STM images of a superconducting Cu-O plane and the corresponding tunneling spectrum in ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$${\mathrm{CaCu}}_2$${\mathrm{O}}_{8+\mathrm{\delta }}$

Clear atomic images of ${\mathrm{Bi}}_2$ ${\mathrm{Sr}}_2$ ${\mathrm{CaCu}}_2$ ${\mathrm{O}}_{8+\mathrm{\delta }}$ cleaved surfaces have been observed at T=6 and 300 K by scanning tunneling microscopy (STM). The atomic images taken for bias voltages much lower than the Bi-O plane semiconducting gap ${\mathit{E}}_{\mathrm{g}}$∼100 meV, corresponding to the Cu-O plane, indicate that the conduction electrons exist mainly in the Cu 3${\mathit{d}}_{{\mathit{x}}^2\mathrm{-}{\mathit{y}}^2}$ and O 2${\mathit{p}}_{\mathrm{\sigma }}$ orbitals. Tunneling spectra have been also measured in the same processes as in the Cu-O plane STM image observations at T=6 K. The low-temperature spectra are in good agreement with that in a d-wave superconductor with an anisotropic Fermi surface on which the normal density of states N(${\mathbf{k}}_{\mathit{F}}$) is largest (${\mathit{N}}_{\max }$) for the maximum gap directions and decreases to ∼${\mathit{N}}_{\max }$/2 for the node directions. This, combined with the result on the N(${\mathbf{k}}_{\mathit{F}}$) anisotropy in photoemission experiments, is consistent with a ${\mathit{d}}_{{\mathit{x}}^2\mathrm{-}{\mathit{y}}^2}$ superconducting gap. © 1996 The American Physical Society.