Thermodynamics of the coupled spin-dimer system TlCuCl3 close to a   quantum phase transition
  We present thermal expansion alpha, magnetostriction and specific heat C measurements of \tal, which 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. Using Ehrenfest's relation, we find huge pressure dependencies of the spin gap for uniaxial as well as for hydrostatic pressure. For T->0 and H->H_{C0} we observe a diverging Grueneisen parameter Gamma(T)=alpha/C, in qualitative agreement with theoretical predictions. However, the predicted individual temperature dependencies alpha(T) and C(T) are not reproduced by our experimental data. 
T. Lorenz, S. Stark, O. Heyer, N. Hollmann, A. Vasiliev, A. Oosawa, H.   Tanaka