Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Competition between Charge Ordering and Superconductivity in Layered
  Organic Conductors $\alpha$-(BEDT-TTF)$_2M$Hg(SCN)$_4$ (M = K, NH$_4$)   While the optical properties of the superconducting salt
$\alpha$-(BEDT-TTF)$_2$NH$_4$Hg(SCN)$_4$ remain metallic down to 2 K, in the
non-superconducting K-analog a pseudogap develops at frequencies of about 200
cm$^{-1}$ for temperatures T < 200 K. Based on exact diagonalisation
calculations on an extended Hubbard model at quarter-filling we argue that
fluctuations associated with short range charge ordering are responsible for
the observed low-frequency feature. The different ground states, including
superconductivity, are a consequence of the proximity of these compounds to a
quantum phase charge-ordering transition driven by the intermolecular Coulomb
repulsion.

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The categories of this paper are: gr-qc
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The categories of this paper are: funct-an
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The categories of this paper are: math
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The categories of this paper are: cmp-lg
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The categories of this paper are: q-bio
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The categories of this paper are: hep-ph
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The categories of this paper are: hep-ex
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The categories of this paper are: acc-phys
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The categories of this paper are: q-fin
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The categories of this paper are: ao-sci
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The categories of this paper are: physics
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The categories of this paper are: cs
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The categories of this paper are: nucl-ex
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The categories of this paper are: eess
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The categories of this paper are: stat
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The categories of this paper are: solv-int