Scaling Analysis of Direct Superconductor-Insulator Transitions in Disordered Ultrathin Films of Metals

Superconductor-insulator (SI) transitions of homogeneously disordered ultrathin quench-condensed films in many instances appear to be direct, without any intervening metallic regime. This is in contrast with what has been found in some other systems. These direct transitions have been analyzed using finite size scaling. The products of the dynamical critical exponent and the coherence length exponent found vary, depending upon the tuning parameter. They are approximately 1.3 for the thickness tuned SI transition, and approximately 0.7 for perpendicular and parallel magnetic field tuning. Charge tuning also yields 0.7. Assuming that the dynamical critical exponent is unity as is anticipated for systems with long range interactions, all of the transitions, except the thickness-tuned transition would appear to belong to the 3D XY universality class. This behavior is different from that observed for magnetic field tuned transitions of compounds such as InO_x or TiN, or other metallic systems. The source of these differences is not known but may be due to differences in carrier density or structural or chemical disorder on a mesoscopic scale.