Tunneling Asymmetry Reveals Atom Alignment in High-Temperature Cuprate Superconductors

JOM, Apr 2007

Researchers at Cornell University have used a highly precise scanning tunneling microscope (STM) to learn why superconductivity-the conduction of'electricity with zero resistance-stalls in certain copper oxides known as cuprates.

Pure cuprates, though normally insulators, become superconductors at temperatures as high as -1250C when doped with small numbers of other atoms. Superconducting was first discovered in metals cooled to-273°C with liquid helium, so by comparison the cuprates offer much higher temperature superconducti vi ty.

Using an STM to study hole-doped cuprate crystals, researchers at Cornell found strong variations in electronic structure with some Cu-O-Cu bonds distributed randomly through the crystal apparently exhibiting holes where electrons are missing. The Cornell researchers studied cuprate crystals in which about 10 percent of the electrons were removed and replaced by holes. At 16 percent hole density, the cuprates display the highest temperature superconductivity of any known material. But if hole density is reduced by just a few percent, the superconductivity vanishes and the materials become highly resistant.

An STM uses an atom-sized tip that moves in atom-sized steps across a surface (Figure 3a). When a voltage is applied between the tip and the surface, a small current known as a tunneling current flows between them. By adjusting the height of the tip above the surface to produce a constant current, researchers can see the shapes of individual atoms. However, according to the Cornell researchers, the technique has limitations in imaging the distribution of holes.

To improve upon past research, the new work compared current flow in opposite directions at each point in the scan. At regions of the crystal containing fewer electrons, and thus, more holes, more electrons can flow down from the tip into the holes than flow up. The process is called tunneling asymmetry, or TA (Figure 3b). Using TA, the researchers imaged two cuprates with different chemistry, crystal structure, and doping characteristics and the results were nearly identical. This was attributed to the spatial arrangement of electrons in the crystal. The areas where TA imaging suggests there are holes appear to be centered on oxygen atoms within the Cu-O-Cu bond.

Over larger areas the holes appear to be arranged in patterns that are rectangular and exactly four crystal lattice spaces wide. These "nanostripes" are aligned with the crystal lattice but otherwise distributed at random. Previous experiments found evidence that long-range patterns of stripes of alternating high and low charge density, spaced four units of the crystal lattice apart, exist in doped cuprates, but no imaging technique had been able to detect them.

"It's plausible that when you increase the number of holes these nanostripes will combine into the orderly stripes seen in other experiments," said J.C. Seamus Davis, Cornell professor of physics who worked on the project with Yuhki Kohsaka. a postdoctoral researcher.

A next step, Davis said, is to use TA imaging on more heavily doped materials that exhibit such stripes to see if they are made of the oxygen-centered holes. The key challenge, he said, is to understand precisely how the process of hole localization into the patterns seen here suppresses superconductivity.

A paper on the work by Kohsaka, Davis, and others was the cover story in the March 9 edition of Science.