Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Observation of the universal jump across the
  Berezinskii-Kosterlitz-Thouless transition in two-dimensional Bose gases   The physics in two-dimensional (2D) systems is very different from what we
observe in three-dimensional (3D) systems. Thermal fluctuations in 2D systems
are enhanced, and they prevent the conventional Bose-Einstein condensation
(BEC) at non-zero temperatures by destroying the long-range order. However, a
phase transition to a superfluid phase is still expected to occur in a 2D
system along with an emergence of a quasi-long-range order, explained by the
Berezinskii-Kosterlitz-Thouless (BKT) mechanism. Within the BKT mechanism, a
universal jump of the superfluid density in a 2D Bosonic system was
theoretically predicted by Nelson and Kosterlitz, and was first observed in 2D
\textsuperscript{4}He films by Bishop and Reppy. Recent experiments in trapped
ultracold 2D Bose gas systems have shown signatures of the BKT transition, and
its superfluidity. However, the universal jump in the superfluid density was
not observed in these systems. Here we report the first observation of the
universal jump in the superfluid density using an optically trapped ultracold
2D Bose gas. The measured superfluid phase space density at the BKT transition
agrees well with the predicted value within our measurement uncertainty.
Additionally, we measure the phase fluctuations in our density profiles to show
that the BKT transition occurs first, followed by the BEC transition.

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The categories of this paper are: plasm-ph
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The categories of this paper are: dg-ga
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The categories of this paper are: nucl-ex
6
The categories of this paper are: astro-ph
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The categories of this paper are: hep-ex
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The categories of this paper are: adap-org
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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: chao-dyn
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The categories of this paper are: alg-geom
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The categories of this paper are: econ
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The categories of this paper are: cmp-lg
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The categories of this paper are: chem-ph
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The categories of this paper are: hep-ph
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The categories of this paper are: nlin
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The categories of this paper are: eess