Variational Model for Low-Dimensional Magnets

The method of nonlocal trial variation function for quantum one-dimensional systems is developed on an example of a spin-1/2XXZchain with an alternating magnetic field. A four-site trial wave function for a fermionic representation of the model is constructed. The results obtained using the model with the extended trial wave function show a considerable improvement of accuracy of the ground state energy calculation in the field of critical behavior in comparison with the solutions obtained earlier. The methods for calculating the experimentally observed spin correlation function are considered.