Nonequilibrium quasiparticles and electron cooling by normal metal - superconductor tunnel junctions

TL;DR

This paper analyzes a double junction NISIN system to understand how resistance ratios affect electron cooling and quasiparticle removal, deriving a generalized cooling power expression considering charge imbalance effects.

Contribution

It introduces a model for a NISIN system with asymmetric junction resistances and derives a generalized cooling power formula including charge imbalance effects.

Findings

Cooling power depends strongly on resistance ratio

Partial removal of quasiparticles enhances cooling efficiency

Generalized expression accounts for charge imbalance effects

Abstract

We consider a model NISIN system with two junctions in series, where N is a normal metal, S is a superconductor and I is an insulator. We assume that the resistance of the first junction is high, while the resistance of the second one is low. In this case the first junction cools the left normal electrode, while the second junction partially removes excited quasiparticles from the superconductor. We consider cooling properties of this double junction structure. It is shown that the cooling power depends strongly on the ratio of the resistances of the two junctions. In conclusion, we derive a generalized expression for the cooling power of a NIS tunnel junction taking into account charge imbalance effects.