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40TH ANNIVERSARY ARTICLE www.rsc.org/chemcomm | ChemComm Contemporary superconducting materials

R. J. Cava

DOI: 10.1039/b512643f

Superconductors currently under active study are briefly described from a chemical perspective. The author’s current views on those superconductors, the role of theory in searching for new superconductors, and possibilities for new superconducting materials that might be found in the future are presented.

Introduction field, and have also found new super- complexity for the kinds of relatively conductors, both in focused searches simple compounds that can support New superconductors are currently being and by accident in synthetic programs . Some materials of discovered in the world at about the with other goals. Many excellent, detailed current interest are described in the rate of 2 to 4 per year. Although the reviews of superconductivity and the following. transition temperature to the zero resis- known superconducting materials are tance state is a critical characteristic, 1–5 available (see e.g. ). This brief essay Sr2RuO4 other characteristics such as critical is directed at the chemical reader inter- The magnetic properties of ruthenium- current, critical magnetic field, and pro- ested in learning about recently dis- based oxides range from ferromagnetism cessability are equally important for covered superconducting materials, to antiferromagnetism to Pauli paramag- possible applications. From a scientific and considering some speculations of netism, and the electronic properties perspective, the underlying cause of mine about the possibilities for new range from insulating to metallic. This superconductivity can often raise fore- superconductors. front issues in condensed matter physics, unusually wide variation comes from the and has been investigated actively for fact that Ru–O networks are poised at Recently discovered the cusp between localized and deloca- many materials—ranging from those superconductors lized electron behavior, with strong with T s as low as 1 K (e.g. Sr RuO ) c 2 4 Ru–O orbital hybridization. Further, to those with Tcs above 100 K (e.g. the From the chemist’s perspective, the most

Downloaded on 05 January 2011 ferromagnetic and antiferromagnetic copper oxide superconductors). The interesting superconductors are those states at low temperatures are close search for new superconductors has for which many chemical or structural Published on http://pubs.rsc.org | doi:10.1039/B512643F in energy. The CaO and SrO-based largely been the domain of condensed variants can be found. Disappointingly, Ruddlesden Popper (AO) (RuO ) matter physicists knowledgeable in the the superconductors found in recent n+1 2 n series embodies all this complexity in synthesis of intermetallic or oxide com- years have either literally been ‘‘one a single family. Ca3Ru2O7 is a Mott– pounds. Chemists have much to offer the of a kind’’ or very nearly so. I remain Hubbard insulator, CaRuO3 is a optimistic that new families remain to be paramagnet that can be tipped into found, but it could be that our searches Department of Chemistry, Princeton Institute ferromagnetism with slight chemical are pushing into the periphery of the for the Science and Technology of Materials, doping, SrRuO3 is a ferromagnet, , Princeton, NJ, USA stability and chemical and structural Sr3Ru2O7 is a metamagnet (becomes ferromagnetic, but only under a modest Robert Cava is Chair of the Depart- applied field) and Sr2RuO4 (Fig. 1) is 6 ment of Chemistry at Princeton and a superconducting at 1 K. There has been Professor in Chemistry and Materials. a remarkable amount of work on this He is a Fellow of the American superconductor in the past decade (see, Physical Society and the American e.g. ref. 7). It looks like the super- Society, and a member of the conducting wave function has p-wave US National Academy of Sciences. He symmetry, implying that the super- was Acting Director of the Princeton conductivity is a result of a ferromag- Materials Institute from 2001 to 2002. netic-like interaction between electrons. He began at Princeton in 1997 after Perhaps surprisingly, this 1 K super- working at Bell Laboratories from 1979 to 1997, where he was a Distinguished conductivity has motivated the study Member of Technical Staff. He was and discovery of many new ruthenium a National Research Council oxides. The delicately sensitive super- Postdoctoral Fellow at NIST from conductivity of this type (a few hundred 1978 to 1979 after receiving his Ph.D. parts per million impurities are sufficient in from MIT in 1978. to kill it—a purity level not commonly achieved in synthesis of complex oxides)

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Fig. 2 The layered crystal structure of MgB2, recently discovered to be superconducting at 39 K, a compound that has been available for purchase from chemical suppliers for many years. (The red spheres are B and the blue spheres are Mg). This structure type, which contains honeycomb layers of boron structurally and electronically analogous to graphite, is

found for many transition metal and lanthanide borides. In the case of MgB2, a critical deviation from full transfer of charge from Mg to B results in the presence of holes in the sigma bonding orbitals of the B honeycomb layer. In contrast, these sigma orbitals are filled in graphite, and superconductivity occurs at lower temperatures when electrons are doped into states that are not as effectively coupled to the lattice vibrations.

