Limitations in cooling electrons using normal-metal-superconductor tunnel junctions

Phys Rev Lett. 2004 Feb 6;92(5):056804. doi: 10.1103/PhysRevLett.92.056804. Epub 2004 Feb 6.


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.