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∑_i(S^x_i) ² -(J/2)∑_(i, j)S_i·S_j -h∑_iS^z_i, 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.