Fig. 1 The layered crystal structure of

the 1 K superconductor Sr2RuO4. (The Sr are shown as large blue spheres and the diboride superconductors Nb12xB2 and transform for example from one to RuO6 octahedra are shown as polyhedra; (Zr,Mo)B2 (with Tcs less than 10 K) have another via simple chemical or physical the Ru are internal to the octahedra and been known since the 1960s, but MgB is means. In recent years fundamental the O are small red spheres). The low 2 special because no d orbitals are involved research has been performed on a hand- temperature superconductivity in this compound is only seen in crystals with in the electronic system. In the crystal ful of materials where the two states are unusually high purity and is believed to structure (Fig. 2), the boron forms a very nearly perfectly balanced. From my be unconventional in character. This honeycomb plane, analogous to the C viewpoint, ZrZn2 is the most interesting structure type is commonly found for layers in graphite. The magnesium forms of these.10 It is very weakly ferromag- transition metal oxides, and many studies a triangular layer between the B layers. If netic, with a small ordered magnetic of two-dimensional conductivity and MgB2 were strictly ionic, then it would be moment and a ferromagnetic ordering Downloaded on 05 January 2011 magnetism have been performed on isoelectronic with graphite. Critically, the temperature of 28 K (note that it is made compounds like this one. The first of the Mg to B charge transfer is not complete, from non-ferromagnetic elements) but Published on http://pubs.rsc.org | doi:10.1039/B512643F ‘‘High Tc superconductors’’ discovered, and the in-plane boron–boron sigma La Ba CuO , has the same basic also, while ferromagnetic, becomes 1.85 0.15 4 states, which are filled in the case of crystal structure, with some subtle but superconducting at 0.3 K (!). Under a important differences due to the differ- graphite, are only partly filled in the case modest applied pressure, both the ferro- ence in d orbital occupancy. of MgB2, leading to metallic conduc- magnetism and superconductivity go tivity. The modulation of the in-plane away at the same time, indicating that B–B bonds by a phonon therefore they are intimately coupled. Related strongly impacts the electronic system, 11 behavior is seen for UGe2, where has so far made Sr2RuO4 one of a kind: and gives rise to the very high Tc. ferromagnetism transforms into 1 K superconducting Sr2RuO4 can only be Surprisingly, there appear to be two superconductivity under applied pres- obtained in crystals grown by the float- superconducting systems of electrons in sure, though the presence of the magnetic ing zone method, a method that naturally MgB2—the pi electron states give rise state in the ambient pressure material results in zone-refining of a crystal’s to a lower energy system of super- is less surprising in this case, as U impurity content. conducting carriers as well! The conse- often displays a local magnetic moment. quences of this are still under study. These materials have suggested an MgB 2 Maddeningly, this important super- interesting new class of superconductors conductor is one of a kind—no addi- MgB is an interesting case, and with a that is likely to have more members— 2 tional new superconductors with high T s T of 39 K8,9 can be considered the c marginally magnetic materials where c grew from its discovery. ultimate electron–phonon coupled super- subtle changes in pressure or temperature conductor. It was discovered to be super- tip the balance to marginal super- ZrZn , UGe conducting in 2001 after being available 2 2 conductivity. These are classic examples as a commercial chemical reagent for Magnetic ordering and superconduc- of materials that display a ‘‘quantum several decades. This is an example of tivity are both cooperative states of critical point’’—materials that display a the surprises in store in this field: who electrons at low temperatures, and there- crossover in physical properties very could imagine that a simple binary fore in some cases can be competitors. near zero Kelvin, where quantum 12 compound with a Tc of nearly 40 K For almost all materials, one state is the effects must rule. Surprisingly, highly could lie undiscovered for so long? The clear winner, and it is not possible to unexpected properties are found in such

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systems at substantially higher tempera- these interactions happen at the Fermi tures, and these, like other quantum energy, the charge carriers are much critical materials, are currently of great heavier than free electrons, by a factor interest. of 1000, and superconductivity can sometimes happen (these are the ‘‘heavy 16 MgCNi3 fermion’’ superconductors ). This can occur for 5f element containing com- MgCNi , a new intermetallic perovskite 3 pounds as well, and has been seen superconductor (T 5 7 K), has been c previously for uranium. My favorite part proposed as another kind of example of of the story is that the disintegrations a superconductor that derives from a from the radioactive decay of the Pu magnet, due to what is believed to be the cause damage to the crystal structure as presence of relatively localized nickel- the samples age after synthesis. The derived orbitals at the Fermi energy.13 In disorder that this causes in the lattice in this case, a clear link between magnetism turn causes the superconducting transi- and superconductivity has not been tion temperature to decrease over time. found, so it may not be as exotic as However, the increased defect con- originally proposed. centration in the compound results in improved trapping of flux vortices in the Na CoO ?1.3H O—the devil x 2 2 superconducting state, so at the same superconductor time the Tc is decreasing, the super- 14 With a Tc of about 4.5 K, this is a conducting critical current is increasing. wonderful example of an unexpected Optimal practical properties (though no superconductor found by accident. one would ever market a superconductor Due to its fundamentally challenging based on plutonium!) would therefore electronic and magnetic character from hypothetically occur at some finite time a theoretical perspective, I believe this after synthesis where the two effects would have been a very important super- balance. conductor except that its character as a material precludes it from being studied Pyrochlores reliably. The structure (Fig. 3) is based Downloaded on 05 January 2011 Fig. 3 The layered crystal structure of The pyrochlore structure, one of the on planes of CoO6 octahedra fully ‘‘Na0.3CoO2?1.3H2O’’, recently discovered most commonly found crystal structures

