Front. Phys., 2011, 6(4): 347–349DOI 10.1007/s11467-011-0226-8 PERSPECTIVE
Gifts from the superconducting curiosity shop
David Mandrus1,2
1Department of Materials Science and Engineering, University of Tennessee, Knoxville, TN 37996 USA 2Materials Science and Engineering Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA Received October 16, 2011; accepted November 20, 2011 Superconductivity has just celebrated its 100th birthday, per Matthias demonstrated that a universal curve could and yet despite its advanced age it has never been more be drawn for Tc as a function of (e/a) for elements and alive. Given that most subfields of materials physics have simple compounds with a maxima at 5 and 7 and a min- a half-life of about seven years, what accounts for the imum at 6 [4]. Although this simple rule worked well enduring popularity of superconductivity? What is it for elements and simple binaries, it eventually became about superconductivity that continues to fascinate? clear that the rule failed for more complex ternary com- The answer, of course, is that “it’s the materials, pounds like the Chevrel phases. In 2001 Pickett observed stupid.” And especially the exciting new materials that that many of the newer superconductors that violated serve to periodically re-energize the field. Superconduct- Matthias’s rules had several things in common: “(i) must ing materials display a nearly zoological level of diversity be multicomponent — more than a binary; (ii) at least and complexity, ranging from elemental metals to multi- one site should have variable occupancy (“dopant”); (iii) nary intermetallics, Zintl phases, organics, and oxides.
large electronegativity difference between constituents; Some are good metals, some are “bad” metals [1], some (iv) proximity to a magnetic or insulating phase; (v) are semimetals, and some are barely metals at all. The mixed antibonding bands at the Fermi level; (vi) there interplay of magnetism and superconductivity is deeply are favorable electron/ion ratios.” [5] It must be said mysterious, as is the relationship between ferroelectric- that although these new rules have stood up rather well ity and superconductivity [2]. Dimensionality also seems so far, they have limited predictive power and are not important in that many of the most interesting supercon- terribly helpful for finding new materials. In fact, theory ductors have layered crystal structures, and yet this, too, has generally not been of much help in finding new su- is mysterious in that there seems to be no good way to perconductors. This has been emphasized by Fisk, who quantify the “2D-ness” of a system in a consistent way.
has pointed out that the graph of maximum Tc vs. time Further mysteries abound, especially when one starts to “sails through 1957 BCS without a glitch.” [6] think about superconductors from a chemical point of It must be recognized, therefore, that at least for now view. Why is it, for example, that some of the most in- we are dependent on serendipity for the discovery of teresting superconductors (e.g., La2CuO4 and BaFe2As2 new superconductors. This is why new families of su- families) have both ionic and covalent bonding in the perconductors so often seem to come from “out of the same material? This is reminiscent of the Zintl concept, blue.” This is actually a dangerous situation, for the in which electrons from element A are donated to ele- kind of exploratory synthesis that is required to find ment B, which then uses the donated electrons to form new families of superconductors is losing ground to a covalent bonds (NaTl is the classic example) [3].
“materials-by-design” mindset in which progress can be Although it is clear that new materials are the “directed” by “high throughput” computing. Something lifeblood of superconductivity, it is not clear where future similar has happened in drug research. Writing in the materials will come from as we have no good heuristics Financial Times, Shaywitz and Taleb explain that the (rules of thumb) for finding interesting new superconduc- dwindling drug pipelines can be understood as a failure tors. It is not for lack of trying. In the 1950s (pre-BCS) to capitalize on “positive uncertainty,” or serendipity: Matthias developed the first set of guidelines for finding . . . The process of drug development is also very new superconductors by emphasizing certain regularities difficult to predict, because of both our limited un- among superconducting elements and compounds, espe- derstanding of disease and our inevitably imperfect cially the valence electron/atom (e/a) ratio. In a 1955 pa- understanding of the effect any new compound will c Higher Education Press and Springer-Verlag Berlin Heidelberg 2011 David Mandrus, Front. Phys., 2011, 6(4) have on the body. While design played a pivotal role have interesting lattice properties because the caged ion in the development of effective HIV drugs, other has a much lower characteristic vibrational frequency modern medications were discovered in the old fash- than atoms making up the cage. This is readily appar- ioned way: by accident. Viagra, for example, was ent in specific heat experiments, where a low frequency originally developed as a treatment for chest pain.
