Key features in the measured band structure of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$: Flat bands at ${\mathit{E}}_{\mathit{F}}$ and Fermi surface nesting

The near-${\mathit{E}}_{\mathit{F}}$ electronic structure and Fermi surface of ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_{8+\mathrm{\delta }}$ have been mapped out with angle-resolved photoemission. Key features of the measured band structure are (1) an extended region of flat ${\mathrm{CuO}}_2$ derived bands very near ${\mathit{E}}_{\mathit{F}}$ and (2) a strong propensity for Fermi surface nesting. Comparative analysis of these data with those from other cuprate superconductors suggests that these features may be responsible for many of the anomalous physical properties of the p-type cuprates, while the absence of these features may be related to the more ‘‘normal’’ physical properties of the n types.