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

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.