NONEQUILIBRIUM STATISTICAL-MECHANICS OF THE SPIN-1/2 VANDERWAALS MODEL .1. TIME EVOLUTION OF A SINGLE SPIN

The spin van der Waals model has received some attention in recent years as a soluble model, useful for demonstrating collective behavior analytically. By solving the Heisenberg equation of motion, we obtain the time evolution of a single spin in this model when N --> infinity, where N is the number of spins in the model. Our solution is characterized by rotations in spin space having the appearance of a Larmor precession. In the accompanying paper, we use this time-evolution solution to obtain the spin autocorrelation function and determine its long-time behavior.