Oxygen doping ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$ superconductors: Variations in the BiO-layer electronic states determined by scanning tunneling microscopy

Scanning tunneling spectroscopy (STS) and microscopy (STM) have been used to elucidate the low-energy electronic states and structure of the BiO layer in high-pressure oxygen-annealed ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_{8+\mathrm{\delta }}$ single crystals. Our STS measurements demonstrate that oxygen doping increases the BiO layer density of states (DOS) near the Fermi level. Atomic resolution images indicate, however, that the BiO layer may not become metallic. These results provide a picture of the electronic changes in the BiO layer due to oxygen doping and elucidate the relationship between variations in the BiO layer DOS and ${\mathit{T}}_{\mathit{c}}$.