EXPERIMENTAL-EVIDENCE FOR STRONG ELECTRON-PHONON COUPLING TO SELECTEDPHONON MODES IN POINT-CONTACT SPECTROSCOPY OF YBA2CU3O7-DELTA

Point-contact spectroscopy measurements on several YBa2Cu3O7-delta sam ples (melt-textured, single crystal, and thin film) reveal Andreev ref lection 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/dI(2)(V) are in good correlation with selected phonon modes whic h, in turn, were reported to renormalize below the superconducting tra nsition temperature T-c. The experimental data suggest a strong electr on-phonon interaction; however, YBCO is still qualitatively different from conventional superconductors since the modulations at high energi es are extremely strong, and only selected phonon modes are coupled.