Penetration Depth and Flux Creep in Thin Superconducting Indium Films

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 $\frac{\Delta H_i}{\Delta H_0}$ 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 $\Delta H_i$ was linear in $\Delta H_0$; a superconducting magnetometer was used to measure $\Delta H_i$. Film thicknesses were between 95 and 160 Å, 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 ${\lambda }_L\mathrm{}\left(0\right)\mathrm{}$ were calculated to be in the range 397 ± 22 Å. This indicates that ${\lambda }_L\mathrm{}\left(0\right)\mathrm{}$ 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 $\Delta H_0$ 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.