Dresden 2011 – scientific programme

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TT: Fachverband Tiefe Temperaturen

TT 44: Poster Session Correlated Electrons

TT 44.29: Poster

Thursday, March 17, 2011, 10:00–13:00, P1

Grüneisen ratios at the magnetic-field-induced quantum phase transition in NiCl₂-4SC(NH₂)₂ — •Manuel Brando¹, Robert Kuechler¹, Luis Pedrero¹, Alexander Steppke¹, Armando Paduan-Filho², Christian Batista³, Franziska Weickert³, Vivien Zapf³, Marcelo Jaime³, and Frank Steglich¹ — ¹Max-Planck-Institut für Chemische Physik fester Stoffe, Nöthnitzer Str. 40, 01187 Dresden, Germany — ²Instituto de Fisica, Universidade de Sao Paulo, Sao Paulo, Brazil — ³Los Alamos National Laboratory, Los Alamos, USA

The tetragonal organic system NiCl₂-4SC(NH₂)₂ (known as DTN) is a quantum paramagnet where the Ni atoms form S=1 dimers which are weakly coupled along the crystallographic c axis. The Ni²⁺ single ion anisotropy D = 8.9 K opens an energy gap between the S_z = 0 ground state and the S_z = ± 1 first excited state. When a magnetic field H is applied along c, it closes the gap and induces a transition into an XY-antiferromagnetic ordered state at low temperatures. For T=0 the critical field Hc₁ ≈ 2 T. Such a quantum critical point belongs to the d=3 and z=2 universality class and the following theoretical laws are predicted: The magnetization M∝ T^d/z, the phase boundary line (H−Hc₁)∝ T_c^d/z, the thermal and magnetic Grüneisen ratios, Γ and Γ_H, are expected to follow T⁻¹. We present measurements of the magnetization, specific heat and thermal expansion in the temperature range 0.05 ≤ T ≤ 5 K across Hc₁ and analyse the critical behavior of Γ and Γ_H at Hc₁.