IMPROVED FINITE LATTICE METHOD FOR ESTIMATING THE ZERO-TEMPERATURE PROPERTIES OF 2-DIMENSIONAL LATTICE MODELS

The well-known finite-lattice method for the calculation of the proper ties of quantum spin systems on a two-dimensional lattice at zero temp erature was introduced in 1978. The method has now been greatly improv ed for the square lattice by including finite lattices based on parall elogram tiles as well as the familiar finite lattices based on square tiles. Dozens of these new finite lattices have been tested and graded using the S=1/2 XY ferromagnet. In the process new and improved estim ates have been obtained for the XY model's ground-state energy per spi n, epsilon(0)=-0.549 36(30) and spontaneous magnetization per spin, m= 0.4349(10). Other properties such as near-neighbour, zero-temperature spin-spin correlations, which appear not to have been calculated previ ously, have been estimated to high precision. Applications of the impr oved finite-lattice method to other models can readily be carried out.