The Superconducting Proximity Effect as a Tool to Investigate Metal   Films and Interfaces
  052<p type="texpara" tag="Body Text" et="abstract" >The superconducting proximity effect is measured in sandwiches of thin Pb films and the alkali metals Cs, Rb, K and Na. The $T_{c}$-dependence provides information about the interface barriers between Pb and the alkalis. Such a barrier is particularly large in Pb/Cs sandwiches. It is not due to impurities or oxydation. In the presence of a sufficiently strong barrier a special form of the Cooper limit can be applied to calculate the transition temperature of the sandwich. 
D. Garrett, M. Zhang and G. Bergmann