Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Negative four-terminal resistance as a probe of crossed Andreev
  reflection   We report on the experimental investigation of electronic transport in
superconductor-ferromagnet spin-valve structures. Our samples consist of two
ferromagnetic iron leads forming planar tunnel contacts to a superconducting
aluminum wire. At energies below the superconducting gap, we observe a negative
four-probe resistance that can be explained by crossed Andreev reflection.

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The categories of this paper are: dg-ga
32
The categories of this paper are: atom-ph
33
The categories of this paper are: plasm-ph
2
The categories of this paper are: math
27
The categories of this paper are: comp-gas
9
The categories of this paper are: nlin
16
The categories of this paper are: stat
14
The categories of this paper are: math-ph
8
The categories of this paper are: hep-ex
20
The categories of this paper are: acc-phys
28
The categories of this paper are: alg-geom
35
The categories of this paper are: mtrl-th
29
The categories of this paper are: funct-an
5
The categories of this paper are: gr-qc
37
The categories of this paper are: supr-con
13
The categories of this paper are: quant-ph