Evidence of Collective Charge Behavior in the Insulating State of Ultrathin Films of Superconducting Metals

Nonlinear I-V characteristics have been observed in insulating quench-condensed films which are locally superconducting. We suggest an interpretation in terms of the enhancement of conduction by the depinning of a Cooper pair charge density wave, Cooper pair crystal, or Cooper pair glass that may characterize this insulating regime. We propose that this is a more likely description than the Coulomb blockade or charge-anticharge unbinding phenomena.