Superconducting Fluctuations and Paraconductivity in Ultrathin a-Pb Films near Superconductor-Insulator Transition

The Aslamazov-Larkin (A-L) equation describes the extra conductance above T_C of a superconductor due to superconducting fluctuations. While agreement with A-L model has been found in some conventional two-dimensional (2D) superconductors, its applicability in ultrathin films near the superconductor-insulator transitions (SITs) has not been fully ascertained. Here we report a detailed examination of superconducting fluctuation and paraconductivity in ultrathin 2D amorphous Pb films near the SITs tuned by in situ variation of the film thickness and incremental deposition of paramagnetic impurities (Cr) on the same sample [1]. The two sets of R(T) were fitted to the A-L equation, both in the form of paraconductance and total conductance [2], with T_C, R_N and magnitude of the paraconductivity, g_exp, as the fitting parameters. In both cases, in the vicinity of the SIT (T_C below ~ 1 K), the paraconductivity increases precipitously, reaching as high as 50 times that of the 2D A-L value. The significant enhancement of superconducting fluctuations above the A-L theory near the SITs suggests pronounced emergent electronic inhomogeneities in the morphologically uniform 2D amorphous films.