Thermal entanglement in a two-dimensional Heisenberg XY model

The thermal entanglement in a two-dimensional Heisenberg XY Model is investigated. The concurrence C   of a 2×22×2 and a 3×33×3 square lattices, respectively, is calculated. The concurrence C is decreased as the temperature T   is increased. Though there is no direct interactions between two diagonal qubits, the thermal entanglement still exists. The saturation effect occurs in the entropy when the 2×22×2 square lattice is reduced and extended to a one-dimensional Heisenberg chain of large number of lattices.