Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The antiferromagnetic spin 1/2 chain with competing dimers and
  plaquettes: Numerical versus exact results   We examine the ground state and the excitations of an one-dimensional
Heisenberg spin 1/2 antiferromagnet with alternating dimers and four-spin
plaquettes (dimer-plaquette chain). The properties of the system depend on the
competing dimer and plaquette bonds. Several exact, exact-numerical and
perturbational results are presented. We find that the system is gapped for all
parameter values. For the latter one we find an effective S=1 Haldane like
behaviour in the limit of dominating dimer bonds. The model considered is
related to the recently found 1/5-depleted square-lattice Heisenberg system
$CaV_4O_9$.

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The categories of this paper are: chao-dyn
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The categories of this paper are: physics
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The categories of this paper are: acc-phys
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The categories of this paper are: hep-lat
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The categories of this paper are: adap-org
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The categories of this paper are: alg-geom
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The categories of this paper are: q-fin
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The categories of this paper are: hep-ph
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The categories of this paper are: math
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The categories of this paper are: ao-sci
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The categories of this paper are: stat
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The categories of this paper are: q-bio
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The categories of this paper are: atom-ph
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The categories of this paper are: patt-sol
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The categories of this paper are: eess
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The categories of this paper are: solv-int