Thermodynamic properties of the spin-1/2 antiferromagnetic ladder Cu-2(C2H12N2)(2)Cl-14 under magnetic field

Specific heat (Cv) measurements in the spin-1/2 Cu-2(C2H12N2)(2)Cl-4 system under a magnetic field up to H = 8.25 T are reported and compared to the r esults of numerical calculations based on the 2-leg antiferromagnetic Heise nberg ladder. While the temperature dependences of both the susceptibility and the low-field specific heat are accurately reproduced by this model, de viations are observed above the critical field H-Cl at which the spin gap c loses. In this Quantum High Field phase, the contribution of the low-energy quantum fluctuations are stronger than in the Heisenberg ladder model. We argue that this enhancement can be attributed to dynamical lattice fluctuat ions. Finally; we show that such a Heisenberg ladder, for H > H-Cl, is unst able, when coupled to the 3D lattice, against a lattice distortion. These r esults provide an alternative explanation for the observed low temperature (T-C similar to 0.5 K-0.8 K) phase (previously interpreted as a 3D magnetic ordering) as a new type of incommensurate gapped state.