Imaging 'Gridlock' in High-temperature Superconductors
Date: Monday, March 05, 2007 @ 20:32:14 EST
Superconductivity -- the conduction of electricity with zero resistance
-- sometimes can, it seems, become stalled by a form of electronic
A possible explanation why is offered by new research at Cornell
University. The research, reported March 5 at the annual meeting of the
American Physical Society in Denver, concerns certain copper oxides --
known as cuprates -- that can become high-temperature superconductors,
but also can, in a slightly different configuration, become stalled by
Understanding how and why that transition takes place is a crucial
question for cuprate superconductivity research because, if it did not,
the maximum temperatures for superconductivity could conceivably be
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. Sťamus
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.
Source: Cornell University
Story from: http://www.physorg.com/news92334606.html