Superconductivity in β-Tungsten Films

Abstract

Evaporated tungsten films were found to have a superconducting transition temperature T_c of 3.2°K as compared to the bulk value of 0.01°K. Structural study of the electron‐beam‐evaporated W films was carried out by both x‐ray and electron diffraction techniques. It was found that films prepared at pressures >10⁻⁷ Torr and evaporation rates T_c of these films is due to the presence of the β phase. Films prepared at 3×10⁻⁹ Torr are pure α‐W and they did not superconduct down to 1°K. The superconducting energy gap Δ(T) of the β‐W films prepared at 5×10⁻⁵ Torr and 25 Å/min was determined by tunneling in W‐B‐M junctions, where B is the plasma‐grown oxide barrier on W and M stands for Sn, Pb, and Au counterelectrodes. The temperature dependence of the energy gap Δ(T)/Δ(0) vs T/T_c followed the BCS curve remarkably well. The ratio 2Δ(0)/KT_c varied between 2.32 to 3.64 as against the BCS value of 3.53, corresponding to the variation in T_c between 3.1° to 3.3°K, respectively. The relative amounts of the α and β phases present were estimated from the tunneling experiments. The variations in T_c, 2Δ(0), and 2Δ(0)/KT_c are related to the relative amounts of α and β phases present in the composite film and their connectivity.