Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Transport properties and electronic states of the thermoelectric oxide
  NaCo_2O_4   Physical properties of the thermoelectric oxide NaCo_2O_4 are briefly
reviewed. The high thermoelectric properties of this material are attributed to
the substantially enhanced effective mass, which comes from the large entropy
of Co$^{4+}$ in the low spin state. The large entropy confined in the CoO_2
block causes a spin-density-wave transition at 22 K upon Cu substitution for
Co, which can be regarded as ``order from disorder''.

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The categories of this paper are: q-alg
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The categories of this paper are: nucl-th
1
The categories of this paper are: cs
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The categories of this paper are: cmp-lg
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The categories of this paper are: q-fin
3
The categories of this paper are: physics
9
The categories of this paper are: nlin
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The categories of this paper are: adap-org
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The categories of this paper are: astro-ph
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The categories of this paper are: eess
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The categories of this paper are: alg-geom
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The categories of this paper are: dg-ga
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The categories of this paper are: acc-phys
16
The categories of this paper are: stat
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The categories of this paper are: hep-lat
13
The categories of this paper are: quant-ph