Superconductive Phenomena in Ultrathin Films

Experiments on very thin films and systems composed of small particles have shown transition temperatures significantly higher than the bulk T_c. Of the numerous ideas advanced to explain the high T_c's in these films, one of the most exciting is the suggestion of Cohen and Douglass that superconductive pairing can occur across dielectric barriers, and thereby lead to high T_c's. Recent results on the T_c of thin superconductors in contact with dielectric barriers, evaporated on cryogenic substrates, are presented. Some representative T_c's obtained in these experiments are Al∼5.7°K, Zn∼1.9°K, Sn∼6°K, In∼4.5°K, and Pb∼7.1°K. In the case of Al, Sn, and Zn these T_c's are much higher than the values found by Buckel and Hilsch for films deposited on quartz at liquid‐helium temperatures. Explanations other than the dielectric barrier idea, such as ``amorphorus structure'' and the suggestion of Ginzburg that surface interactions may modify T_c, are also discussed. In the course of these experiments to investigate high T_c's in films it was found that very thin continuous films (∼20 Å or less) could be prepared at cryogenic temperatures. An investigation of superconductivity in these ultrathin films showed that T_c was much lower than the T_c of the thicker films. This occurred in all the metals measured, which included Pb, Al, Sn, and Bi. An explanation of these results in terms of the destruction of long‐range order in two‐dimensional systems due to fluctuations in the Ginzburg‐Landau order parameter will be discussed. By using this idea and numerical values from the data, estimates can be made of the possibility of obtaining very high T_c's in films approaching a few atomic layers.