Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Pressure induced emission enhancement and bandgap narrowing:
  experimental investigations and first principles theoretical simulations on a
  model halide perovskite   We report high-pressure photoluminescence, Raman scattering, and x-ray
diffraction measurements on a lead-free halide perovskite $Cs_3Sb_2Br_9$. At
about 3 GPa, an electronic transition manifests itself through a broad minimum
in linewidth, a maximum in the intensity of $E_g$, $A_{1g}$ Raman modes, and
the unusual change in the $c/a$ ratio of the trigonal lattice. The large
compressibility and observed Raman anomalies indicate to a soft material with
strong electron-phonon coupling. The observed below bandgap broadband emission
in the photoluminescence measurement indicates the recombination of
self-trapped excitons. The initial blueshift of the photoluminescence peak
reinforces itself to the redshift at around 3 GPa due to the change in the
electronic landscape. A first order trigonal to a monoclinic structural
transition is also seen at 8 GPa. The first-principles density functional
theory (DFT) calculations reveal that the electronic transition is associated
with direct-to-indirect bandgap transition due to changes in the hybridization
of $Sb-5s$ and $Br-4p$ orbitals near the Fermi level in the valence band. The
experimentally observed Raman modes are assigned to their symmetry using the
density functional perturbation theory. In addition, the DFT calculations
predict a 27.5\% reduction of the bandgap in the pressure range 0-8 GPa.

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The categories of this paper are: q-alg
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The categories of this paper are: econ
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The categories of this paper are: q-fin
10
The categories of this paper are: q-bio
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The categories of this paper are: adap-org
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The categories of this paper are: hep-th
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The categories of this paper are: supr-con
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The categories of this paper are: chao-dyn
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The categories of this paper are: hep-ex
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The categories of this paper are: nlin
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The categories of this paper are: ao-sci
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The categories of this paper are: eess
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The categories of this paper are: mtrl-th
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The categories of this paper are: hep-ph
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The categories of this paper are: chem-ph
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The categories of this paper are: plasm-ph