Size dependence of the superconducting transition temperature and low-temperature electronic transport in niobium ultrathin wires and thin films

The author has performed an extensive study of the size dependence of the superconducting transition temperatures in niobium ultra-thin wires and thin films, both in the dirty and clean limits. With increasing disorder and decreasing size, Tc drops dramatically in a systematic way. For the wires, the findings fairly well agree with the Fukuyama model. In the case of the films, the suppression of Tc increases linearly as the sheet resistance Rsq increases, which is in good agreement with the theory. Measurements of the temperature and magnetic field dependence of the resistance for the niobium ultra-thin wires and thin films have been made. In the absence of the magnetic field, the variation of the resistance with temperature is explained within the frame-work of the theories of localization and electron-electron interactions. The effect of the magnetic field on the wires accounts for the appearance of the anomalous magnetoresistance. This magnetoresistance is mainly due to the magnetic field dependent contributions from electron localization and electron-electron interactions as well as electron scattering by superconducting fluctuations. The contribution from the superconducting fluctuations. The contribution from the superconducting fluctuations dominates as T approaches Tc. From the magnetoresistance (MR) data, the inelastic scattering time and its temperature dependence were determined. The voltage-current characteristics for Nb wires and films were measured and the calculated Jc0 (current density at T = 0K) was compared with the Ginsburg-Landau theory. Results agree with the GL theory. He has also observed magnetoresistance oscillations in the clean Nb wires