||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.
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 superconductors, these 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.