Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Sine-Gordon description of Beresinskii-Kosterlitz-Thouless physics at
  finite magnetic field   The Beresinskii-Kosterlitz-Thouless (BKT) physics of vortices in
two-dimensional superconductors at finite magnetic field is investigated by
means of a field-theoretical approach based on the sine-Gordon model. This
description leads to a straightforward definition of the field-induced
magnetization and shows that the persistence of non-linear effects at low
fields above the transition is a typical signature of the fast divergence of
the correlation length within the BKT theory.

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The categories of this paper are: supr-con
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The categories of this paper are: atom-ph
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The categories of this paper are: dg-ga
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The categories of this paper are: eess
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The categories of this paper are: comp-gas
9
The categories of this paper are: nlin
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The categories of this paper are: mtrl-th
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The categories of this paper are: nucl-ex
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The categories of this paper are: chem-ph
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The categories of this paper are: gr-qc
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The categories of this paper are: hep-th
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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: q-bio
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The categories of this paper are: solv-int
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The categories of this paper are: chao-dyn