Origin of the electronic states near the Fermi level in high-Tcsuperconductors

Abstract

We have studied superconducting and insulating Cu-oxide systems by photoemission spectroscopy. The emission feature near the Fermi level $E_F$ for ${\mathrm{Bi}}_2$(Sr,Ca${)}_3$ ${\mathrm{Cu}}_2$ ${\mathrm{O}}_{8+\mathrm{\delta }}$ remains unchanged when hole carriers are depleted by Y substitution for Ca. A similar feature is observed also for insulating ${\mathrm{Sr}}_2$ ${\mathrm{CuO}}_2$ ${\mathrm{Cl}}_2$. These facts suggest that the feature near $E_F$ in the Bi compounds is due to states which are split off out of the oxygen p band, possibly through hybridization with Cu $d^8$ states, and cannot be due to a Kondo-type resonance and resulting renormalized heavy-electron band states. Other origins for the states near $E_F$ in ${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$ ${\mathrm{Sr}}_{\mathrm{x}}$ ${\mathrm{CuO}}_4$ and ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ are also discussed.