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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 80and 300 K. All eleven $A_g$ Raman modes expected for the Ortho-II structure aswell some modes of $B_{2g}$ and $B_{3g}$ symmetry were identified in closecomparison with predictions of lattice dynamical calculations. The electronicscattering from the $ab$ planes is strongly anisotropic and decreases between200 and 100 K within the temperature range of previously reported pseudogapopening. The coupling of phonons to Raman active electronic excitationsmanifested by asymmetric (Fano) profiles of several modes also decreases in thesame range. Among the new findings that distinguish the Raman scattering ofOrtho-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 apexoxygen $A_g$ mode.

Raman study of Twin Free YBa$_2$Cu$_3$O$_{6.5}$ (Ortho-II) Single Crystals