Dresden 2006 – scientific programme

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TT: Tiefe Temperaturen

TT 25: Correlated Electrons - Poster Session

TT 25.61: Poster

Wednesday, March 29, 2006, 14:30–18:30, P1

Magnetocaloric effect and Grüneisen parameter of thespin-gap system TlCuCl₃ — •S. Stark¹, N. Johannsen¹, T. Zabel¹, O. Heyer¹, A. Oosawa², H. Tanaka³, A. Vasiliev⁴, and T. Lorenz¹ — ¹II. Physikalisches Institut, Universität zuKöln, Germany — ²Advanced Science Research Center,Japan Atomic Energy Research Institute, Japan — ³Dep. ofPhysics, Tokyo Institute of Technology, Japan — ⁴LowTemp. Physics Dep., Moscow State University, Russia

TlCuCl₃ is a S = 1/2 quantum system with a nonmagnetic singlet ground state and a small energy gap to the excited triplet states. A magnetic field H > 6 T induces 3D antiferromagnetic order with a staggered magnetization perpendicular to the applied field. This transition can be described by a Bose-Einstein condensation of magnons and represents an example for a field-induced quantum phase transition. We present a study of the magnetocaloric effect, the thermal expansion α, the specific heat c_p and the magnetostriction. There exist clear predictions for the behavior of these quantities near a quantum critical point [1]. The differential magnetocaloric effect θ = dT/dB and the Grüneisen parameter Γ = α/c_p are very convenient to determine a possible quantum critical behavior of TlCuCl₃, since one anticipates that both Γ and θ/T diverge (∼ 1/T) for T → 0 and B → B_c. Using Ehrenfest relations we also derive the uniaxial pressure dependence of the phase boundary [2]. We performed our measurements down to 0.3 K and compare our results to the existing theory.

[1] L. Zhu et al., Phys. Rev. Lett. 91 (2003) 066404.

[2] N. Johannsen et al., Phys. Rev. Lett. 95 (2005) 017205.

This work was supported by the DFG through SFB 608