Pulsed-Field Ultrasonic Experiments in the Quasi-2D Antiferromagnet Cs2CuBr4

The magnetic insulators Cs₂CuCl₄ and Cs₂CuBr₄ are model systems for a frustrated quasi-two-dimensional triangular-lattice spin-1/2 Heisenberg antiferromagnet with a weak interlayer coupling. The quasi-two-dimensional character manifests itself in a broad maximum at T _max in the magnetic susceptibility, which for Cs₂CuCl₄ has been found to mark the upper bound for a spin-liquid regime (R. Coldea et al. in Phys. Rev. B 68:134424, 2003). Motivated by the observation of characteristic B ² dependencies in the elastic constants and ultrasound attenuation in the spin-liquid regime for this material, we look for corresponding signatures in the related Cs₂CuBr₄ compound at low temperatures T≤4.2 K and fields B≤50 T. We observe a softening of the elastic constant up to B _s ～32 T (B∥a) and a maximum in the sound attenuation at 0.86 B _s . Both quantities exhibit the same characteristic B ² dependence as found for Cs₂CuCl₄, suggesting the existence of a spin-liquid phase in Cs₂CuBr₄ as well.