Abstract
The polarized Raman scattering spectra from freshly cleaved ab, ac, and bc surfaces of high quality twin-free YBa2Cu3O6.5 ortho-II single crystals Tc=57.5 K and T=0.6 K were studied between 80 and 300 K. All 11 Ag Raman modes expected for the ortho-II structure as well as some modes of B2g and B3g symmetries were identified in close comparison to 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 a previously reported pseudogap opening. The coupling of phonons to Raman active electronic excitations manifested by asymmetric Fano profiles of several modes also decreases within the same range. Among the findings that distinguish the Raman scattering of the ortho-II phase from that of the ortho-I phase is the unusual relationship xx − yy between the elements of the Raman tensor of the apex oxygen Ag mode.
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