Limitations in cooling electrons by normal metal - superconductor tunnel junctions

TL;DR

This paper investigates fundamental limitations in electron cooling using normal metal-superconductor tunnel junctions, highlighting the effects of high injection rates and low-temperature states on cooling efficiency.

Contribution

It provides both theoretical and experimental insights into two key factors limiting cooling performance in such junctions, including non-equilibrium effects and subgap states.

Findings

High injection rates lead to non-Fermi-Dirac electron distributions.

Subgap states cause anomalous heating at low temperatures.

Theoretical limits on minimum achievable temperature are identified.

Abstract

We demonstrate both theoretically and experimentally two limiting factors in cooling electrons using biased tunnel junctions to extract heat from a normal metal into a superconductor. Firstly, when the injection rate of electrons exceeds the internal relaxation rate in the metal to be cooled, the electrons do no more obey the Fermi-Dirac distribution, and the concept of temperature cannot be applied as such. Secondly, at low bath temperatures, states within the gap induce anomalous heating and yield a theoretical limit of the achievable minimum temperature.