Photoemission studies of the high-$T_c$ superconductor ${\mathrm{Ba}}_2$${\mathrm{YCu}}_3$${\mathrm{O}}_{9\mathrm{-}\mathrm{\delta }}$

Valence-band photoemission measurements have been made both above and below the $T_c$ of ${\mathrm{Ba}}_2$ ${\mathrm{YaCu}}_3$ ${\mathrm{O}}_{9\mathrm{-}\mathrm{\delta }}$ where δ=2.1. We find similarities to recent calculations of the electronic properties. However, the experimental binding energies are between 1.0 and 1.4 eV further from the Fermi level than the calculation. We suggest that this difference may affect attempts to predict $T_c$ from tight-binding modeling of the electron-phonon mechanism.