The seemingly unrelated fields of plutonium metallurgy and unconventional superconductivity have a long and coupled history at Los Alamos. In the early 1980s Jim Smith, Zachary Fisk, and Sig Hecker developed much of the present understanding of the role of f electrons in influencing the metallurgical properties of elemental plutonium. The three were working in what was then Physical Metallurgy (CMB-5), parts of which now exist in both Nuclear Materials Science (NMT-16) and Condensed Matter and Thermal Physics (MST-10).
At the same time, Fisk and Smith discovered "heavy fermion superconductivity"-an unconventional type of superconductivity driven by the magnetic properties of f electrons-in UPt3 and UBe13, two uranium compounds that superconduct near 1 Kelvin and still attract international attention. This achievement is arguably the most significant in the history of experimental condensed-matter physics at Los Alamos and netted Fisk and Smith a major international prize, the American Physical Society International Prize for New Materials, in 1990.
Recently, a renaissance in the interaction of plutonium metallurgy and condensed-matter physics has culminated in the discovery of the first plutonium-based superconductor-PuCoGa5-a compound containing plutonium, cobalt, and gallium, which has an unexpectedly high superconducting transition temperature of 18.5 Kelvin (or 255 degrees Celsius). The discovery was reported in the Nov. 21, 2002, issue of the prestigious journal Nature.
Superconductivity is an unusual state of matter in which electrical current flows without resistance as a result of the material's electrons acting in pairs. Although the temperatures at which superconductivity is observed are usually quite low, superconductors are of interest both from a fundamental scientific perspective as well as for applications such as superconducting magnets. Only a handful of intermetallic compounds display superconductivity above 18 Kelvin.
Los Alamos
researchers Luis Morales (at microscope) and John Sarrao have discovered
a plutonium-based superconductor.
A team led by Luis Morales of Nuclear Materials Science (NMT-16) and John Sarrao of Condensed Matter and Thermal Physics (MST-10) has established a capability within Wing 2 of the Chemistry and Metallurgy Research (CMR) Building to grow single crystals of transuranic intermetallic compounds using flux-growth techniques (see sidebar, page 4). The research is part of a Laboratory Directed Research and Development (LDRD) project whose goal is to advance the first-principles understanding of the electronic structure of plutonium.
Single crystals are valuable when trying to understand the intrinsic properties of materials, and this is especially true for compounds that display radiation-induced self-damage, like those containing plutonium. This project is complementary to one led by Albert Migliori of the Materials Science and Technology (MST) Divisionıs National High Magnetic Field Laboratory to advance the thermodynamic understanding of plutonium. Taken together, these efforts represent a significant investment in elaborating plutonium's fundamental properties.
The synthesis of new compounds is often serendipitous, and the case of the newly discovered plutonium-based superconductor was no exception. The researchers were exploring plutonium-cobalt-gallium ternary solutions as a precursor to growing single crystals of gallium-stabilized d-phase plutonium when surprisingly large crystals resulted.
Catalysis, and Separations Chemistry (C-SIC), the researchers determined that the crystal structure was 5. The compound is a layered relative of d-phase plutonium and plutonium-trigallium, in which layers of plutonium-trigallium and cobalt-digallium are alternately stacked along the crystallographic c axis.
Even more surprising to the researchers was the fact that their new compound displayed superconductivity at 18 Kelvin. Elemental plutonium is poised on the border between localized and itinerant f-electron behavior. This leads both to the complex metallurgy and significant differences that exist between a-phase plutonium (the phase at room temperature) and d-phase plutonium (the phase stable at high temperatures) and makes it unlikely to display conventional superconductivity.
(For a general introduction to the properties of metallic plutonium, see the article by A. Michael Boring and James L. Smith in Los Alamos Science No. 26, 2000.)
The strong electron correlation effects that are present in plutonium tend to favor magnetic order that is generally harmful for superconductivity. The researchers' current understanding of the plutonium-based superconductor suggests that the superconductivity they observe may be one of a very small handful of superconductors (the copper oxide-based high-temperature superconductors are the most famous representative of this family) whose superconductivity actually derives from magnetic correlations. The heavy fermion superconductors are another example of these materials.
In fact, the researchers think they have come full circle with their new plutonium-based superconductor in revisiting the problems pioneered by Fisk, Smith, and Hecker in understanding the role of f electrons in the metallurgy of plutonium.
The cubic crystal structure of d-phase plutonium is quite similar to
that of plutonium-trigallium (PuGa3). In the tetragonal
crystal of
plutonium-cobalt-pentagallium (PuCoGa5), layers of
plutonium-trigallium and cobalt-digallium are alternately stacked along
the crystallographic c axis. The relationship of the superconductor's
crystal structure to the others can be simplistically appreciated as the
insertion of a cobalt layer at the location indicated by the dotted red
line in PuGa3.
Superconductivity and plutonium metallurgy
Superconductivity is an unusual phenomenon in which the electrical resistivity of a material drops to zero at some transition temperature-as seen, for example, in the accompanying plot for plutonium-cobalt-pentagallium (PuCoGa5). In essence, this means that the material offers no resistance to the flow of electrical current. Although this behavior is observed at rather low temperatures, it nonetheless holds a great deal of potential for technological applications, in addition to its considerable interest for scientific researchers.
