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TT: Fachverband Tiefe Temperaturen
TT 29: Superconductivity: Mechanisms, Phase Diagram, Competing Order
TT 29.6: Vortrag
Donnerstag, 28. Februar 2008, 10:45–11:00, H 3010
Phonon anomalies in detwinned YBa2Cu3O6+x: Evidence for in-plane anisotropy in the electronic system — •Mohammed Bakr, Clemens Ulrich, Chengtian Lin, Manuel Cardona, and Bernhard Keimer — Max Planck Institute, Stuttgart, Germany
The pairing symmetry of high Tc superconductors is predominantly of d-wave character with a probable small s-wave component. In detwinned YBCO, this results in an ab-anisotropy of the 2Δ gaps. Different experiments have revealed controversial ab- discrepancies in the 2Δ gaps and therefore the s-wave contribution. Raman light scattering (RLS) allows us to probe the electronic signals directly and also anisotropies in the phonon lineshapes. We have performed RLS experiments on detwinned YBa2Cu3O6.95 single crystals using different laser lights. Within the experimental error of 4 meV, the electronic gaps along a- and b- axes show no differences. Phonons are quite sensitive to changes in the electronic system. Therefore, some phonons show a pronounced Fano-type line shape which implies a strong electron-phonon coupling. In the normal state, the Fano asymmetry parameter q−1 shows a sizable ab-anisotropy which can be understood by basic band structure calculations. Below Tc, q−1 changes drastically and in a distinguishable behavior for the a- and b-axes. For instance the B1g-like mode at 340 cm−1 shows a substantial ab-dissimilarity whereas the apical oxygen phonon at 501 cm−1 shows the largest anisotropy. Furthermore, we have found that the anisotropy in the q−1 is highly dependent on the exciting laser energy. Our results shed more light on the strong electronic correlations in high-Tc superconductors.