Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: EuFe$_2$As$_2$ under high pressure: an antiferromagnetic bulk
  superconductor   We report the ac magnetic susceptibility $\chi_{ac}$ and resistivity $\rho$
measurements of EuFe$_2$As$_2$ under high pressure $P$. By observing nearly
100% superconducting shielding and zero resistivity at $P$ = 28 kbar, we
establish that $P$-induced superconductivity occurs at $T_c \sim$~30 K in
EuFe$_2$As$_2$. $\rho$ shows an anomalous nearly linear temperature dependence
from room temperature down to $T_c$ at the same $P$. $\chi_{ac}$ indicates that
an antiferromagnetic order of Eu$^{2+}$ moments with $T_N \sim$~20 K persists
in the superconducting phase. The temperature dependence of the upper critical
field is also determined.

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The categories of this paper are: astro-ph
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The categories of this paper are: q-bio
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The categories of this paper are: stat
1
The categories of this paper are: cs
32
The categories of this paper are: atom-ph
34
The categories of this paper are: chem-ph
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The categories of this paper are: patt-sol
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The categories of this paper are: solv-int
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The categories of this paper are: hep-ph
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The categories of this paper are: eess
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The categories of this paper are: hep-ex
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The categories of this paper are: alg-geom
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The categories of this paper are: nucl-th
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The categories of this paper are: hep-th
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The categories of this paper are: quant-ph
37
The categories of this paper are: supr-con