Excess Currents Larger than the Point Contact Limit in Normal Metal -Superconducting Junctions

TL;DR

This paper investigates how finite superconductor width in NS junctions enhances excess current beyond the traditional point contact limit due to superfluid flow effects, revealing a maximum excess current at specific width ratios.

Contribution

It introduces a model accounting for finite superconductor width, showing increased excess current and identifying the optimal width ratio for maximum excess current.

Findings

Maximum excess current of approximately (7/3)(2eΔ/h)

Optimal superconductor width is about 7/3 times the normal metal width

Superfluid flow extends the energy range for Andreev reflection

Abstract

In a point contact NS junction, perfect Andreev reflection occurs over a range of voltages equal to the superconducting energy gap, producing an excess current of $I_{exc} = (4/3)(2e\Delta/h)$. If the superconductor has a finite width, rather than the infinite width of the point contact, one cannot neglect superfluid flow inside the superconducting contact. The energy range available for perfect Andreev reflections then becomes larger than the superconducting gap, since superfluid flow alters the dispersion relation inside the finite width superconductor. We find a maximum excess current of approximately $(7/3)(2e\Delta/h)$ when the width of the superconductor is approximately 7/3 times the width of the normal metal.