Abstract
Point-contact spectroscopy measurements on several YBa 2Cu 3O 7−δ samples (melt-textured, single crystal, and thin film) reveal Andreev reflection with a rather sharp gap edge as well as strong modulations at yet higher bias. The boson energies obtained by the second derivative d 2 V/d I 2( V) are in good correlation with selected phonon modes which, in turn, were reported to renormalize below the superconducting transition temperature T c. The experimental data suggest a strong electron-phonon interaction; however, YBCO is still qualitatively different from conventional superconductors since the modulations at high energies are extremely strong, and only selected phonon modes are coupled.
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