Einstein contribution is clearly visible. Other clathrate In the face of declining productivity, pharma (and crypto-clathrate) superconductors include Al10V companies have been trying to boost output by in- [10], KOs2O6 [11], and Ba8Si46 [12].
creasing efficiency, narrowing their focus to a hand- The next one is polysulfur nitride (SN)x, which was ful of disease areas, shelving safe but ineffective discovered to superconduct at 0.26 K by Greene and co- compounds without fully exploring their scientific workers in 1975 [13]. (SN)x is a quite curious material potential and trying to ensure that each project the because it is an inorganic polymer, and in fact the first company is working on is carried out with a clearly polymer discovered to superconduct. It is highly one- defined market segment in mind. Unfortunately, for dimensional, and grows as golden fibrous crystals with new medicines in particular, this strategy often fails metallic luster. The fibers are several hundred Angstroms significantly to reduce exposure to negative uncer- across and are composed of highly oriented (SN)x chains.
tainty — all the bad things that can happen during Polysulfur nitride in some sense presages the explosion drug development — and eliminates much of the of interest in low dimensional metals, organics, and even exposure to positive uncertainty (serendipity) that nanowires. It is also notable that two of the precursors in the synthesis, S2N2 and S4N4, are both explosive and I think the lesson that should be drawn is that there thus great care must therefore be taken in the prepara- should always be room for serendipity in science, and particularly in the search for new materials.
The third material is a superconductor discovered A “curiosity shop” conjures up the image of a large, in 2007 by Hosono’s group, 12CaO·7Al2O3:e[14]. By disorganized, and slightly musty store in an older part of a chemical reduction treatment oxygen ions in sub- town that has accumulated so many odds and ends over nanometer cages are removed, allowing doped electrons the years that nobody is really sure what is in there any- to occupy the former oxygen sites. Hosono calls the resul- more. I think in many respects such an image is fitting for tant material an inorganic “electride,” which is typically the vast literature on superconductivity, and I thought I composed of an anionic electron and the cationic frame- would briefly mention three of the more unusual super- work. As the authors amusingly point out, oxides such as conductors I have come across while rummaging around Al2O3 are typically used as “architectural materials.” In there. These superconductors are not fashionable and do other words, this material is essentially superconducting not have very high transition temperatures, but they are cement, and is about as far away from Matthias’s rules fascinating nevertheless because they are oddities and serve to highlight the sheer chemical diversity of super- Perhaps the secret to eternal youth is simply to re- main curious? In that case the future of superconduc- The first one is a silver clathrate salt, Ag7O8NO3.
tivity looks bright because there is little doubt that new Superconductivity in this material (and some relatives) superconductors will continue to be found as long as we was reported by Robin and co-workers in 1966 [8]. The argentic oxide salts (Ag7O8)+X(X= NO 4 ) are cubic clathrates consisting of face- sharing, 26-sided Ag6O8 polyhedra with the anions lo- References
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C. Zheng, R. Hoffmann, R. Nesper, and H. G. Vonschnering, First, silver is immediately under copper in the peri- odic table and the silver is mixed valent just like cop- B. T. Matthias, Phys. Rev., 1955, 97(1): 74 per is mixed valent in the cuprate superconductors. Fur- W. E. Pickett, Physica B, 2001, 296(1–3): 112 thermore, the coordination of the silver is 4-fold square planar, again like the cuprates. After that the analogy D. Shaywitz and N. Taleb, Drug Research Needs Serendipity, breaks down, because in the silver clathrates the square- planar units link to form clusters, which in turn link M. B. Robin, K. Andres, T. H. Geballe, N. A. Kuebler, and to form a cubic structure. The other reason these ma- D. B. McWhan, Phys. Rev. Lett., 1966, 17(17): 917 terials are interesting is that they are the first exam- K. Kawashima, M. Kriener, M. Ishii, Y. Maeno, and J.
ples of superconducting metallic clathrates. Clathrates David Mandrus, Front. Phys., 2011, 6(4) T. Claeson and J. Ivarsson, Commun. Phys., 1977, 2(3): 53 R. L. Greene, G. B. Street, and L. J. Suter, Phys. Rev. Lett., S. Yonezawa, Y. Muraoka, Y. Matsushita, and Z. Hiroi, J.
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