Quantum Fluctuations and Anomalous Insulating Phase in Disordered Two-Dimensional Superconductors

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We have studied the superconductor-insulator (SI) transition driven by disorder and field for two-dimensional (2D) systems. For thin indium films we have found that the critical field B_<xyC> determined from the Hall resistance R_<xy> is always larger than that B_<xxC> determined from the longitudinal resistance R_<xx>. The field region expressed as B_<xxC><B<B_<xyC> corresponds to an unusual insulating region where R_<xx> diverges and R_<xy> is small or zero at zero temperature. Since this region turns out to be more pronounced as the film becomes dirtier and/or thinner, we propose a possibility that this insulating region corresponds to the Bose-glass insulator where quantum fluctuations enhanced in 2D play an essential role. We have also observed the anomalous insulating region near the SI transition for ultrathin films of amorphous Mo_xSi_<1-x>. For insulating films the magnetoresistance is always positive, while for superconducting films it shows an peculiar peak and a subsequent decrease with increasing B at T<0.1K.This suggests the presence of the localized Cooper pairs on the insulating side of the field-driven SI transition.We have also investigated the presence of the vortex-glass (VG) transition in thick (3D) indium films. Near the onset of the dc linear resistivity in B=0.1T,we can clearly determine the VG transition temperature from the temperature and frequency dependence of the phase of the ac resistivity. Within a limited critical regime about the phase transition, the isotherms of the current-and frequency-dependent resistivity collapse onto the universal scaling functions, which well resemble those found in high-T_C superconductors. These results offer strong evidence for the VG transition in the indium film and the universality of the VG transition in disordered type-II superconductors.