Penetration Depth and Flux Creep in Thin Superconducting Indium Films

Abstract

We have measured the absolute value of the penetration depth in superconducting indium films. Applying a dc axial magnetic field to the exterior of a hollow superconducting thin-film cylinder, we determined the ratio $\frac{\ensuremath{\Delta}{H}_{i}}{\ensuremath{\Delta}{H}_{0}}$ of the variation of the magnetic field penetrating into the interior region to the variation of the applied magnetic field. Applied magnetic fields of small amplitude were used so that $\ensuremath{\Delta}{H}_{i}$ was linear in $\ensuremath{\Delta}{H}_{0}$; a superconducting magnetometer was used to measure $\ensuremath{\Delta}{H}_{i}$. Film thicknesses were between 95 and 160 \AA{}, and the temperature range was from 1.3 K to the critical temperature near 4.0 K. From the actual penetration depths deduced from these measurements, which were performed on four samples, values of the London penetration depth at absolute zero ${\ensuremath{\lambda}}_{L}(0)$ were calculated to be in the range 397 \ifmmode\pm\else\textpm\fi{} 22 \AA{}. This indicates that ${\ensuremath{\lambda}}_{L}(0)$ in thin films is larger than typical published measurements for bulk indium, in accordance with other evidence. Except for one sample and for temperature within about 0.3 K of the critical temperature, the data were fitted moderately well by the theoretical temperature dependence. Flux creep depending logarithmically on elapsed time was also observed for values of $\ensuremath{\Delta}{H}_{0}$ sufficiently large to induce currents in the film exceeding the critical-state current. The temperature dependence of the logarithmic flux-creep rate was approximately that of the product of the critical current, the absolute temperature, and the square of the penetration depth over a wide range of temperatures.