Superconductor-insulator transitions in 2D: The experimental situation

Superconductor-insulator (SI) transitions in ultrathin films have attracted significant attention over the last decade because of the possibility that they are quantum phase transitions. Magnetic field, film thickness, or car rier concentration can be used as control parameters. The bosonic pictures of these transitions proposed some years ago are only in qualitative agreem ent with experiment. In particular, the critical resistance appears not to be universal, and there are variations in the values of critical exponents. It has been concluded that in real films fermionic degrees of freedom must be taken into account. There are also indications that the phase diagram m ay include a significant metallic phase separating the superconducting and insulating phases, and that the transition may have a significant percolati ve aspect. The experimental situation will be broadly reviewed with attenti on paid to issues relating to materials and measurements.