Angle-resolved photoelectron spectra of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ and their line-shape analysis

We measure the high-resolution photoelectron spectra of twinned ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ single crystals cleaved at about 10 K in ultrahigh vacuum perpendicular to the c axis for various angles of detection along the Γ-X(Y) azimuth. The spectra in general and the 1-eV peak in particular strongly depend on the cleavage plane investigated, with correlated changes observable in the low-energy-electron-diffraction pattern: For the surface showing a distorted c(2×2) superlattice the 1-eV surface-state peak is missing; the electronic structure at this surface seems to show the smallest deviations from that of the bulk. The spectral features measured at this surface can be assigned to at least four different branches of the calculated energy bands. A quadratic dependence of the linewidth on the binding energy, a symmetrical line shape of the spectra, and the identification of a clear Fermi edge above ${\mathit{T}}_{\mathit{c}}$ also support a Fermi-liquid description. At temperatures well below ${\mathit{T}}_{\mathit{c}}$ the spectra of this surface show some indications of BCS-type modifications close to ${\mathit{E}}_{\mathit{F}}$, while no hint for them could be detected for the numerous cleavage planes showing the 1-eV peak.