Quantum Monte Carlo study of a Heisenberg spin system in two dimensions with long-range interactions

The thermodynamic properties and critical behavior of a Heisenberg ferromag net in two dimensions are studied using the Handscomb quantum Monte Carlo m ethod. The spins (with quantum number S = 1/2) are arranged on a square lat tice and have a long-range interaction with a power-law decrease, J(r) prop ortional to r(-p), with distance r. The parameter p is taken to be in the r ange 2 < p less than or equal to 6. The system has an ordering transition a t a nonzero temperature T-c when 2 < p < 4. The dependence of T-c on p and the critical behavior of the susceptibility are investigated. Properties of spin correlation functions in different regimes (of p and T) are discussed . The results are compared with previous approximate analytic quantum theor ies. (C) 2001 American Institute of Physics.