SUPERCONDUCTIVITY IN BETA-TUNGSTEN FILMS

Evaporated tungsten films were found to have a superconducting transition temperature Tc 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 <30 Å/min are a mixture of both the α and β phases, the β phase being the A15 structure, and the α phase being the bcc structure. It was established that the enhancement in Tc 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/Tc followed the BCS curve remarkably well. The ratio 2Δ(0)/KTc varied between 2.32 to 3.64 as against the BCS value of 3.53, corresponding to the variation in Tc 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 Tc, 2Δ(0), and 2Δ(0)/KTc are related to the relative amounts of α and β phases present in the composite film and their connectivity. © 1968 The American Institute of Physics.