Thermodynamic properties of the field-induced Neel order of TlCuCl3
  TlCuCl3 shows a quantum phase transition from a spin-gap phase to a Neel-ordered ground state as a function of magnetic field around H_{C0}~4.8T.   From measurements of the specific heat c and the linear thermal expansion alpha_i we calculate the Grueneisen parameter Gamma_i(T)=alpha_i/c. Close to H_{C0} we find a diverging Gamma_i(T->0, in qualitative agreement with theoretical predictions. However, the predicted individual temperature dependencies of alpha(T) and c(T) are not reproduced by our experimental data. 
S. Stark, O. Heyer, A. Vasiliev, A. Oosawa, H. Tanaka, T. Lorenz