Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Posttreatment Effects on the Crystal Structure and Superconductivity of
  Ca-Free Double-Layered Cuprate Sr$_2$SrCu$_2$O$_{4+y}$F$_{2-y}$   We report the effects of low-temperature postannealing on the structural and
superconducting properties of the recently discovered Ca-free double-layered
cuprate, Sr$_2$SrCu$_2$O$_{4+y}$F$_{2-y}$. Although the as-synthesized sample
prepared under high pressure has a tetragonal structure with a rock-salt-type
blocking layer (the so-called $T$-phase), we found that the symmetry of the
structure lowered to that of an orthorhombic system when annealed with CuF$_2$.
The structural refinements reveal that such a topochemical reaction leads not
only to the removal of excess O$^{2-}$ from the apical site but also to the
intercalation of extra F$^-$ into the interstitial site. The orthorhombic phase
exhibits bulk superconductivity at a critical temperature of 107 K, which is
significantly higher than that of the $T$-phase ($\sim$50 K). Meanwhile, the
$T$-phase turns into another structure possessing a fluorite-type blocking
layer without apical fluorine (known as the $T^\prime$-phase) by annealing
without CuF$_2$. Density functional theory calculations show that the
$T^\prime$-phase is more stable than the $T$-phase. This is the first report on
the formation of a $T^\prime$-type double-layered cuprate. Furthermore, the
structural stability of the three phases of Sr$_2$SrCu$_2$O$_{4+y}$F$_{2-y}$ is
discussed in terms of lattice matching between the blocking and conducting
layers.

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The categories of this paper are: funct-an
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The categories of this paper are: math-ph
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The categories of this paper are: chem-ph
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The categories of this paper are: cs
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The categories of this paper are: stat
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The categories of this paper are: ao-sci
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The categories of this paper are: bayes-an
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The categories of this paper are: math
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The categories of this paper are: hep-lat
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The categories of this paper are: dg-ga
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The categories of this paper are: supr-con
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The categories of this paper are: solv-int
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The categories of this paper are: atom-ph
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The categories of this paper are: nucl-th
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The categories of this paper are: q-fin
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The categories of this paper are: comp-gas