GROUND-STATE PROPERTIES FOR AN EASY-PLANE SPIN-ONE FERROMAGNET - THE COUPLED-CLUSTER METHOD

In this paper we have investigated the ground state of an easy-plane spin-one ferromagnet: H = D-SIGMA-i(S(i)x)2 - (J/2)SIGMA(i,j)S(i)S(j) - h-SIGMA-iS(i)z, using the coupled-cluster method. The mean-field approximation is commonly used in calculations of thermodynamic quantities for such systems because of the complexities caused by the single-ion anisotropy term. However, in the mean-field approximation both quantum and spin fluctuation correlations are neglected. Above the critical point, the zero-temperature series expansion method has been used to obtain estimates of the critical crystal field potential (D(c)) and critical exponent of the susceptibility. Using the coupled-cluster method we obtained the ground-state energy, magnetization and critical point for both two- and three-dimensional lattices.