Quantum thermodynamics of easy-plane ferromagnetic chains

A classical effective Hamiltonian for the quantum easy-plane ferromagnetic chain with an in-plane applied field is introduced by means of a path-integral variational method. It then becomes possible to include quantum effects of the out-of-plane fluctuations in an easily valuable phase-space integral which approximates the quantum partition function. A comparison with experimental data for the excess specific heat of CsNiF3 shows much improvement in the description of this system. In addition, new light is shed on the old problem of defining the classical counterpart of a spin Hamiltonian.