Mesoscopic noise studies of atomic motions in cold amorphous conductors

The dynamics of slow low-energy atomic motions in the amorphous conductors C-Cu and Si-Au were probed using conductance fluctuations in nanometer-scale samples in the temperature range 4--30 K. Ample evidence was found of isolated fluctuators with both tunneling and thermally activated kinetics, with the estimated density of sites at 4 K close to that expected from the anomalous heat capacity of amorphous solids. However, a detailed statistical analysis revealed that the conductance-fluctuation data were inconsistent with a picture of independent randomly fluctuating two-level systems. Direct evidence of non-two-state behavior was occasionally seen in discrete switching in the voltage traces. It was also found that anneals up to 100 K were usually sufficient to randomize features in the spectral density. The statistical analyses and the strong sensitivity to relatively weak anneals suggest that intersite interactions are prevalent in these systems.