FROM LOCALIZED TO ITINERANT SPIN-GLASSES - GRASSMANN FIELD-THEORY ANDMEAN-FIELD SOLUTIONS

We introduce a new method to describe (quantum) spin glasses which is based on a Grassmann field representation of spins. Five spin glass mo dels are considered in detail. We distinguish between Ising/Heisenberg spin glass models (I(s)/H(s)) on spin space and Ising/Heisenberg spin glasses (I(f)/H(f)) on Fock space. To demonstrate the effect of the t wo different underlying spaces we calculate T(c), the spin glass order parameter q, and the replica-diagonal average q = [sigma2]. The free energy is derived both for the replica-symmetric theory (SK solution) and for broken replica permutation symmetry (Parisi solution). For mod el I(f) we also calculate the entropy, a modified Almeida-Thouless lin e depending on q(T(c)), and the dependence on chemical potential and/o r filling. Finally we define an itinerant spin glass model RHI(f), whi ch consists of model I(f) and an additional random hopping hamiltonian . The self-consistency equations for q, q, and for the electron Green function are determined. Above T(c), the spin glass susceptibility is derived by expanding the action to second order in the order parameter fluctuation fields. The critical temperature for the onset of metalli c spin glass order is found to be T(c) = Jq(T(c)) with q(T(c)) = max(4 /pi2 - 1/2piJtau, 0), where J denotes the variance of the spin-spin co upling and T refers to the elastic scattering time. This result leads to a criterion for the suppression of spin glass order which is remini scent of the Stoner criterion for itinerant ferromagnetism.