Superconducting quantum phase transitions tuned by magnetic impurity and magnetic field in ultrathin a -Pb films

Superconducting quantum phase transitions tuned by disorder ( d ), paramagnetic impurity ( MI ) and perpendicular magnetic field ( B ) have been studied in homogeneously disordered ultrathin a -Pb films. The MI -tuned transition is characterized by progressive suppression of the critical temperature to zero and a continuous transition to a weakly insulating normal state with increasing MI density. In all important aspects, the d -tuned transition closely resembles the MI -tuned transition and both appear to be fermionic in nature. The B -tuned transition is qualitatively different and probably bosonic. In the critical region it exhibits transport behavior that suggests a B -induced mesoscale phase separation and presence of Cooper pairing in the insulating state.