Surface Energy Effects at the Boundary between a Superconductor and a Normal Conductor

Films of tin ranging from 500 to 1400 Å in thickness have been prepared in pairs by vacuum deposition onto rotating cylindrical glass and metal substrates. The temperature dependence of the critical fields has been measured. The critical temperatures of the films on metal substrates are all depressed. Their critical fields at absolute zero, however, are usually not depressed but can be higher than those of the corresponding films on glass. This increase can be explained by a negative surface energy arising from a short mean free path of the superconducting electrons in an incidental oxide layer at the surface of the metal. Most of the films on glass had an abnormally small value of the critical field at absolute zero.