  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.
