Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Raman study of Twin Free YBa$_2$Cu$_3$O$_{6.5}$ (Ortho-II) Single
  Crystals   The polarized Raman scattering spectra from freshly cleaved $ab$, $ac$, and
$bc$ surfaces of high quality twin free YBa$_2$Cu$_3$O$_{6.5}$ (Ortho-II)
single crystals ($T_c$=57.5 K and $\Delta T = 0.6$ K) were studied between 80
and 300 K. All eleven $A_g$ Raman modes expected for the Ortho-II structure as
well some modes of $B_{2g}$ and $B_{3g}$ symmetry were identified in close
comparison with predictions of lattice dynamical calculations. The electronic
scattering from the $ab$ planes is strongly anisotropic and decreases between
200 and 100 K within the temperature range of previously reported pseudogap
opening. The coupling of phonons to Raman active electronic excitations
manifested by asymmetric (Fano) profiles of several modes also decreases in the
same range. Among the new findings that distinguish the Raman scattering of
Ortho-II from that of Ortho-I phase is the unusual relationship ($\alpha_{xx}
\approx -\alpha_{yy}$) between the elements of the Raman tensor of the apex
oxygen $A_g$ mode.

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The categories of this paper are: gr-qc
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The categories of this paper are: hep-th
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The categories of this paper are: math
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The categories of this paper are: q-bio
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The categories of this paper are: q-alg
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The categories of this paper are: cmp-lg
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The categories of this paper are: ao-sci
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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: stat
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The categories of this paper are: solv-int
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The categories of this paper are: astro-ph
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The categories of this paper are: cs
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The categories of this paper are: q-fin
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The categories of this paper are: hep-ex
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The categories of this paper are: math-ph