SITE-DISORDERED SPIN SYSTEMS IN THE GAUSSIAN VARIATIONAL APPROXIMATION

We define a replica held theory describing finite dimensional site-dis ordered spin systems by introducing the notion of grand canonical diso rder, where the number of spins in the system is random but quenched. A general analysis of this field theory is made using the Gaussian var iational or Hartree-Fock method, and illustrated with several specific examples. Irrespective of the form of interaction between the spins t his approximation predicts a spin-glass phase. We discuss the replica symmetric phase at length, explicitly identifying the correlator that diverges at the spin-glass transition. We also discuss the form of con tinuous replica symmetry breaking found just below the transition. Fin ally we show how an analysis of ferromagnetic ordering indicates a bre akdown of the approximation.