Discrepancies between experiment and theory in the superconducting proximity effect

The superconducting proximity effect is investigated for SN double layers in a regime where the resulting transition temperature Tc does not depend on the mean free paths of the films and, within limits, not on the transparency of the interface. The experimental results for Tc are compared with a numerical evaluation which was recently developed in our group. The results for the SN double layers can be divided into three groups. i When N=Cu, Ag, Au, Mg a disagreement between experiment and theory by a factor of the order of 2.5 is observed. ii When N=Cd, Zn, Al the disagreement between experiment and theory is reduced to a factor of about 1.5. iii When N=In, Sn a reasonably good agreement between experiment and theory is observed.