The superconductor-insulator transition in ultrathin Pb: the effects of disorder, magnetic field, and magnetic impurities

Using ultrathin quench-condensed Pb films we have performed a systematic comparative study of the superconductor-insulator transition (SIT) driven by disorder ($d)$, magnetic field ($B)$, and magnetic impurities (\textit{MI}). The Pb films were quench-condensed at low temperature onto an Sb buffer layer. The $d$-tuned transition was studied by increasing the thickness of the same film in small steps and performing \textit{in situ} transport measurements. The film was driven back into the insulating state by a perpendicular magnetic field and then later in zero field by magnetic impurities deposited in small increments. We observed that the $d$- and \textit{MI}-tuned transitions showed similar features across the SIT, while the $B$-tuned transition appeared qualitatively different. In particular, the $B$-field induced a quasi-reentrant behavior near the critical field and activated transport immediately on the insulating side, indicating possible $B$-induced mesoscale phase separation across the $B$-tuned SIT. These are distinguishing features for the SIT in granular films and were absent in the $d$- and \textit{MI}-tuned transitions which exhibited sharp well-defined phase boundaries.