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Cornell STM probes mystery of high-temp superconductor

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  • RemyC
    From: http://www.physorg.com/news962.html An incredibly sensitive Cornell STM probes the mystery of a high-temperature superconductor August 26, 2004 With
    Message 1 of 1 , Nov 26 5:51 AM

      An incredibly sensitive Cornell STM probes the mystery of a high-temperature

      August 26, 2004

      With equipment so sensitive that it can locate clusters of electrons,
      Cornell University and University of Tokyo physicists have -- sort of --
      explained puzzling behavior in a much-studied high-temperature
      superconductor, perhaps leading to a better understanding of how such
      superconductors work.

      It turns out that under certain conditions the electrons in the material
      pretty much ignore the atoms to which they are supposed to be attached,
      arranging themselves into a neat pattern that looks like a crystal lattice.
      The behavior occurs in a phase physicists have called a "pseudogap," but
      because the newly discovered arrangement looks like a checkerboard in
      scanning tunneling microscope (STM) images, J.C. Seamus Davis, Cornell
      professor of physics, calls the phenomenon a "checkerboard phase."

      Davis, Hidenori Takagi, professor of physics at the University of Tokyo, and
      co-workers describe the observations in the Aug. 26, 2004, issue of the
      journal Nature. An article about the work also is scheduled to appear in the
      September issue of Physics Today.

      Image: In a standard scanning tunneling microscope image, left, the atoms in
      a cuprate crystal the bright blobs) are not in a particularly orderly
      arrangement. But an image of the probable distribution of electrons, right,
      shows that clouds of them have arranged themselves in what amounts to an
      electronic crystal. The brighter areas seem to contain more electrons, but
      the reason for this is unknown.

      "In at least one cuprate high-temperature superconducting material that
      phase is an electronic crystal," Davis reports. "We don't understand what
      we've found, but we have moved into unknown territory that everyone has
      wanted to explore. Many people have believed that to understand
      high-temperature superconductivity we have to look in this territory."

      A superconductor is a material capable of conducting electricity with
      virtually no resistance. Modified crystals of copper oxide, known as
      cuprates, can become superconductors at temperatures up to about 150 Kelvin
      (-123 degrees Celsius or --253 degrees Fahrenheit) when they are doped with
      other atoms that create "holes" in the crystal structure where electrons
      would ordinarily be. These superconductors are widely used in industry,
      although there is still no clear explanation of how they work. Their
      superconducting behavior begins when about 10 percent of the electrons have
      been removed, but for over a decade physicists have been puzzled by what
      happens when somewhat fewer electrons are removed: The material conducts
      electricity, but just barely. In theory it shouldn't conduct at all.

      Davis has now been able to observe this phase with a specially modified STM
      that measures, in effect, the quantum wave functions of the electrons in a

      The famous Heisenberg uncertainty principle says that we can never tell
      exactly where an electron is. Rather than thinking of electrons orbiting the
      nuclei of atoms like little planets, scientists today imagine "clouds" of
      electrons somewhere in the vicinity. An STM uses a needle so fine that its
      tip consists of just one atom, scanning across a small surface and measuring
      current flow between the surface and the tip. Conventional STMs scan with
      enough precision to image individual atoms. Davis has increased the scanning
      precision to a point where he can resolve details smaller than atoms. His
      new instrument, located in the basement of the Clark Hall of Science on the
      Cornell campus, is so sensitive that it has been built in a room mounted on
      heavy concrete pillars and isolated by air springs. For these experiments,
      it scans a sample and reads the probability that electrons are in certain
      locations, based on current flow through the STM tip.

      Davis's team studied a copper oxide containing calcium and chlorine that was
      doped by replacing some of the calcium atoms with sodium to remove, in
      various samples, from 8 to 12 percent of the electrons. The material was
      cooled to about 100 milliKelvins, or a hundredth of a degree above absolute

      What they found was that the electrons in the sample arranged themselves in
      tiny squares, all in turn arranged in a neat checkerboard pattern. The same
      pattern was found at the highest level of doping tested, where the material
      begins to become superconducting. Whether or not it continues at higher
      levels remains to be seen, Davis says.

      The discovery only leads to more questions. Theoretically, Davis says, this
      arrangement should not conduct electricity at all, because the electrons are
      locked into their crystal-like pattern. "It's always been a mystery, how do
      you get from an insulator through a tiny change to a superconductor," he
      notes. "Having empirical knowledge of what this phase is may help us to get
      from here to there."

      The Nature paper is titled "Discovery of a 'Checkerboard' Electronic Crystal
      State in Lightly Hole-Doped Ca2-xNaxCuO2Cl2." Along with Davis and Takagi,
      co-authors include Cornell post-doctoral researchers Christian Lupien and
      Yuki Kohsaka; Dung-Hai Lee, University of California-Berkeley professor of
      physics; and Tetsuo Hanaguri of the RIKEN Institute in Japan. The cuprate
      material used in the experiments were prepared by Yuki Kohsaka at the
      University of Tokyo in collaboration with the scientists who developed it in
      1995, Masaki Azuma and Mikio Takano of Kyoto University.

      Source: Cornell University
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