Eur. Phys. J. B 7, 171-174 (1999)
https://doi.org/10.1007/s100510050600

Thermodynamic properties of the spin-1/2 antiferromagnetic ladder Cu₂(C₂H₁₂N₂)₂Cl₄ under magnetic field

¹ DRFMC, Service de Physique Statistique, Magnétisme et Supraconductivité, CENG, 38054 Grenoble Cedex 9, France
² Laboratoire de Physique Quantique (UMR-CNRS 5626) , Université Paul Sabatier, 31062 Toulouse, France
³ Instituto de Física Rosario y Departamento de Física, Universidad Nacional de Rosario, 2000-Rosario, Argentina
⁴ Laboratoire de Spectrométrie Physique, Université Joseph Fourier, B.P. 87, 38402 St Martin d'Hères, France
⁵ Grenoble High Magnetic Field Laboratory, CNRS and MPI-FKF, B.P. 166, 38042 Grenoble, France
⁶ Dipartimento di Chimica, Università di Perugia, 06100 Perugia, Italy

Received: 23 July 1998
Accepted: 24 August 1998
Published online: 15 January 1999

Abstract

Specific heat (C_V) measurements in the spin-1/2 Cu₂(C₂H₁₂N₂)₂Cl₄ system under a magnetic field up to H=8.25 T are reported and compared to the results of numerical calculations based on the 2-leg antiferromagnetic Heisenberg ladder. While the temperature dependences of both the susceptibility and the low-field specific heat are accurately reproduced by this model, deviations are observed above the critical field H_C1 at which the spin gap closes. 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 fluctuations. Finally, we show that such a Heisenberg ladder, for H > H_C1, is unstable, when coupled to the 3D lattice, against a lattice distortion. These results provide an alternative explanation for the observed low temperature (T_C ~ 0.5 K-0.8 K) phase (previously interpreted as a 3D magnetic ordering) as a new type of incommensurate gapped state.