The effect of magnetic field on superconductivity in ultrathin amorphous Pb films with paramagnetic impurities.

We report on a systematic study of the effect of applied magnetic fields on superconductivity in ultrathin amorphous Pb films containing various amounts of paramagnetic impurities. The Pb film, along with a 1 nm thick Sb buffer layer, was quench-condensed onto a Si substrate with pre-deposited Au contacts in a modified dilution refrigerator. Cr impurities were then deposited onto the Pb film by heating a NiCr wire at a fixed current. Both the Pb thickness (thus its T$_{C})$ and the Cr density can be varied, and electrical measurements can be performed at each step in perpendicular magnetic fields up to 8 T, all\textit{ in situ}. The reduction of the Pb T$_{C}$ with increasing Cr density is well described by the Abrikosov-Gorkov theory. The application of perpendicular magnetic fields did not result in any suppression of the pair-breaking effect by the Cr impurities, i.e., field enhanced superconductivity, on several samples covering a wide range of Pb thicknesses and Cr densities. The pronounced reentrant behavior found in the magnetic field-tuned transitions in pure Pb films$^{1}$ was progressively suppressed by increasing Cr impurities. $^{1}$ J.S. Paker et al., Europhys. Lett. 75, 950 (2006).