Published on http://pubs.rsc.org | doi:10.1039/B512643F sharing edges, creating triangular layers. to be superconducting at 4.5 K. (CoO6 in ternary oxides, had not been known Between these layers is a double layer of octahedra are shown as polyhedra, the to yield a superconductor until very H O that also contains the Na. At the 2 oxygens as small red spheres, the O from recently. This structure is of substantial optimal Na content, the triangular CoO the water molecules as large blue spheres, 2 interest from a magnetic perspective layer contains a mixture of spin K and and the Na as purple spheres.) Like since the metal sublattices are arranged spin zero Co and is electrically conduct- Sr2RuO4, this superconductor is of inter- as corner sharing tetrahedra, a geometry ing. The motion of charge in a layered est because there may be a link between that frustrates the long range magnetic triangular lattice based on spin K is the magnetism usually observed in Co- ordering of antiferromagnetically coupled new territory in experimental condensed based compounds and its superconduc- tivity. The chemical instability of this spins at low temperatures.17 The first of matter physics, and many strange compound is at the root of a continuing these discovered was Cd Re O ,18 and characteristics are expected and indeed 2 2 7 controversy concerning even its formula: subsequent materials have been osmates found. The superconducting compound information that is critical for determin- like KOs O .19 There are enough is just barely stable under ambient 2 6 ing the most fundamental of charac- examples of these that they can now be conditions, decomposing at room tem- teristics of the superconductor. considered a family, but the fact that perature at relative humidities of less they are Os oxides has restricted their than 40% (e.g. lab air), and at any PuCoGa study. The evidence so far is that the humidity level for temperatures above 5 superconductivity, at temperatures below 30 uC. I believe that though some reliable This compound was discovered to be 10 K, has nothing to do with magnetism papers have been published on this super- superconducting at a very high 18 K, not or the potential importance of the conductor, there are a larger number of surprisingly, by materials physicists at triangle-based geometry of the lattice. experimental studies that are incorrect. Los Alamos.15 They were investigating Unless another, chemically stable, super- the implications of the properties of a Elements conductor is discovered in this family, similar compound based on Ce rather the confusion surrounding the charac- than Pu, special due to the interactions In the past decade, the non-supercon- terization of the properties of this mate- of localized electrons in the rare earth ducting elements have been studied rial dooms it to a future as muddy as the 4f orbitals with delocalized electrons systematically under high pressure material itself. derived from the other metals. When to determine whether they can be

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made superconducting. High pressure ‘valence skippers’ (e.g. Bi3+ and Bi5+), features of the band structures in many decreases the distance between atoms, look at materials for which polaronic superconducting materials. We have and orbital overlap is improved drama- charge transport is important, look at found several examples of such occur- tically. New solid phases form as mole- the boundary between metallic and rences in the calculated band structures cules break down to form extended insulating phases that can be crossed of new materials that unluckily we were solids. Under sufficiently high pressures by doping’’, and others. The physics not ever able to make superconducting. many nonmetallic elements become behind each of these suggestions is quite Obviously, real theorists would have to metallic conductors, and many have interesting—spin K systems obey quan- figure out the potential for coupling these been found to be very impressive super- tum mechanical not classical physics, electronic states to the lattice and other conductors (e.g. S at 17 K under a triangular lattices frustrate simple mag- important factors to get a real prediction pressure of 2 million atmospheres20 and netic ordering schemes, valence skipping for superconductivity. My guess is that oxygen at 0.6 K under 1 million atmo- reflects an intrinsic electronic instability, most chemists would not interested spheres of pressure21). This kind of charge transport by polarons indicates in going that far into the theoretical experimental work is not for the timid. strong electron-lattice coupling, and the calculations—one might as well just crossover point between different elec- make the compound and test it. Guidance from theory tronic states is right where opposing forces come into balance—motivating Speculations on the future In my view, one of the joys of solid state many studies in the properties of chemistry is its unpredictability; both in extended solids beyond looking for new It’s best not to try to predict the future what kinds of crystal structures form superconductors.22 of new superconducting materials, since from a particular set of elements, and Predictions for specific materials are we seem to average about one totally how to go about trying to make a generally based on looking in detail at the new and unexpected superconductor particular compound by a rational syn- properties of a known superconductor discovered every two years. The best thetic route. I realize that this is the and then predicting that a related phase opportunities in well studied materials reason that ‘‘real’’ chemists are wary of may do equivalently well or better. In my classes such as the oxides lie in the us, but most of us have learned to live opinion, the most dramatic success of compounds of marginal stability or with that. For my part, I love the fact this type of prediction is that the analysis syntheses too involved to be attempted

