The current status of the experimental study of the superconductor-to-insulator transition is reviewed for systems which are two-dimensional and nominally homogeneous. Interest in this problem has been heightened by the prospect that the transition may be representative of a new class of quantum phase transitions at zero temperature. An ubiquitous feature of the experiments is that films, depending on their properties, or the value of an applied magnetic field, exhibit either insulating or superconducting behavior in the limit of zero temperature. In particular, there appears to exist a limiting behavior which is associated with a finite zero-temperature resistance. In the case of the zero-field transition the value of the limiting resistance may be universal and very close to h/4e2, the quantum resistance for pairs. The experimental results on both the zero- and finite-field transitions will be reviewed and their implications for a particular theoretical picture, the dirty boson model, will be discussed. It has been argued that this model is relevant to the superconductor-insulator transitions.
Competing correlated states around the zero-field Wigner crystallization transition of electrons in two dimensions
