Classes of superconductor-insulator transitions in high-T_c films

The superconductor-insulator transition has been studied in disordered DyBa₂Cu₃O_7-x ultrathin films. Near the transition there is an anomalous magnetoresistance, which results in a re-entrant superconductor-insulator transition as a function of magnetic field. The highest-field transition is consistent with the expected field-induced transition. The low-field transitions may result from field modulation of the resistance through critical values which are higher than h/4e².