Researchers have used scanning tunneling microscopy (STM) to make the first-ever nanometer-scale maps of "granular" superconductivity in a high-temperature superconductor.
The work was done by scientists at Lawrence Berkeley National Laboratory and the University of California at Berkeley.
They verified their discovery with a second innovative use of STM, employing individual nickel atoms as probes to distinguish superconducting from nonsuperconducting regions in the material, Bi-2212, an important representative of the copper oxide superconductors.
"In underdoped Bi-2212, we found nanoscale grains of apparent superconductivity embedded in an electronically distinct background," says J. C. Sťamus Davis of Berkeley Lab's Materials Sciences Division, a professor of physics at UC Berkeley. "Although this background state appears to be nonsuperconducting," he says, "macroscopic superconductivity may still occur through Josephson tunneling," a quantum-mechanical phenomenon.
A major challenge to understanding the mysterious high-Tc superconductors is that superconducting, insulating, and other electronic states can exist in them cheek-by-jowl at the same time.
Davis and his colleagues have shown that even a high-Tc superconductor with essentially perfect crystal structure can exhibit granular superconductivity, its regions of superconductivity spatially separated from one another. He and his coauthors announce their results in the 24 January 2001 issue of the journal Nature.
Davis explains that all the highest-temperature superconductors found so far are cuprate ceramics, with layers of copper and oxygen sandwiched between layers of other atoms such as bismuth.
"The cuprates are normally insulators, but some become superconducting if they are doped with other atoms. For example, additional oxygen can introduce positive charges or 'holes' into the copper-oxygen layers."
A cuprate achieves its highest-temperature transition to the superconducting state with just the right amount of doping, different for each compound. When the cuprate is underdoped -- that is, if there are too few dopant atoms in the material -- theoreticians have long predicted that a phenomenon called "frustrated electronic phase separation" (FEPS) might occur.
If this happens, regions of the sample develop different electronic phases even though they are separated from each other by mere nanoscale distances. Electronic phases like superconducting and insulating have been compared to different physical phases like liquid water and ice, or different chemical phases like oil and vinegar.
There is significant evidence for nanoscale FEPS in some cuprates. When Joseph Orenstein of Berkeley Lab and UC Berkeley performed studies of the conductance of bulk high-Tc BSCCO ?? a compound of bismuth, strontium, calcium, copper, and oxygen, of which Bi-2212 (Bi2Sr2CaCu2O8+delta) is one type -- he and his colleagues found puzzling evidence of variation in the density of the superconducting electronic "fluid."
One form of FEPS is the proposed "stripe phase," in which charge carriers are thought to flow along one-dimensional lines like rivers through insulating regions.
"Another possibility is that superconducting domains are separated like islands in an insulating sea," says Davis, which could give rise to the kind of electronic granularity his group observed directly.
Davis's group cleaved perfect single crystals of B-2212, which split cleanly along the bismuth-oxygen plane lying immediately over the copper-oxygen plane.
Their scanning tunneling microscope (STM) was able to image individual atoms in the plane; in ultra-high vacuum at very low temperature, the electronic states of the underlying copper-oxygen plane could also be sensed.
As the probe tip scanned over the plane, it measured differences in the current reaching the tip, a function of the voltage between the tip and the surface. Two kinds of regions of different conductance were revealed: alpha regions exhibited relatively small energy gaps, typical of superconductivity; beta regions had larger gaps.
From these spectral scans, "gapmaps" were constructed showing that, in the underdoped crystal, the alpha regions were roughly circular areas less than three nanometers (billionths of a meter) across, separated from one another and surrounded by narrow beta regions approximately two nanometers wide.
In "as-grown," slightly overdoped crystals, however, this nanoscale segregation was not evident.
"One question we couldn't answer with the initial STM spectra was whether the alpha regions really were superconducting," says Davis. "But we had recently developed a new atomic-scale tool, one we'd already used to study magnetic impurities in superconductors, that could address this question."
Nickel atoms introduced into the copper-oxygen planes of Bi-2212 stand out because of the orientation of their surrounding clouds of charge: cross shapes reveal the density of negative charges ?? electrons -- and x shapes that of positive charges -- holes. These patterns, or resonances, result when the impurity atom scatters entities known as quasiparticles.
Quasiparticles, which can be thought of as unpaired charge carriers, do not participate directly in superconductivity. Superconductivity is carried by pairs (Cooper pairs) of either electrons or holes; superconductivity in Bi-2212 and most other high-Tc superconductors is carried by holes.
Quasiparticles too may be either particle-like or hole-like. An overall balance, or symmetry, between the particle-like and hole-like quasiparticle resonances created by impurity atoms is a requirement of local superconductivity.
"Particle-hole symmetry of an impurity resonance indicates the superconducting state," says Davis. "It is predicted to decrease in other states and may disappear altogether in nonsuperconducting regions."
The Davis team surveyed B-2212 crystals a second time with the STM, this time looking not for energy gaps but for the cross- and x-shaped resonances that were signatures of the individual nickel atoms they had introduced into the sample -- and signposts of the superconducting state. In the underdoped compound they found nickel impurities centered in alpha regions but none in beta regions.
"A likely explanation is that nickel atoms are indeed present in other regions. But because these are not superconducting, there is no symmetrical particle-hole scattering to reveal the nickel impurities," Davis says.
Taken together, these two new STM techniques -- high-resolution spectral surveys, and use of impurity resonances as local markers of superconductivity -- not only show that superconductivity is segregated into discrete domains in underdoped B-2212 but also strongly suggest that this material displays granular superconductivity due to frustrated electronic phase separation.
"Since the domains are so close together," Davis says, "quantum-mechanical Josephson tunneling across the nonsuperconducting regions that separate them is probably what supports the long-range superconducting properties of this material."
K.M. Lang, V. Madhavan, J. Hoffman, E.W. Hudson, H. Eisaki, S. Uchida and J.C. Davis are the authors of "Using impurity atoms to search for granular superconductivity in underdoped Bi2Sr2CaCu2O8+delta," which appears in the 24 January 2002 issue of Nature.
Kristine Lang, Vidya Madhavan, and Joan Hoffman are present members of the Davis group. Former member Eric Hudson is a National Research Council fellow at the National Institute of Standards and Technology. Shin-ichi Uchida, Davis's principal collaborator, is professor of physics at Tokyo University; he and his colleague Hiroshi Eisaki made essential materials available. - By Paul Preuss
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