Spin-glass fluctuation statistics : mesoscopic experiments in CuMn

Mesocopic resistance fluctuation experiments on the spin glass CuMn are compared with expectations for different classes of spin-glass theories. The experiments show fluctuating units of approx 10 exp 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 dilute 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, the data are used to calculate a coherence size ( approx 10 exp 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.