Free at last: Bose metal uncaged
A metallic phase disrupts the classic two-state superconductor-insulator picture Even the particle world is not immune to identity politics. Bosons have been in a bit of an identity crisis, or so it has seemed since 1989 (1). Quantum mechanics requires bosons made of two paired electrons to either condense into a superfluid with a well-defined phase with zero electrical resistance or localize in an insulating state with infinite resistance. The direct transition from superconducting to insulating states was widely observed in a range of thin films (2–4). The most popular model for explaining these observations (5) claims that the destruction of superconductivity occurs when the resistance of the thin film exceeds a critical value. For bosons on the brink of localization, electrically insulating behavior is observed if the resistance is greater than the quantum of resistance, Rq = h/4e2, otherwise superconductivity persists, where h is Planck's constant and e is the electric charge. On page 1505 of this issue, Yang et al. (6) offer a counterexample by establishing that a bosonic metallic phase disrupts the superconductor-insulator transition (SIT) in the high-temperature superconductor YBa2Cu3O7–x (YBCO).