Hard gap in a normal layer coupled to a superconductor

TL;DR

This paper investigates how the excitation gap in a normal layer coupled to a superconductor depends on the layer's thickness, revealing that a sizable gap can be induced even in layers much thicker than the Fermi wavelength.

Contribution

It provides a quantum-mechanical analysis showing the induced gap persists and remains sizable for thicker normal layers, bridging the gap between quasiclassical and atomically thin regimes.

Findings

The induced gap decreases with increasing normal layer thickness.

A crossover scale exists where the gap remains comparable to the superconductor's bulk gap.

A sizable gap can be induced in layers much thicker than the Fermi wavelength.

Abstract

The ability to induce a sizable gap in the excitation spectrum of a normal layer placed in contact with a conventional superconductor has become increasingly important in recent years in the context of engineering a topological superconductor. The quasiclassical theory of the proximity effect shows that Andreev reflection at the superconductor/normal interface induces a nonzero pairing amplitude in the metal but does not endow it with a gap. Conversely, when the normal layer is atomically thin, the tunneling of Cooper pairs induces an excitation gap that can be as large as the bulk gap of the superconductor. We study how these two seemingly different views of the proximity effect evolve into one another as the thickness of the normal layer is changed. We show that a fully quantum-mechanical treatment of the problem predicts that the induced gap is always finite but falls off with the thickness of the normal layer, $d$. If $d$ is less than a certain crossover scale, which is much larger than the Fermi wavelength, the induced gap is comparable to the bulk gap. As a result, a sizable excitation gap can be induced in normal layers that are much thicker than the Fermi wavelength.