Relaxational dynamics of the Edwards-Anderson model and the mean-field theory of spin-glasses

Langevin equations for the relaxation of spin fluctuations in a soft-spin version of the Edwards-Anderson model are used as a starting point for the study of the dynamic and static properties of spin-glasses. An exact uniform Lagrangian for the average dynamic correlation and response functions is derived for arbitrary range of random exchange, using a functional-integral method proposed by De Dominicis. The properties of the Lagrangian are studied in the mean-field limit which is realized by considering an infinite-ranged random exchange. In this limit, the dynamics are represented by a stochastic equation of motion of a single spin with self-consistent (bare) propagator and Gaussian noise. The low-frequency and the static properties of this equation are studied both above and below Tc. Approaching Tc from above, spin fluctuations slow down with a relaxation time proportional to |T-Tc|^-1 whereas at Tc the damping function vanishes as ω1/2. We derive a criterion for dynamic stability below Tc. It is shown that a stable solution necessarily violates the fluctuation-dissipation theorem below Tc. Consequently, the spin-glass order parameters are the time-persistent terms which appear in both the spin correlations and the local response. This is shown to invalidate the treatment of the spin-glass order parameters as purely static quantities. Instead, one has to specify the manner in which they relax in a finite system, along time scales which diverge in the thermodynamic limit. We show that the finite-time correlations decay algebraically with time as t^-ν at all temperatures below Tc, with a temperature-dependent exponent ν. Near Tc, ν is given (in the Ising case) as ν(T)~1/2-π^-1(1-TTc)+σ(1-TTc)². A tentative calculation of ν at $T=0$ K is presented. We briefly discuss the physical origin of the violation of the fluctuation-dissipation theorem.