Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Crystal structure of the Mg$_{1-x}$Al$_x$B$_2$ superconductors near
  x=0.5   Precise structural information on the Mg1-xAlxB2 superconductors in the
vicinity of x 0.5 is derived from high-resolution synchrotron X-ray powder
diffraction measurements. We find that a hexagonal superstructure, accompanied
by doubling of the c-axis, ordering of Mg and Al in alternating layers, and a
shift of the B layers towards Al by ~0.1 A, is formed. The unusually large
width of the (001/2) superlattice peak implies the presence of microstrain
broadening, arising from anisotropic stacking of Al and Mg layers and/or
structural modulations within the ab plane. The ordered phase survives only
over a limited range of compositions away from the optimum x= 0.5 doping level.

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The categories of this paper are: cs
27
The categories of this paper are: comp-gas
16
The categories of this paper are: stat
5
The categories of this paper are: gr-qc
33
The categories of this paper are: plasm-ph
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The categories of this paper are: hep-lat
0
The categories of this paper are: hep-ph
14
The categories of this paper are: math-ph
28
The categories of this paper are: alg-geom
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The categories of this paper are: supr-con
34
The categories of this paper are: chem-ph
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The categories of this paper are: adap-org
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The categories of this paper are: astro-ph
26
The categories of this paper are: bayes-an
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The categories of this paper are: q-fin
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The categories of this paper are: nlin