Ultrasonic investigation of the quasi-2D quantum antiferromagnet Cs₂CuCl₄

We report on results of sound-velocity and sound-attenuation measurements in the triangular-lattice quasi-2D spin-1/2 antiferromagnet (AFM) Cs₂CuCl₄ (T_N = 0.6 K), in magnetic fields up to 18 T applied along the a axis and at low temperatures from 5 down to 0.3 K. Below T_N this material displays a 3D incommensurate spiral longrange AFM order, which is stable up to B_s ≈ 8.5 T for fields applied along the a axis. Above this field all spins are polarized. For the AFM phase a possibility for the Bose-Einstein condensation of magnons has been suggested whereas beyond the AFM phase at low temperatures a proximity to the spin liquid (SL) state is considered in this compound. The longitudinal c₁₁ acoustic mode, which has a propagation direction along the a axis, shows pronounced anomalies in sound velocity and attenuation in discussed temperature and field range indicating spinstrain interaction. It also demonstrates frequency-dependent effects indicating the presence of relaxation processes. The ultrasonic results are analyzed with a theory based on exchange-striction coupling. There is a good qualitative agreement between theory and experiment.