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تسجيل مستخدم جديدRaman study of Twin Free YBa$_2$Cu$_3$O$_{6.5}$ (Ortho-II) Single Crystals
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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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In this paper we explore whether the quantum oscillation signals recently observed in ortho-II YBa$_2$Cu$_3$O$_{6.5}$ may be explained by conventional density functional band-structure theory. Our calculations show that the Fermi surface of YBa$_2$Cu
$_3$O$_{6.5}$ is extremely sensitive to small shifts in the relative positions of the bands. With rigid band shifts of around 30 meV small tubular pockets of Fermi surface develop around the Y point in the Brillouin zone. The cross-sectional areas and band masses of the quantum oscillatory orbits on these pockets are close to those observed. The difference between the bandstructure of YBa$_2$Cu$_3$O$_{6.5}$ and YBa$_2$Cu$_4$O$_{8}$ are discussed.
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