up to 15 T applied along the same direction and for temperatures between 0.3 and 7 K. We found that below 1.5 K this mode softens with increasing magnetic field, whereas in the vicinity of the critical field, at about 8.9 T, the sound velocity increases again. Near this field, the sound attenuation attains a maximum and both acoustic characteristics depend strongly on frequency. We analyzed our results in the framework of an effective quantum theory based on the exchange-striction coupling. The theoretical results agree qualitatively with our data.