First Atomic-Scale Imaging of a CuO2 Plane,

First Atomic-Scale Imaging of a
CuO₂ Plane in a Superconductor

The peculiar behavior of high-temperature superconductors has baffled scientists for many years. Many of the unusual characteristics of these compounds are believed to arise from the behavior of electrons in the crystallographic planes of these compounds, which consist of copper and oxygen atoms. In a paper appearing in the August 19, 2002, issue of Physical Review Letters (S. Misra, S. Oh, D. J. Hornbaker, T. DiLuccio, J. N. Eckstein, and A. Yazdani, "Atomic Scale Imaging and Spectroscopy of a CuO₂ Plane at the Surface of Bi₂Sr₂CaCu₂O_8+d," Phys. Rev. Lett. 89, 087002-1 [2002]), physicists at the University of Illinois at Urbana-Champaign have for the first time imaged a single copper-oxide plane at the surface of one of the high-Tc cuprates. This work has not only uncovered some surprises in the nature of electronic states in these planes, but it has also created a new methodology for probing electrons in these unusual materials.

The work has been a collaborative effort between the research groups of Professor Ali Yazdani and Professor James Eckstein. Using atomically engineered molecular beam epitaxy (MBE) samples grown by Eckstein's group, Yazdani and his graduate student, Shashank Misra, have been able to demonstrate that a single copper-oxide plane can form a stable layer at a superconductor's surface.

To image the surface of thin films of a superconducting crystal, Yazdani's group uses a low-temperature scanning tunneling microscope that they built at Illinois. By exploring large areas of the sample and correlating the STM topographic images with X-ray crystallographic data, the researchers were able to identify individual layers of copper oxide and of bismuth oxide and then measure their discrete electronic properties. To their surprise, they found that these two surfaces exhibit very different electronic properties. In particular, they find that the rate of electron tunneling into a CuO₂ plane at the surface is strongly suppressed at low energies—unexpected behavior in a d-wave superconductor that demonstrates the dramatic influence of the layered structure on the surface electronic properties.

To understand their findings, the Illinois researchers consider a model of how electrons from an STM tip could couple with the atomic orbitals of the CuO₂ plane (see illustration above). The paper describes how the orbital symmetry of the CuO₂ plane, which resembles a cloverleaf, could strongly constrain the electrons' tunneling from the STM tip. At low energies, electrons from the tip are constrained by the orbital symmetry of the plane's electronic wave function; this directional dependence of the current could explain the suppressed tunneling.

Previous measurements had been performed on surfaces terminated with other layers—bismuth oxide, for example—where the copper-oxide plane was buried under the surface. In those experiments, however, it was not apparent how the STM tip was coupling to the copper-oxide plane. "You could theorize that the other layers had no effect on the measurement, but that flies in the face of our experiment," Yazdani said. "From our results, it is clear that what you put at the surface makes a huge difference in what you measure."

Other collaborators on the project were graduate students Seongshik Oh, postdoctoral research associate Tiziana DiLuccio, and Daniel Hornbaker. The work was funded by the National Science Foundation, Office of Naval Research and the U.S. Department of Energy.