Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Limitations in cooling electrons by 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. 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.

24
The categories of this paper are: dg-ga
28
The categories of this paper are: alg-geom
3
The categories of this paper are: physics
30
The categories of this paper are: q-alg
9
The categories of this paper are: nlin
37
The categories of this paper are: supr-con
35
The categories of this paper are: mtrl-th
2
The categories of this paper are: math
5
The categories of this paper are: gr-qc
14
The categories of this paper are: math-ph
31
The categories of this paper are: ao-sci
36
The categories of this paper are: cmp-lg
8
The categories of this paper are: hep-ex
20
The categories of this paper are: acc-phys
7
The categories of this paper are: hep-th
11
The categories of this paper are: hep-lat