Thermodynamics of the coupled spin - dimer system TlCuCl 3 close to a quantum phase transition

We present thermal expansion α, magnetostriction and specific heat C measurements of TlCuCl3, which shows a quantum phase transition from a spin-gap phase to a Néel-ordered ground state as a function of magnetic field around HC0 ≃ 4.8 T. 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 ≃ HC0 we observe a diverging Grüneisen parameter Γ(T) = α/C, in qualitative agreement with theoretical predictions. However, the predicted individual temperature dependencies α(T) and C(T) are not reproduced by our experimental data.