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

Abstract

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.The relaxation rate temperature dependence follows an activation law having an energy scale in agreement with the average level splitting. In weakly metallic films the relaxation rate is smaller and decreases faster with temperature. Overall our observations are in line with the presence of a Kubo spin in Al nano-size grains due to quantum size effects.

Granular superconductors for high kinetic inductance and low loss quantum devices

“…That spacing is indeed small in the Al-Ge system, in which the grain size is about 10 nm 39 . The spacing being then of the order of 1 K. But when the grain size is about 2 nm, the inter-level spacing is about 100 K 40 .…”