Atomic Scale Imaging and Spectroscopy of a $\mathrm{C}\mathrm{u}{\mathrm{O}}_{\mathrm{2}}$ Plane at the Surface of $\mathrm{B}{\mathrm{i}}_{\mathrm{2}}\mathrm{S}{\mathrm{r}}_{\mathrm{2}}\mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{u}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{8}\mathrm{+}\mathrm{\delta }}$

We have used a scanning tunneling microscope to demonstrate that a single $\mathrm{C}\mathrm{u}{\mathrm{O}}_{\mathrm{2}}$ plane can form a stable and atomically ordered layer at the surface of $\mathrm{B}{\mathrm{i}}_{\mathrm{2}}\mathrm{S}{\mathrm{r}}_{\mathrm{2}}\mathrm{C}\mathrm{a}\mathrm{C}{\mathrm{u}}_{\mathrm{2}}{\mathrm{O}}_{\mathrm{8}\mathrm{+}\mathrm{\delta }}$. In contrast to previous studies on high-$T_c$ surfaces, the $\mathrm{C}\mathrm{u}{\mathrm{O}}_{\mathrm{2}}$-terminated surface exhibits a strongly suppressed tunneling conductance at low voltages. We consider a number of different explanations for this phenomena and propose that it may be caused by how the orbital symmetry of the $\mathrm{C}\mathrm{u}{\mathrm{O}}_{\mathrm{2}}$ plane’s electronic states affects the tunneling process.

Figure 1

(A) A $100\AA \times 100\AA$ STM topograph of a step edge, rendered to display the atomic corrugation on both the top (right) and bottom (left) of the step ($V=-200\mathrm{m}\mathrm{V}$ and $I=50\mathrm{p}\mathrm{A}$). (B) Average of the topographic line scans in (A) and the known relative distances between the BiO plane and other crystallographic planes.

Figure 2

The STM spectra measured at the indicated distance perpendicular to the step edge shown in Fig. 1a, offset for clarity. The topmost and bottommost spectra are taken on the BiO and $\mathrm{C}\mathrm{u}{\mathrm{O}}_{\mathrm{2}}$ planes, respectively, far away from the step edge.

Figure 3

(A) A $100\AA \times 100\AA$ STM topograph of the BiO plane. (B) STM topographs of the $\mathrm{C}\mathrm{u}{\mathrm{O}}_{\mathrm{2}}$ terminated surface. Main image ($64\AA \times 64\AA$) and the inset ($8\AA \times 8\AA$). (C) STM spectra of the BiO plane taken at the indicated distance along a straight line (offset for clarity). (D) STM conductance spectra taken along a straight line, at the indicated distances, on the $\mathrm{C}\mathrm{u}{\mathrm{O}}_{\mathrm{2}}$ plane (offset for clarity). All data taken with $V=200\mathrm{m}\mathrm{V}$ and $I=200\mathrm{p}\mathrm{A}$.

Figure 4

(A) The relevant orbital in the $\mathrm{C}\mathrm{u}{\mathrm{O}}_{\mathrm{2}}$ plane: O $2p$ (blue and purple), Cu $3d$ (brown and blue), and Cu $4s$ (yellow). A model of tunneling matrix elements based on these orbitals is introduced in the text. (B) Comparison of the model with tunneling spectra of the $\mathrm{C}\mathrm{u}{\mathrm{O}}_{\mathrm{2}}$ plane (red) for parameter values $\Delta =60\mathrm{m}\mathrm{V}$ and $\sigma \sim 12\mathrm{m}\mathrm{V}$.