Copper-oxide plane at surface of superconductor has surprising properties

The peculiar behavior of high-temperature superconductors has baffled scientists for many years. Now, by imaging the copper-oxide plane in a cuprate superconductor for the first time, researchers at the University of Illinois at Urbana-Champaign have found several new pieces to this important puzzle.

As reported in the Aug. 19 issue of Physical Review Letters, physics professor Ali Yazdani, graduate student Shashank Misra, and colleagues used a scanning tunneling microscope to demonstrate that a single copper-oxide plane can form a stable layer at the superconductor’s surface. This plane behaves differently when exposed at the surface than when buried inside the crystal, the researchers discovered, offering additional insight into the behavior of high-temperature superconductors.

“In contrast to previous studies, we found that this copper-oxide layer exhibits an unusual suppression of tunneling conductance at low energies,” Yazdani said. “We think the orbital symmetry of the plane’s electronic states may be influencing the tunneling process and is responsible for the strange behavior we observed at the surface.”

Surface-sensitive techniques, such as electron tunneling and photoemission, have been crucial in gleaning information about high-temperature superconductors, Yazdani said. But it hasn’t always been clear from which layer the information was coming. By imaging at the atomic scale and probing on the nanoscale, the researchers achieved much higher precision.

“High-temperature superconductors are layered compounds containing one or more copper-oxide planes and other layers that act as charge reservoirs,” Yazdani said. “Like dopants in a semiconductor, these layers donate charge carriers to the copper-oxide planes, making them conducting. The strong electronic interactions in the copper-oxide planes are responsible for the material’s unusual electronic properties.”

To image the surface of thin films of superconducting crystal, Yazdani and his colleagues used 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 electronic properties.

“With the STM, we can send electrons through the tip and measure the rate at which they flow into the surface,” Yazdani said. “We found a very strong contrast in the spectra taken on the two surfaces. The electron tunneling in the copper-oxide plane was strongly suppressed at low energies.”

This behavior is unexpected in a d-wave superconductor, Yazdani said, and could demonstrate the dramatic influence of the layered structure on the surface electronic properties. The observations can best be explained by the way in which the STM tip couples to the electronic states of the copper-oxide plane, the researchers concluded.

“At low energies, electrons from the tip are constrained by the orbital symmetry of the plane’s electronic wave function, which resembles a cloverleaf pattern,” Yazdani said. “This directional dependence of the current can 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, Yazdani said.

“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.”

Having direct access to the surface means scientists can begin manipulating its properties by changing what’s under the surface. The Illinois work also opens a new methodology for probing electrons in the copper-oxide plane.

Collaborators on the project were physics professor James Eckstein, postdoctoral research associate Tiziana DiLuccio, and graduate students Seongshik Oh and Daniel Hornbaker. The National Science Foundation, Office of Naval Research and the U.S. Department of Energy funded the work.