Why does a metal-superconductor junction have a resistance?

TL;DR

This tutorial explains why a metal-superconductor junction exhibits resistance despite Andreev reflection, highlighting fundamental differences between optical phase conjugation and electronic processes.

Contribution

It clarifies the paradox of resistance in metal-superconductor junctions by contrasting electronic Andreev reflection with optical phase conjugation.

Findings

Disordered metal-superconductor junction retains normal resistance.

Andreev reflection differs fundamentally from optical phase conjugation.

The paradox is resolved by understanding electronic versus optical phase conjugation.

Abstract

This is a tutorial article based on a lecture delivered in June 1999 at the NATO Advanced Study Institute in Ankara. The phenomenon of Andreev reflection is introduced as the electronic analogue of optical phase-conjugation. In the optical problem, a disordered medium backed by a phase-conjugating mirror can become completely transparent. Yet, a disordered metal connected to a superconductor has the same resistance as in the normal state. The resolution of this paradox teaches us a fundamental difference between phase conjugation of light and electrons.