Physics of High Temperature Superconductivity Elucidated
Important new insights into the phenomenon known as high-temperature superconductivity have been reported by a team of researchers who used their own customized Scanning Tunneling Microscope (STM) to study the effects of doping a "high-Tc superconductor" with a single "impurity" atom.
The research team included scientists with the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab), the University of California at Berkeley, and the University of Tokyo. Their results are reported in the February 17, 2000 issue of the journal Nature.
FIRST-EVER STM IMAGES OF A 2-D LAYER OF COPPER OXIDE DOPED WITH
A SINGLE ZINC IMPURITY ATOM SHOW A CROSS-SHAPED QUASI-PARTICLE
(FREE-ELECTRON) CLOUD CENTERED DIRECTLY ABOVE THE ATOM OF ZINC.
THESE ELECTRON-CLOUD IMAGES ARE PROVIDING VALUABLE NEW
INFORMATION ON THE PHYSICS BEHIND HIGH-TEMPERATURE
SUPERCONDUCTIVITY.
The discovery in 1986 of a new class of superconductors -- materials in which electrical resistance drops to zero when cooled below a critical temperature symbolized as Tc) -- conjured visions of magnetically levitated high-speed trains and dirt-cheap electrical power. Made from ceramic copper oxides rather than the metal alloys of previous new high-Tc superconductors hold promise for superconductivity at room temperatures. For room-temperature materials to ever be realized, however, scientists need a better understanding of the physics behind high-Tc superconductivity.
A key finding in the new report from Berkeley is the first real-space observation of "d-wave symmetry" in a high-Tc superconductor. This is consistent with theoretical models which hold that high-Tc superconductivity involves fluctuations of the magnetic spins of atomic nuclei rather than the phonon-mediated mechanism (vibrations in the atomic lattice) of classical low-Tc superconductors.
"Researchers all over the world have searched for these phenomena for over a decade," says Séamus Davis, who leads the team and holds a joint appointment with Berkeley Lab's Materials Sciences Division and UC Berkeley's Physics Department. "Ours is the first STM study of the effects on high-Tc superconductivity of individual impurity atoms."
In addition to observations consistent with existing theories, Davis and his colleagues also observed phenomena that theorists have not predicted. These surprising observations strongly suggest the need for new and more sophisticated models to explain the physics of high-Tc superconductivity at the atomic scale.
Other authors of the Nature paper, in addition to Davis, were Shuheng Pan (who is now a professor at Boston University), Eric Hudson (now an NRC Fellow at NIST), Kristine Lang of UC Berkeley's Physics Department, and Hiroshi Eisaki and Shin-ichio Uchida of the University of Tokyo's Department of Superconductivity.
The Berkeley researchers worked with a perovskite-type ceramic copper oxide called BSCCO (pronounced bis-ko) because it contains bismuth, strontium, and calcium in addition to copper and oxygen. Like most high-Tc superconductors, BSCCO is a layered material which can be mechanically cleaved to reveal two-dimensional crystal planes that contain only copper and oxygen atoms. In these 2-D copper oxide planes, the researchers had previously substituted single atoms of zinc for single atoms of copper during the crystal-making stage.
High-Tc superconductivity is believed to originate from strongly
interacting or "paired" electrons moving through copper oxide layers. A
single atom of zinc, a strong scatterer of electrons, substituted for an atom
of copper, which would be the source of any paired electrons, proved to be
an ideal probe for studying the underlying physics of high-Tc
superconductivity.
"Associated with the zinc impurities in the cuprate oxide plane of our
samples, we find intense quasi-particle resonances consistent with unitary
scattering in a d-wave superconductor," the authors report in their Nature
paper. "Density-of-state imaging at the resonance energy shows a highly
localized 'quasi-particle cloud' which has a clear four-fold symmetry
aligned with the d-wave gap nodes, in qualitative agreement with theory."
Quasi-particles are states of electron excitation that collectively act like a
free electron, with energy and angular momentum. D-waves are a
function of the angular momentum of the quasi-particles. According to
theory, impurity atoms create quasi-particle scattering resonances with
characteristic spatial and spectroscopic signatures.
The Berkeley STM images showed intensely bright, cross-shaped,
quasi-particle clouds centered directly above the zinc atoms and extending
out to about 10 angstroms. These bright crosses are consistent with
d-wave symmetry theories which hold that high-Tc superconductivity is
mediated by distortions of the magnetic spins in the atomic lattice of the
copper oxide layers. The images also validate other theories such as the
so-called "Swiss cheese model" which predicts the size and shape of
non-superconducting regions around each impurity atom.
"This is the first demonstration of quasi-particle imaging and tunneling
spectroscopy at individual impurity atoms in complex materials like the
cuprate-oxides," says Davis. "The experimental idea is simple – put one
impurity atom at an important site and see what happens – but the
technique is so powerful it opens completely new avenues of research
including the potential to develop exotic new materials. We've shown that
even materials which are structurally and electronically very complex can
be studied one atom at a time."
The STM used in this experiment was designed and constructed by Davis
and his colleagues Pan and Hudson. Operable at temperatures as low as
0.25 degrees above absolute zero and capable of simultaneously measuring
both the surface topography and the density of state of a sample with
atomic resolution, this STM is optimized for the study of high-Tc
superconducting materials. Images are recorded when the ultra-fine tip of
the STM (only a few atoms wide) is passed over a sample about a billionth
of a meter (one nanometer) above the surface. An electrical current,
generated between the atoms on the sample surface and the STM tip,
through which electrons can "tunnel," causes displacements of the tip that
can be recorded and translated into topographic images. The STM can
also be used to detect physical phenomena such as electrostatic and
magnetic forces.
Among the findings by Davis and his colleagues not predicted by theorists
was a second cross-shaped quasi-particle cloud, about three times larger
but much less bright than the first and rotated about 45-degrees relative to
it. More surprises are expected when the Berkeley group replaces copper
atoms with impurity atoms other than zinc. Davis' research group has also
constructed the first known superconducting tip for an STM. Made from
niobium and operable in a powerful magnetic field (7.2 Tesla), this
superconducting tip could give Davis and his colleagues the ability to study
the magnetic spins of individual atoms which would be a major advance
towards unlocking the secrets of high-Tc superconductivity.
"No one knows the precise recipe for making new higher temperature
superconductors," says Davis. "To find that recipe it would be
tremendously helpful to understand how high-Tc superconductivity works
at the atomic level."
Berkeley Lab is a U.S. Department of Energy national laboratory located
in Berkeley, California. It conducts unclassified scientific research and is
managed by the University of California.
Contact Information:
Séamus Davis can be reached at (510) 642-4505 or
(510)-643-9090, or via email at jcdavis@socrates.berkeley.edu
Séamus Davis group Web site
UC Berkeley news release