A new metastable superconducting modification of bismuth attainable in ultrathin films
Abstract
The superconductor properties of ultrathin (18−70 Å) bismuth films were studied, these films having been produced by condensation of metal vapor on a surface cooled down to 2°K. It has been established that the superconducting transition temperature becomes higher after annealing of such films and that the magnitude of this upward shift is a function of the film thickness. The maximum shift is Δ Tc = 0.9°K for a film 18 Å thick, the shift is smaller for films thicker than that, until there is no shift for films thicker than 40 Å. The highest superconducting transition temperature after annealing of all specimens of all thicknesses did not exceed 3.6°K. These data as well as the previously known anomalous dependence of dHc/dT, electrical resistivity, and coordination structure of freshly condensed thin bismuth films on their thickness suggest that such films thinner than 40 Å form as a different modification than that of thicker films. The hypothesis is proposed here that this modification is a distorted variant of the high-pressure Bi-II phase.