Excess conductance in normal-metal/Anderson insulator/superconductor junctions

The current-voltage characteristics of (Formula presented) tunnel junctions exhibit peculiar zero-bias anomalies. At low temperatures the zero-bias resistance attains saturation values that are smaller than the normal-state resistance by a factor that often exceeds 2. This zero-bias anomaly (ZBA) is also characterized by voltage (Formula presented) where Δ is the energy gap of lead. δ depends on the thickness of the (Formula presented) layer. In the presence of a microwave field the ZBA features are affected in a nontrivial way. The possibility that these features arise from the nature of the barrier being an Anderson insulator is discussed.