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Indirect Magnetic-Field-Tuned Superconductor-Insulator Transitions and Weak Localization of Bosons of Quasi-Two Dimensional Metal Films

Magnetic field and electrostatically tuned superconductor-insulator (SI) transitions of ultrathin metal films with levels of disorder that place them near the disorder-tuned SI transition appear to be direct, continuous quantum phase transitions. When films with lower levels of disorder are subjected to a perpendicular magnetic field, instead of a direct transition, a mixed superconductor-nonsuperconductor regime emerges at the lowest temperatures. The zero temperature limit of the resistance is either insulating or superconducting, depending upon the value of the field, suggesting that the behavior in this limit is governed by percolation physics. At high fields and low temperatures, in the nominally insulating regime, the resistance rather than the conductance is found to be a logarithmic function of temperature corresponding to predicitons for the weak localization of bosons.