Thermodynamic properties of the field-induced Néel order of TlCuCl3

Abstract TlCuCl 3 shows a quantum phase transition from a spin-gap phase to a Neel-ordered ground state as a function of magnetic field around H C 0 ≃ 4.8 T . From measurements of the specific heat c and the linear thermal expansion α i we calculate the Gruneisen parameter Γ i ( T ) = α i / c . Close to H C 0 we find a diverging Γ i ( T → 0 ) , in qualitative agreement with theoretical predictions. However, the predicted individual temperature dependencies of α ( T ) and c ( T ) are not reproduced by our experimental data.