Spin-glass fluctuation statistics: mesoscopic experiments in CuMn

Abstract Mesoscopic resistance fluctuation experiments on the spin glass Cu Mn are compared with expectations for different classes of spin-glass theories. The experiments show fluctuating units of about 10 4 spins, much larger than expected for spin-density-wave domain pictures. The noise statistics do not resemble those from independent two-state systems, so a dillute droplet model is not applicable. Scaling properties of the ‘second spectrum’ appear inconsistent with droplet pictures. These scaling properties could generally be fit with a hierarchical picture. Within a hierarchical picture, we use the data to calculate a coherence size (about 10 6 spins), the number of branches per hierarchical tree vertex and some features of the overlap distribution function, which seems to show more ‘replica symmetry breaking’ than expected from the mean-field theory.

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