Vapor-Deposited Superconductive Films of Nb, Ta, and V

Abstract

The strong gettering action for oxygen or oxygen‐containing gases displayed by Nb, Ta, and V was used to reduce the partial pressure of these gases in a vacuum system; and allowed deposition from the vapor of superconducting thin films of these metals even at pressures of the residual gases higher than 10⁻⁵ Torr provided elevated substrate temperatures were used. Critical temperatures of the films were as high or higher than those of the bulk materials; the highest values were T(Nb)=9.81°±0.05°K, T(Ta)=4.51°±0.05°K, T(V)=6.02°±0.10°K. Electron tunneling experiments demonstrated that the size and temperature dependence [ε(0)=3.5T_c] of the energy gap of these films is in good agreement with the BCS theory. Critical current vs magnetic field measurements at 4.2°K indicate that Ta films go normal at 220–240 Oe and Nb films at 9–15 kOe. Films of niobium prepared at low‐substrate temperatures displayed inflated lattice parameters and were not superconductors.