Inverse slow relaxation in granular hopping systems

We present experimental results that demonstrate a glassy behavior in the conductance of quench-condensed insulating granular metals that is different from that observed in continuous disordered systems. Exciting the granular system by biasing the sample with a high electric field results in a slow conductance change both during the excitation and during the relaxation back to its steady state. The time scales for these processes are many orders of magnitude longer than the typical hopping time. We find that, initially, this conductance change has an opposite sign to that observed in similar experiments performed on continuous films. Only after relatively long times is this trend reversed and the samples exhibit conventional behavior. We suggest that the granular systems exhibit a different glassy process related to charge redistribution among the grains. This process combines with the relaxation processes that are characteristic of disordered systems to give rise to a unique relaxation profile.