Andreev reflection spectroscopy in strongly paired superconductors

TL;DR

This paper investigates how Andreev reflection spectroscopy can reveal the BCS to BEC evolution in low-carrier-density superconductors, highlighting the limitations and potential experimental signatures of BEC superconductivity.

Contribution

It demonstrates that standard Andreev reflection cannot exceed a conductance bound in BEC superconductors but proposes tunneling setups to recover and detect BEC signatures.

Findings

Andreev reflection in BEC superconductors is limited to e^2/h conductance.

A duality links the conductance of BCS and BEC superconductors.

Tunneling through potential wells can reveal perfect Andreev reflection in BEC regimes.

Abstract

Motivated by recent experiments on low-carrier-density superconductors, including twisted multilayer graphene, we study signatures of the BCS to BEC evolution in Andreev reflection spectroscopy. We establish that in a standard quantum point contact geometry, Andreev reflection in a BEC superconductor is unable to mediate a zero-bias conductance beyond $e^2/h$ per lead channel. This bound is shown to result from a duality that links the sub-gap conductance of BCS and BEC superconductors. We then demonstrate that sharp signatures of BEC superconductivity, including perfect Andreev reflection, can be recovered by tunneling through a suitably designed potential well. We propose various tunneling spectroscopy setups to experimentally probe this recovery.