Experimental search for excitonic superconductivity

A search for excitonic superconductivity in a thin-film structure of the type proposed by Ginsberg and Allender, Bray, and Bardeen was undertaken. Ultrathin layers of Pb, In, and Tl were deposited on PbTe, and layers of Pb and In were deposited on Te. Substrate preparation and film deposition were accomplished in an ultrahigh-vacuum system with integral cryostat, and were monitored by low-energy electron diffraction and Auger electron spectroscopy. In situ measurement of crystalline, epitaxial Pb films as thin as 10-15 Å deposited on PbTe showed a strong film-substrate interaction; however, no excitonic enhancement of $T_c$ was observed. A simple proximity-effect model is presented to explain the observed decrease in $T_c$ for the thinner films, and the influence of the substrate on the superconductivity of ultrathin films is discussed. Several aspects of these experiments involving the transition temperatures of In and Tl films are unexplained.