Exact properties of spin glasses. II. Nishimori's line : new results and physical implications

Abstract
An exact result for spin glasses, due to Nishimori, is reconsidered. The existence, in the phase diagram, of a subspace where some exact properties can be derived, is generalized to a larger class of models. This clarifies the role of the gauge symmetry. A new upper bound for the quenched free energy on this subspace is obtained. Recent numerical simulations are confronted with these results for S.K., 3d and 2d Ising spin glasses. This suggests the possible existence of a tricritical point for the 2d Ising spin glass at pc ≃ 0.9, kTc/J ≃ 0.9