Limitations in cooling electrons using normal-metal-superconductor tunnel junctions.
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. First, when the injection rate of electrons exceeds the internal relaxation rate in the metal to be cooled, the electrons do not obey the Fermi-Dirac distribution, and the concept of temperature cannot be applied as such. Second, at low bath temperatures, states within the gap induce anomalous heating and yield a theoretical limit of the achievable minimum temperature.
Pekola
J P
JP
Low Temperature Laboratory, Helsinki University of Technology, P.O. Box 3500, 02015 HUT, Finland.
Heikkilä
T T
TT
Savin
A M
AM
Flyktman
J T
JT
Giazotto
F
F
Hekking
F W J
FW