Despite the hurdles that remain for practical implementation, superconductors offer the possibility of widespread delivery of electrical current without frictional losses. This could significantly lower the cost of electricity.
Nor are the applications of this phenomenon entirely futuristic: Magnets made from super-resonance-imaging) instruments. For such magnets, the temperature at which a material superconducts is often less important than how much current the material can carry (the "critical current") before it loses its superconductivity.
The current-carrying capacity of a superconductor depends in large part on its microstructure and defect properties. It turns out that the damage mechanisms associated with radioactive decay of plutonium, which are well understood from studies of plutonium aging, are quite well suited for high critical currents in PuCoGa5. If not for the health and safety issues associated with plutonium, PuCoGa5 would be a rather ideal candidate for commercial use in superconducting magnets.
Most superconductors are understandable within the BCS (Bardeen-Cooper-Schrieffer) theory of superconductivity, a Nobel Prize-winning theory of the 1950s) and have transition temperatures below 10 Kelvin. In the last decade or so, a new family of superconductors, the high-Tc cuprates, have been discovered with much higher transition temperatures, about 100 Kelvin.
The higher transition temperature results from the presence of magnetic interactions in the material. Magnetism usually destroys superconductivity, but in this case (and in the case of PuCoGa5), it appears that the higher transition temperatures are a consequence of the magnetism. The physics underlying this phenomenon is directly related to the degree of electron hybridization (for example, of the f electrons in plutonium), which also drives plutoniumıs complex metallurgy.
There are several more practical consequences of the fact that the superconductor contains plutonium. The superconducting transition temperature decreases as a function of time at a rate of about 0.25 degrees Celsius per month. However, this aging effect also has a positive benefit.
The critical current that a superconductor can support, which is important for applications, derives from the materialıs ability to trap and hold magnetic flux. The radiation-induced damage that causes the decrease in transition temperature also increases its ability to pin magnetic flux and leads to a "critical current" that is quite large and increases with time. Critical current is the amount of electrical current that a superconductor can support and still display zero resistance.
If not for the fact that this property derives from the presence of plutonium, PuCoGa5 would be an outstanding material from which to produce superconducting magnets.
Because researchers have learned so much about damage mechanisms from studying plutonium aging in other contexts, PuCoGa5 promises to be an important test material in the research community's understanding of the so-called mixed state of superconductors, even if its commercial viability is limited.
(For more on damage mechanisms in aging plutonium, see the articles by Wilhelm G. Wolfer in Los Alamos Science, No. 26, 2000, and by Thomas Zocco and collaborators in Actinide Research Quarterly, 4th Quarter, 2001.)
Photographed through a glovebox window, this
single crystal of the superconductor plutonium-cobalt-pentagallium was
formed using the flux-growth technique.
The method used to grow the single crystals discussed in this article is the flux-growth technique, which was initially championed by Zachary Fisk in the early 1980s. The technique involves dissolving the constituent elements of the desired compound in an excess of a low-melting metala flux; a process analogous to growing sugar crystals from supersaturated water solutions in high school chemistry. The Los Alamos researchers grew their plutonium superconductor from excess gallium, but they have also grown single crystals from excess indium and antimony. To grow the crystals, the researchers place the starting material, including the excess flux, in an alumina crucible that is sealed in an evacuated quartz ampoule.
The sealed ampoule is heated to high temperature (about 1,000 degrees Celsius) and then cooled slowly over one or two days to an intermediate temperature of about 600 degrees Celsius. At this point, a centrifuge is used to separate the solid crystals from the excess liquid flux. The resulting crystals are well faceted, large (about 1 gram of total mass is not difficult to achieve), and of high quality.
While PuCoGa5 may be an interesting compound unto itself, what prospects does it raise for advancing the general understanding of actinide materials?
Although the researchers have only been studying this material for a short while, it is already clear that it, like elemental plutonium, is poised on the boundary between localized and itinerant f-electron behavior, in which the f electrons can't decide whether they want to contribute to structural bonding, yielding complex low-symmetry structures, or remain uncoupled and only influence magnetic properties.
The ability of electronic structure calculations to correctly predict which limit is realized in real materials challenges the state of the art in the field. This uncertainty is a central factor in limiting understanding of the equation of state of plutonium.
PuCoGa5 has already attracted attention within the international condensed-matter physics community, and electronic structure calculations have already been reported in Germany and Japan, as well as at Los Alamos. The accuracy of these calculations, which can be validated experimentally by further measurements of the properties of PuCoGa5, will directly benefit the understanding of elemental plutoniumıs electronic structure.
The superconductivity in PuCoGa5 has also been confirmed experimentally by a group at the Institute for Transuranic E'ements in Karlsruhe, Germany. Stimulated by the Los Alamos researchersı discovery, this growing community promises to not only improve the understanding of PuCoGa5, but also will engage a new generation of materials scientists in the challenges of plutonium.
And, because the properties of plutonium-containing intermetallic compounds are unexplored, only time will tell what additional surprises await scientists in their continuing synthesis of single-crystal compounds.
This article was contributed by Luis A. Morales of Nuclear Materials Science (NMT-16), and John L. Sarrao and Joe D. Thompson of Condensed Matter and Thermal Physics (MST-10)
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