Ground states of quantum matter can often be distinguished by their zero temperature electrical properties. This way, superconductors, metals and insulators have zero, finite, and infinite resistance respectively, at T=0. In two dimensions, it is widely believed that only superconductors, insulators, and quantum Hall states are viable ground states of electronic systems. Contrary to this notion, several experiments (including those from Stanford in A. Kapitulnik's group) involving disordered thin films have suggested that metallic ground states occur. Such metallic states are found in the vicinity of a T=0 phase transition between a superconducting and insulating ground states accessed by tuning the external magnetic field. The explanation for such metallic behavior has remained unclear for several decades.
Motivated by a well-established analogy between the superconductor-insulator transition and quantum Hall transitions, and invoking arguments of duality between particles and vortices, the authors suggest a possible origin for the metallic behavior seen in these experiments. In essence, they have suggested that composites of Cooper pairs and superconducting vortices act as fermions and give rise to the metallic behavior. The metallic phase seen in these experiments would then be analogous to the metallic behavior seen near quantum Hall transitions where a Landau level is half-filled.