Kubo spins in nanoscale aluminum grains: A muon spin relaxation study

We report muon spin relaxation rate measurements on films composed of aluminum grains having a size of a few nm, with a large energy level splitting of the order of 100 K. The films range from weakly metallic to insulating. In the insulating case, the low-temperature relaxation rate is consistent with the presence of single electron spins in grains having an odd number of electrons, known as Kubo spins. The relaxation rate temperature dependence follows an activation law having an energy scale in agreement with the average level splitting. In weakly metallic films, inter-grain junction spins may contribute substantially to the smaller relaxation rate.