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.
@article{arxiv.cond-mat/0312401,
title = {Limitations in cooling electrons by normal metal - superconductor tunnel junctions},
author = {J. P. Pekola and T. T. Heikkila and A. M. Savin and J. T. Flyktman and F. Giazotto and F. W. J. Hekking},
journal= {arXiv preprint arXiv:cond-mat/0312401},
year = {2009}
}
Comments
4 pages, 4 figures, added Ref. [6] + minor corrections