Universal conductance fluctuations in spin glasses

Results of numerical studies of conductance of a two-dimensional tight-binding system with random potentials and coupled to a three-dimensional Ising spin glass are presented. Numerical values of the conductance dispersion, in an ensemble of the potentials, support results of diagrammatic calculations for universality classes relevant for this system. Conductance of a sample is studied as a function of ‘‘time’’ for various Monte Carlo trajectories of the spin glass. The dispersion of the conductance noise decreases on cooling of the spin system without a noticeable anomaly at T_c. Power spectra of the noise are power laws in frequency with the exponent of -0.5 (at least in the vicinity of T_c) and the amplitude of a spectrum decreases monotonically with T. The fluctuations in the noise are only statistically related to the number of spin flips involved. The dispersion of the fluctuations at a given temperature depends on initial spin configurations. Locations of spins that affect conductance must also depend on the spin configuration. History-dependent effects are present both above and below T_c. Conductance noise in ferromagnets is compared to that occurring in spin glasses.