Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Superconductivity in undoped CaFe2As2 single crystals   Single crystals of undoped CaFe2As2 were grown by a FeAs self-flux method,
and the crystals were quenched in ice-water rapidly after high temperature
growth. The quenched crystal undergoes a collapsed tetragonal structural phase
transition around 80 K revealed by the temperature dependent X-ray diffraction
measurements. Superconductivity below 25 K was observed in the collapsed phase
by resistivity and magnetization measurements. The isothermal magnetization
curve measured at 2 K indicates that this is a typical type-II superconductor.
For comparison, we systematically characterized the properties of the furnace
cooled, quenched, and post-annealed single crystals, and found strong internal
crystallographic strain existing in the quenched samples, which is the key for
the occurrence of superconductivity in the undoped CaFe2As2 single crystals.

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The categories of this paper are: acc-phys
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The categories of this paper are: supr-con
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The categories of this paper are: funct-an
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The categories of this paper are: ao-sci
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The categories of this paper are: patt-sol
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The categories of this paper are: bayes-an
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The categories of this paper are: nucl-ex
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The categories of this paper are: cmp-lg
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The categories of this paper are: hep-ph
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The categories of this paper are: adap-org
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The categories of this paper are: gr-qc
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The categories of this paper are: hep-ex
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The categories of this paper are: q-bio
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The categories of this paper are: hep-th
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The categories of this paper are: quant-ph
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The categories of this paper are: mtrl-th