Berlin 2008 – wissenschaftliches Programm

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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 YBa₂Cu₃O_6+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 T_c 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 YBa₂Cu₃O_6.95 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 T_c, q⁻¹ changes drastically and in a distinguishable behavior for the a- and b-axes. For instance the B_1g-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-T_c superconductors.