SPIN-GLASS FLUCTUATION STATISTICS - MESOSCOPIC EXPERIMENTS IN CUMN

Mesoscopic 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 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 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, 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.