Magnetic field enhanced order parameter amplitude fluctuations in ultrathin films near the superconductor-insulator transition

A magnetic field tuned superconductor-insulator transition (SIT) in ultrathin PbBi/Ge films has been observed. Electron tunneling measurements in a magnetic field near the SIT show the presence of a large number of quasiparticle states near the Fermi energy. A simple analysis of these data shows that the average number of Cooper pairs in a coherence volume is on the order of 1 at the SIT. Combined with transport measurements, the data imply that strong fluctuations in the amplitude of the superconducting order parameter occur at this field tuned SIT.