Tunneling spectra of unconventional quasi-two-dimensional superconductors

TL;DR

This paper explains unusual tunneling spectra in low-carrier-density quasi-2D superconductors by considering large pairing fluctuations, showing their behavior aligns with a crossover from BCS to Bose-Einstein condensation.

Contribution

It introduces a tunneling theory incorporating pairing fluctuations that accounts for strong coupling features in unconventional superconductors.

Findings

Strong coupling behavior observed in tunneling spectra.

Agreement with a crossover theory from BCS to Bose-Einstein condensation.

Proposes strong coupling as a generic property of quasi-2D low-carrier-density superconductors.

Abstract

Superfluid condensation can fundamentally be different from that predicted by the Bardeen-Cooper-Schrieffer (BCS) theory. In a broad class of low-carrier-density superconductors, such as granular aluminum, doped nitrides, and high-Tc cuprates, tunneling experiments reveal strong rather than weak coupling, as well as a conductance that does not return to that of the normal state upon approaching the critical temperature Tc. Here, we show that this behavior is in quantitative agreement with a tunneling theory that takes into account the large pairing-fluctuation effects that occur in the crossover region from weak-coupling BCS to strong-coupling Bose-Einstein condensation, provided the coherence energy scale rather than the single-particle energy gap is used to evaluate the coupling ratio. We also propose that the tendency toward strong coupling is a generic property of quasi-2D low-carrier-density superconductors.