Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Crystal structure and phase transitions across the metal-superconductor
  boundary in the SmFeAsO1-xFx (0 < x < 0.20) family   The fluorine-doped rare-earth iron oxyarsenides, REFeAsO1-xFx (RE =rare
earth) have recently emerged as a new family of high-temperature
superconductors with transition temperatures (Tc) as high as 55 K (refs 1-4).
Early work has provided compelling evidence that the undoped parent materials
exhibit spin-density-wave (SDW) antiferromagnetic order and undergo a
structural phase transition from tetragonal to orthorhombic crystal symmetry
upon cooling.5 Both the magnetic and structural instabilities are suppressed
upon doping with fluoride ions before the appearance of superconductivity.6,7
Here we use high-resolution synchrotron X-ray diffraction to study the
structural properties of SmFeAsO1-xFx (0 < x < 0.20) in which superconductivity
emerges near x ~ 0.07 and Tc increases monotonically with doping up to x ~
0.20.8 We find that orthorhombic symmetry survives through the
metal-superconductor boundary well into the superconducting regime 2 and the
structural distortion is only suppressed at doping levels, x > 0.15 when the
superconducting phase becomes metrically tetragonal. Remarkably this crystal
symmetry crossover coincides with reported drastic anomalies in the resistivity
and the Hall coefficient8 and a switch of the pressure coefficient of Tc from
positive to negative,9 thereby implying that the low-temperature structure
plays a key role in defining the electronic properties of these
superconductors.

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The categories of this paper are: cmp-lg
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The categories of this paper are: stat
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The categories of this paper are: plasm-ph
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The categories of this paper are: hep-th
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The categories of this paper are: patt-sol
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The categories of this paper are: hep-ph
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The categories of this paper are: alg-geom
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The categories of this paper are: econ
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The categories of this paper are: chao-dyn
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The categories of this paper are: dg-ga
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The categories of this paper are: hep-ex
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The categories of this paper are: nucl-th
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The categories of this paper are: quant-ph
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The categories of this paper are: astro-ph
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The categories of this paper are: solv-int
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The categories of this paper are: ao-sci