Chovpan A. Energy spectrum and thermodynamics low-dimensional magnetics with anisotropiс exchange interactions.

Object of investigation: energy spectrum and thermodynamics of low-dimensoinal magnets. Aim of investigation: to study the influence of nonhomogeneities on the spectrum and thermodynamics of low-dimensoinal magnets. Methods of investigations: Jordan-Wigner transformation, DMRG method, calculations of thermodynamical middles, static and dynamical correlation functions.Theoretical and practical results, novelty: the exact energy spectra for two 1-D spin-1/2 models are found. It is shown that localized energy levels effect strongly on local thermodynamic characteristics and their relaxation. The energy spectrum and some finite temperature properties of two-leg spin-1/2 are studied. The effective low energy spin Hamiltonians in strong inter-chain coupling limit in different longitudinal magnetic fields are derived. It is shown that for a strong ferromagnetic interchain interaction, there is a first-order transition between ferromagnetic and nonmagnetic phases. In the same limit for XY exchange in legs approximate formulae for thermodynamic characteristics are found. It is shown that the anisotropic ladder in magnetic field may have up to four low-temperature maxima in temperature dependence of specific heat. Field of application: to prognose low-temperature behaviour low-simensional spin systems.