Inelastic transport through normal-metal–superconductor interfaces

A theory for inelastic transport through normal-metal–superconductor (NS) interfaces is presented that is applicable throughout the entire range of interface transmittances from the tunneling (low-interface transmittance) to the point-contact (high-interface transmittance) limits. This theory predicts that the excitation of a particular mode of energy ħω leads to a step up in conductance at a voltage eV=ħω+Δ in the tunneling limit, and a step down in conductance at a voltage eV=ħω-Δ in the point-contact limit. A broad, flat distribution of possible energy-loss modes in the interface region results in a linear conduction increase for high-resistance contacts, and a linear conductance decrease for low-resistance contacts. This is in qualitative agreement with recent experiments on the high-${\mathit{T}}_{\mathit{c}}$ superconductors.