Modification of normal metal transport by the superconducting proximity effect

When a normal (N) metal makes intimate contact with a superconductor (S), the density of states in the normal metal is modified by the proximity effect. As a consequence, if a point contact is made with the normal metal the resulting tunnel conductance,G, is a function of the distance,x, from the N-S interface. We show that, for allx, G possesses a minimum at zero voltage and at finite voltage,G equals that of the structure without superconductivity present. The conductance is independent of the magnitude of the superconducting order parameter but depends upon the amount of normal scattering at the N-S interface.