Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Internal Transitions of Two-Dimensional Charged Magneto-Excitons X-:
  Theory and Experiment   Internal spin-singlet and spin-triplet transitions of charged excitons X- in
magnetic fields in quantum wells have been studied experimentally and
theoretically. The allowed X- transitions are photoionizing and exhibit a
characteristic double-peak structure, which reflects the rich structure of the
magnetoexciton continua in higher Landau levels (LL's). We discuss a novel
exact selection rule, a hidden manifestation of translational invariance, that
governs transitions of charged mobile complexes in a magnetic field.

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The categories of this paper are: gr-qc
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The categories of this paper are: alg-geom
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The categories of this paper are: mtrl-th
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The categories of this paper are: nlin
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The categories of this paper are: q-alg
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The categories of this paper are: eess
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The categories of this paper are: astro-ph
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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: adap-org
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The categories of this paper are: math-ph
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The categories of this paper are: hep-lat
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The categories of this paper are: comp-gas
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The categories of this paper are: plasm-ph
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The categories of this paper are: acc-phys
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The categories of this paper are: solv-int