Collapse of the Cooper pair phase coherence length at a superconductor to insulator transition

TL;DR

This study investigates the superconductor-insulator transition in a-Bi films, revealing that Cooper pair phase coherence length abruptly collapses at the transition, challenging existing theories of a gradual phase fluctuation-driven SIT.

Contribution

It provides experimental evidence that the phase coherence length drops suddenly at the SIT, indicating different classes of disorder-tuned transitions.

Findings

Oscillations in magnetoresistance vanish near the critical point.

Cooper pair phase coherence length becomes less than interhole spacing at the transition.

Contrasts with previous observations of persistent oscillations in insulating phase.

Abstract

We present investigations of the superconductor to insulator transition (SIT) of uniform a-Bi films using a technique sensitive to Cooper pair phase coherence. The films are perforated with a nanohoneycomb array of holes to form a multiply connected geometry and subjected to a perpendicular magnetic field. Film magnetoresistances on the superconducting side of the SIT oscillate with a period dictated by the superconducting flux quantum and the areal hole density. The oscillations disappear close to the SIT critical point to leave a monotonically rising magnetoresistance that persists in the insulating phase. These observations indicate that the Cooper pair phase coherence length, which is infinite in the superconducting phase, collapses to a value less than the interhole spacing at this SIT. This behavior is inconsistent with the gradual reduction of the phase coherence length expected for a bosonic, phase fluctuation driven SIT. This result starkly contrasts with previous observations of oscillations persisting in the insulating phase of other films implying that there must be at least two distinct classes of disorder tuned SITs.