SIMULATION OF AN ENSEMBLE WITH VARYING MAGNETIC-FIELD - A NUMERICAL DETERMINATION OF THE ORDER-ORDER INTERFACE TENSION IN THE D=2 ISING-MODEL

In analogy with a recently proposed multicanonical ensemble we introduce an ensemble where the partition function is simulated with a term in the action containing a varying magnetic field. Using this ensemble we demonstrate on lattices with periodic boundary conditions that it is possible to enhance the appearance of order-order interfaces by many orders of magnitude. To perform a stringent test of the method we consider the D = 2 Ising model at beta = 0.5 and simulate square lattices up to size 100 x 100. By a finite-size scaling analysis, the order-order interface tension per unit area is obtained. Our best infinite-volume extrapolation is in excellent agreement with Onsager's exact result.