Improved finite lattice method for estimating the zero-temperature properties of two-dimensional lattice models

The well-known finite-lattice method for the calculation of the properties of quantum spin systems on a two-dimensional lattice at zero temperature was introduced in 1978. The method has now been greatly improved for the square lattice by including finite lattices based on parallelogram 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 estimates have been obtained for the XY model's ground-state energy per spin, 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 previously, have been estimated to high precision. Applications of the improved finite-lattice method to other models can readily be carried out.