Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Composition gradients and their effects on superconductivity in Al-doped
  MgB2   (Abridged abstract) Alloyed MgB2 differs from pure forms in that diffusion is
needed to distribute the alloying elements homogeneously. Williamson-Hall
analyses of x-ray diffraction peaks showed that Mg1-xAlxB2 samples made by a
typical reaction A had higher crystalline strain than when thoroughly annealed
by reaction B. The strain and other analyses indicate that reaction A produced
substantial Al gradients across the individual grains while reaction B did not.
The gredients skew the apparent superconducting behavior: properties appeared
to be distinct when plotted vs. x (e.g. two Tc(x) curves), but all of the data
merged when analyzed in terms of the unit cell volume v (e.g. one Tc(v) curve).
Since v is derived from x-ray diffraction, it captures the average Al content
actually present inside the grains and better reflects the behavior intrinsic
to the addition of Al. These analyses show that it is important to coordinate
structural and property characterizations to remove artifacts of composition
gradients and uncover the intrinsic trends. Because the standard
characterizations of the superconducting properties above gave no clear
indication that the two sample sets had different homogeneity, the structural
information was vital to make a correct assessment of the effects of Al doping
on superconductivity.

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The categories of this paper are: physics
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The categories of this paper are: plasm-ph
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The categories of this paper are: acc-phys
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The categories of this paper are: chao-dyn
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The categories of this paper are: bayes-an
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The categories of this paper are: econ
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The categories of this paper are: ao-sci
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The categories of this paper are: solv-int
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The categories of this paper are: hep-ex
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The categories of this paper are: chem-ph
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The categories of this paper are: mtrl-th
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The categories of this paper are: eess
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The categories of this paper are: math
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The categories of this paper are: stat
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The categories of this paper are: cs
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The categories of this paper are: supr-con