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TT: Fachverband Tiefe Temperaturen

TT 6: Postersession Correlated Electrons: (General) Theory, Low-Dimensional Systems, Kondo Physics, Heavy Fermions, Quantum-Critical Phenomena

TT 6.17: Poster

Monday, March 23, 2009, 13:00–16:45, P1A

Ultrasonic investigation of the quasi-2D quantum antiferromagnet Cs₂CuCl₄ — •A. Sytcheva¹, S. Zherlitsyn¹, J. Wosnitza¹, O. Chiatti¹, A. A. Zvyagin^2,3, and R. Coldea⁴ — ¹Hochfeld-Magnetlabor Dresden, Forschungszentrum Dresden-Rossendorf, Germany — ²MPl für Physik komplexer Systeme, Dresden, Germany — ³Verkin Institute for Low Temperature Physics and Engineering, Kharkov, Ukraine — ⁴Wills Physics Laboratory, University of Bristol, United Kingdom

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 long-range 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 spin-strain 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.