Arlington, Virginia – Researchers funded by the Office of Naval Research have developed an imaging technique that gives them a direct view of electron distribution in superconducting layered copper oxide ceramics. Examples of these images appear on the cover of the March 9 issue of Science, and their research is published in this issue. The images, which map the quantum mechanical probability of adding or removing electrons at specific positions and energies, provide an important clue to solving the 20-year-old puzzle of how electrons can travel so freely through these so-called "high temperature superconductors."
J. C. Séamus Davis and Yuhki Kohsaka of Cornell University´s Department of Physics led the team that produced these images using a technique called atomic-resolution tunneling-asymmetry imaging. They studied two very different cuprate materials that shared one common characteristic—the intrinsic electronic structure of the copper oxide planes in their crystal structures.
High temperature cuprate superconductors are formed by sandwiching flat layers of copper–oxygen networks between blocks composed of other oxides, which can take on a wide variety of chemical compositions. The resulting layered solids do not conduct appreciable electrical currents until impurity elements with fewer electrons are added to the copper–oxide layers. The resulting "electron holes" enable electrical currents to flow through these materials without any electrical resistance. Precisely how this happens has been a matter of some controversy.
Davis´ team found that at low doping concentrations, electron holes are partially localized in an electronic "glass" within the cuprate structure. At higher hole densities, the holes delocalize completely, enabling the onset of superconductivity. They tracked electronic correlation changes induced by hole doping by producing scanning tunneling microscope (STM) images of electron tunneling current asymmetries over an area of the material´s surface. The images represent a mapping of the ratios of filled and empty electronic states for given locations and energies. They set up their experimental conditions in such a way that the unmeasurable effects due to the tunneling matrix elements, tunnel-barrier height, and z variations from electronic heterogeneity are cancelled out in the calculations.
They found that cuprate antiferromagnetism disappears at a density of about 2–3 doped holes per hundred CuO2 units. At intermediate hole concentrations, the electronic configuration may be thought of as an "electronic cluster glass" that gradually gives way to superconductivity as the hole density reaches 5–10%.