Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Excitonic Instability in the Transition from the Black Phase to the
  Golden Phase of SmS under Pressure Investigated by Infrared Spectroscopy   We report the pressure-dependent optical reflectivity spectra of a strongly
correlated insulator, samarium monosulfide (SmS), in the far- and
middle-infrared regions to investigate the origin of the pressure-induced phase
transition from the black phase to the golden phase. The energy gap becomes
narrow with increasing pressure in the black phase. A valence transition from
Sm2+ in the black phase to mainly Sm3+ in the golden phase accompanied by
spectral change from insulator to metal were observed at the transition
pressure of 0.65 GPa. The black-to-golden phase transition occurs when the
energy gap size of black SmS becomes the same as the binding energy of the
exciton at the indirect energy gap before the gap closes. This result indicates
that the valence transition originates from an excitonic instability.

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The categories of this paper are: plasm-ph
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The categories of this paper are: bayes-an
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The categories of this paper are: hep-ph
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The categories of this paper are: econ
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The categories of this paper are: q-bio
3
The categories of this paper are: physics
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The categories of this paper are: solv-int
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The categories of this paper are: atom-ph
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The categories of this paper are: funct-an
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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: dg-ga
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The categories of this paper are: astro-ph
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The categories of this paper are: acc-phys
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The categories of this paper are: cmp-lg
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The categories of this paper are: nlin