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Scanning lightly hole-doped cuprate crystals with a
highly precise scanning tunneling microscope (STM) has revealed strong
variations in electronic structure with some copper-oxygen-copper
(Cu-O-Cu) bonds distributed randomly through the crystal apparently
exhibiting "holes" where electrons are missing. The researchers also
found larger rectangular regions with missing electrons that were
spaced four units of the crystal lattice apart, and may represent the
first direct observation of long-sought electronic "stripes" in
cuprates.
Yuhki Kohsaka, a postdoctoral researcher working
with J.C. Séamus Davis, Cornell professor of physics, reported on the
research. A paper on the work by Kohsaka, Davis and others is the
cover story in the March 9 edition of Science.
The superconducting phenomenon was first discovered
in metals cooled to less than about 4 degrees Celsius above absolute
zero (-273 degrees Celsius or -459 degrees Fahrenheit) with liquid
helium. Recently, superconductivity at much higher temperatures was
discovered in cuprates. Pure cuprates are normally insulators, but
when doped with small numbers of other atoms they become
superconductors at temperatures as high as 148 degrees above absolute
zero (-125 Celsius). The impurities break up the orderly crystal
structure and create "holes" where electrons ought to be.
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 precipitously and the materials become highly resistant.
Previous experiments have given 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.
An STM uses an atom-sized tip that moves in
atom-sized steps across a surface. 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. And with the exceptional precision of the STM
operated by Davis and colleagues at Cornell, the spatial arrangement
of electronic states can be imaged. However, the researchers explain
in their paper, this technique has serious limitations in imaging the
distribution of holes.
The innovation in the new research, based on a
suggestion by Nobel laureate Philip W. Anderson, professor emeritus at
Princeton University, is to compare current flow in opposite
directions at each point in the scan. In simple terms, at regions of
the crystal containing fewer electrons (more holes), more electrons
can flow down from the tip into these voids than up. The process is
called TA-imaging, for tunneling asymmetry.
The Cornell researchers studied cuprate crystals in
which about 10 percent of the electrons in the crystal lattice were
removed and replaced by holes. The researchers imaged two cuprates
with very different chemistry, crystal structure and doping
characteristics and found virtually identical results, which they
attribute entirely to the spatial arrangement of electrons in the
crystal. The areas where TA-imaging suggests that there are holes
appear to be centered on oxygen atoms within the Cu-O-Cu bond. This is
what has long been expected based on X-ray scattering studies. But "the
big surprise," Davis said, "is that when you map this stuff for large
distances across the surface no orderly patterns are observed. We had
no picture of this before." Perhaps even more exciting, he said, is
the discovery that over larger areas the holes do appear to be
arranged in patterns that are rectangular and exactly four crystal
lattice spaces wide. These so called "nanostripes" are aligned with
the crystal lattice but otherwise distributed at random.
"It's plausible that when you increase the number
of holes these 'nanostripes' will combine into the orderly stripes
seen in other experiments," Davis said. A next step, he said, is to
use TA-imaging on more heavily doped materials that exhibit such
stripes to see if they are made up of these oxygen-centered holes. But
the key challenge, he added, is to understand precisely how the
process of hole localization into the patterns seen here suppresses
superconductivity. |
