Raman study of Twin Free YBa2Cu3O6.5 (Ortho-II) Single Crystals

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 eleven Ag Raman modes expected for the Ortho-II structure as well some modes of B2g and B3g 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 (αxx ≈ -αyy) between the elements of the Raman tensor of the apex oxygen Ag mode.