The scaling limit of the Heisenberg XXZ spin chain at zero magnetic field is studied in the gapped antiferromagnetic phase. For a spin-chain ring having Nx sites, the universal Casimir scaling function, which characterises the leading finite-size correction term in the large-Nx expansion of the ground state energy, is calculated by numerical solution of the nonlinear integral equation of the convolution type. It is shown that the same scaling function describes the temperature dependence of the free energy of the infinite XXZ chain at low enough temperatures in the gapped scaling regime.
I. INTRODUCTIONIntegrable models of statistical mechanics and field theory [1, 2] provide us with a very important source of information about the thermodynamic and dynamical properties of the magnetically ordered systems. Of particular importance is any progress in solutions of such models in the scaling region near the continuous phase transition points, since, due to the universality of critical fluctuations, it does not only yield the exact and detailed information about the model itself but also about the whole universality class it represents.In this paper we address the universal finite-size and thermodynamic properties of the anisotropic spin-1/2 XXZ chain in the massive antiferromagnetic phase in the critical region close to the quantum phase transition at the isotropic point. The Hamiltonian of the model has the form H = J 2 arXiv:1904.11324v3 [cond-mat.stat-mech]