Scanning tunneling microscopy and spectroscopy of Bi-Sr-Ca-Cu-O 2:2:1:2 high-temperature superconductors

We have used scanning tunneling microscopy and spectroscopy to investigate the surface-topographic and electronic properties of Bi-Sr-Ca-Cu-O 2:2:1:2 compounds. Even though there are two atoms (Bi and O) per lattice point, only one corrugation maximum per lattice point is observed. Polarity-dependent images show that the corrugations of the images taken at opposite polarities are in phase. We discuss possible explanations for this observation of in-phase corrugations at opposite polarities. Spectroscopic data were obtained at both high and low sample biases. Our data show that the density of surface electronic states near the Fermi level is about 3--4 orders of magnitude smaller than that of a typical metal. These states are only detectable when the stabilization voltage of the tunnel junction is low ({lt}1.5 V). The conductivity near zero bias is extremely nonlinear, consistent with a nonmetallic surface layer. Vacuum resonant tunneling studies show that at these low-bias voltages the tip-to-sample distance is very small ({similar to}3--6 A). This small tip-to-sample distance implies that the conductivity we detect near zero bias might result from the underlying CuO layer. We find evidence of bias-field penetration into the sample, implying that the surface density of states near the Fermi level is too small to screen out the electric field.