Cooper pair insulator in amorphous films induced by nanometer-scale thickness variations

TL;DR

This paper demonstrates that nanometer-scale thickness variations in amorphous Bi films induce inhomogeneous phases with localized Cooper Pairs, revealing the role of inhomogeneities in the insulator-superconductor transition.

Contribution

It provides experimental evidence linking nanoscale thickness variations to the formation of Cooper pair insulators in amorphous films, a novel insight into the transition mechanism.

Findings

Inhomogeneities induce Cooper pair localization.

IST occurs when link resistance approaches quantum resistance.

Weak links between superconducting islands dominate transport.

Abstract

Unusual transport properties of superconducting (SC) materials, such as the under doped cuprates, low dimensional superconductors in strong magnetic fields, and insulating films near the Insulator Superconductor Transition (IST), have been attributed to the formation of inhomogeneous phases. Difficulty correlating the behaviors with observations of the inhomogeneities make these connections uncertain. Of primary interest here are proposals that insulating films near the IST, which show an activated resistance and giant positive magnetoresistance, contain islands of Cooper Pairs (CPs). Here we present evidence that these types of inhomogeneities are essential to such an insulating phase in amorphous Bi (a-Bi) films deposited on substrates patterned with nanometer-sized holes. The patterning induces film thickness variations, and corresponding coupling constant variations, that transform the composition of the insulator from localized electrons to CPs. Analyses near the thickness-tuned ISTs of films on nine different substrates show that weak links between SC islands dominate the transport. In particular, the ISTs all occur when the link resistance approaches the resistance quantum for pairs. These observations lead to a detailed picture of CPs localized by spatial variations of the superconducting coupling constant.