Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Optical spectroscopy on the photo-response in multiferroic BiFeO$_3$ at
  high pressure   The pressure dependence of light-induced effects in single-crystalline
BiFeO$_3$ is studied by optical spectroscopy. At low pressures, we observe
three light-induced absorption features with energies just below the two
crystal field excitations and the absorption onset, respectively. These
absorption features were previously ascribed to excitons, possibly connected
with the ultra-fast photostriction effect in BiFeO$_3$. The pressure-induced
redshift of the absorption features follows the pressure dependence of the
corresponding crystal field excitations and absorption onset, suggesting the
link between them. Above the structural phase transition at
$P_{\mathrm{c1}}\approx{}3.5$ GPa the three absorption features disappear,
suggesting their connection to the polar phase in BiFeO$_3$. The
pressure-induced disappearance of the photo-induced features is irreversible
upon pressure release.

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The categories of this paper are: astro-ph
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The categories of this paper are: patt-sol
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The categories of this paper are: mtrl-th
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The categories of this paper are: acc-phys
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The categories of this paper are: econ
29
The categories of this paper are: funct-an
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The categories of this paper are: nlin
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The categories of this paper are: gr-qc
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The categories of this paper are: math
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The categories of this paper are: comp-gas
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The categories of this paper are: dg-ga
32
The categories of this paper are: atom-ph
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The categories of this paper are: hep-lat
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The categories of this paper are: hep-ex
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The categories of this paper are: math-ph
31
The categories of this paper are: ao-sci