ANALYTIC STUDY OF O(3) NONLINEAR SIGMA-MODEL

With the variational-cumulant expansion method, the internal energy an d the specific heat of O(3) nonlinear sigma model is calculated up to the fourth order. The variational parameter is determined by the varia tional method of first order free energy and accumulation point method respectively. It is shown that a more agreeable internal energy curve and specific heat curve with the MC results can be obtained by the la tter.