Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Structural phase transition below 250 K in superconducting
  K$_{0.75}$Fe$_{1.75}$Se$_{2}$   Vibrational properties of iron-chalcogenide superconductor
K$_{0.75}$Fe$_{1.75}$Se$_{2}$ with $T_{c}\sim$ 30 K have been measured by Raman
and optical spectroscopies over temperature range of 3-300 K. Sample undergoes
\textit{I4/m} $\to $ \textit{I4} structural phase transition accompanied by
loss of inversion symmetry at $T_{1}$, below 250 K, observed as appearance of
new fully-symmetric Raman mode at $\sim$ 165 cm$^{-1}$. Small vibration mode
anomalies are also observed at $T_{2}\sim$ 160 K. From first-principles
vibrational analysis of antiferromagnetic K$_{0.8}$Fe$_{1.6}$Se$_{2}$ utilizing
pseudopotentials all observed Raman and infrared modes have been assigned and
the displacement patterns of the new Raman mode identified as involving
predominantly the Se atoms.

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The categories of this paper are: nlin
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The categories of this paper are: acc-phys
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The categories of this paper are: nucl-ex
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The categories of this paper are: bayes-an
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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: atom-ph
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The categories of this paper are: stat
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The categories of this paper are: solv-int
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The categories of this paper are: gr-qc
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The categories of this paper are: quant-ph
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The categories of this paper are: hep-ex
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The categories of this paper are: hep-th
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The categories of this paper are: q-bio
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The categories of this paper are: math-ph
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The categories of this paper are: ao-sci