Fluctuation Rounding of the Resistive Superconducting Transition in Thin Lead Films

The effect of fluctuations on the resistive transition to superconductivity has been studied in thin (100-200 Å) Pb films. Although the temperature dependence of the fluctuation contribution is given by ${(T-T_c)}^{-1\mathrm{}}$, and the width of the transition increases with increasing film resistance as predicted by Aslamazov and Larkin, this width is found to be smaller than the theoretical value by factors of 2-14. The transition from two-to three-dimensional behavior, expected theoretically in one of the samples studied, was not observed. A similar failure is found on analyzing published data for Al and (possibly for) amorphous Bi films. The various approximations used to derive the theory, and their effect on the analysis of the data (which has been carried out with no fitted parameters), are discussed in detail. The quantitative applicability of the Aslamazov-Larkin theory is considered in the light of its discrepancies with experiment. However, no explanation is found consistent with all published data. For one sample, measurements were performed in a magnetic field; a discussion of additional broadening mechanisms in a magnetic field is presented.