Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Enhanced Optical Gap in Bi-layered Manganites
  La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ near $x=0.4$   We have systematically investigated the optical conductivity spectra of
La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ ($0.3 \leqslant x \leqslant 0.5$). We
find that just above the magnetic ordering temperatures, the optical gap shows
an enhancement up to $\sim 0.3$ eV near $x=0.4$. Based on a $x$-dependent
comparison of the nesting vector of the hypothetical Fermi surface and the
superlattice wave-vector, we suggest that the peculiar $x$-dependence of the
optical gap can be understood in terms of charge and lattice correlation
enhanced by the charge density wave instability in nested Fermi surface.

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The categories of this paper are: acc-phys
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The categories of this paper are: funct-an
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The categories of this paper are: alg-geom
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The categories of this paper are: hep-ex
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The categories of this paper are: physics
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The categories of this paper are: ao-sci
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The categories of this paper are: econ
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The categories of this paper are: nucl-th
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The categories of this paper are: comp-gas
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The categories of this paper are: chem-ph
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The categories of this paper are: bayes-an
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The categories of this paper are: gr-qc
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The categories of this paper are: cmp-lg
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The categories of this paper are: supr-con
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The categories of this paper are: nlin
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The categories of this paper are: dg-ga