Magnetic entropy landscape and Grüneisen parameter of a quantum spin ladder

We present measurements of the magnetic entropy landscape and Grüneisen parameter of the Cu2+ complex (C5 H12 N)2 CuCl4 in a magnetic field. Our thermodynamic measurements are in very good agreement with a theoretical description by a S = 1/2 Heisenberg ladder model. Due to its excellent experimental access, the compound crosses two quantum critical points in the applied range of the magnetic field, first from a gapped unpolarized state to a critical phase and then to a gapped fully polarized state. This behavior is reflected directly in the magnetic entropy map. Due to the remarkable properties of the magnetic Gru ̈neisen parameter, we are able to discuss the validity of critical behavior with respect to temperature and magnetic field for this model quasi-one-dimensional system