SPIN-WAVE-SPIN-WAVE INTERACTION AND THE THERMODYNAMICS OF THE HEISENBERG SPIN CHAIN

The low-temperature free energy of the spin S quantum Heisenberg ferromagnetic chain in a strong magnetic field is obtained within the two-particle approximation by using the exact solution of the two-spin-wave problem. The result is beyond perturbation theory because it incorporates both the bound and scattering state contributions, and the scattering effect is essential, as well as the bound state one. In particular, main temperature renormalization of the exchange constant is found to be temperature-linear, instead of T-3/2 dependence corresponding to the perturbation theory result.