Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Bosonic stimulation of cold 1s excitons into a harmonic potential
  minimum in Cu$_2$O   Density distribution of cold exciton clouds generated into a strain-induced
potential well by two-photon excitation in Cu$_2$O is studied at 2 K. We find
that an anomalous spike, which can be interpreted as accumulation of the
excitons into the ground state, emerges at the potential minimum. The
accumulation can be due to stimulated scattering of cold excitons, mediated by
acoustic phonon emission. Possibility of the formation of the thermodynamic
Bose-Einstein condensate of paraexcitons has been discussed.

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The categories of this paper are: physics
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The categories of this paper are: mtrl-th
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The categories of this paper are: q-bio
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The categories of this paper are: dg-ga
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The categories of this paper are: solv-int
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The categories of this paper are: gr-qc
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The categories of this paper are: cs
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The categories of this paper are: acc-phys
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The categories of this paper are: bayes-an
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The categories of this paper are: stat
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The categories of this paper are: supr-con
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The categories of this paper are: nucl-ex
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The categories of this paper are: nlin
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The categories of this paper are: plasm-ph
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The categories of this paper are: comp-gas
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The categories of this paper are: patt-sol