that there are almost as many ways of of BaPb0.75Bi0.25O3 led to the suggestion by condensed matter physicists. Syntheses looking at the way solid state chemistry that (Ba12xKx)BiO3 would be a better of air sensitive materials or at high works as there are solid state chemists: superconductor (it is). The detailed ana- pressures fall into this category. In

I am often surprised by the report of a lysis of MgB2, on the other hand, intermetallics, it seems to me that the Downloaded on 05 January 2011 remarkable synthesis or structure that I suggested that isoelectronic LiBC would opportunities for finding a new super-

Published on http://pubs.rsc.org | doi:10.1039/B512643F never imagined would be possible but also be superconducting if successfully conductor by conventional synthesis was obvious to someone else. Trying to chemically doped with electrons (so far, generally lie in quaternary systems, design a new compound with a particular it isn’t). There is one interesting aspect of which are largely unexplored. The fact set of electronic or magnetic properties is this possible route to superconductivity. that MgB2 was missed in the 1960s was a grand challenge in our field. Given all Most chemists unluckily do not have the due to the fact that it is made with Mg, this, I don’t think it is reasonable to ask luxury of having a theorist interested in which requires somewhat more attentive theorists to be able to predict from first band structures as an available colla- synthetic procedures than were typically principles what crystal structure a parti- borator at their institution. However, used by the people working in the field cular combination of elements will form there are a few band structure programs at the time. In ternary systems, more and whether that combination will be now available that are sufficiently ‘‘che- involved syntheses will still likely bring superconducting or not. mist friendly’’ and accurate enough to be surprises. My personal view is that The prediction of the properties of used internally in a synthetic group to concepts such as ‘‘valence skipping’’ have compounds with known or hypothesized help guide which materials to look not yet been used to their full advantage crystal structures is a promising avenue at most carefully as potential super- in looking for new superconductors, and for theoretical support, however. This conductors. There are two ways such I think that interesting materials remain kind of guidance has taken two forms in programs might be useful: if they predict to be found in that area. Quite possibly the search for new superconducting the presence of a peak in the density of because I don’t know enough about it, materials: it has presented general prin- states at or near the Fermi energy at a I think that there must be a goldmine ciples for what to try to embody in a particular electron count, or if the pre- of new organic and molecular super- material, and also has given recommen- dicted E vs. k dispersion relations show conductors left to find, as many of the dations for looking at specific materials. the presence of a Van Hove singularity (a known superconductors are chemically Some of the general theoretical sugges- saddle point in the electron energy vs. and structurally similar. Finally, like tions that I personally have followed up wavevector relations for an electronic many in the field, I think that the

on are: ‘‘look for spin K systems that band) in the vicinity of EF. These two links between superconductivity and you can make into metallic conductors, criteria are neither necessary nor suffi- magnetism are where the richest phe- look for conducting triangular lattices, cient to predict that a compound will be nomena and most surprising super- try to make continuous lattices out of superconducting, but they do occur as conductors likely lie, and that there will

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be many more like those described above 5 B. Raveau, C. Michel, M. Hervieu, 14 K. Takada, H. Sakurai, E. Takayama- D. Groult, in Chemistry of Superconductor Muromachi, F. Izumi, R. A. Dilanian and to find in the future. I am an optimist: Materials, T. A. Vanderah, ed., Noyes T. Sasaki, , 2003, 422, 53. just when it looks like all the possibilities Publications, NJ, 1992, pp. 106–145. 15 J. L. Sarrao, L. A. Morales, have been tried, someone will find a new 6 Y. Maeno, H. Hashimoto, K. Yoshida, J. D. Thompson, B. L. Scott, superconductor that will change the S. Nishizaki, T. Fujita, J. G. Bednorz and G. R. Stewart, F. Wastin, J. Rebizant, F. Lichtenberg, Nature, 1994, 372, 532. P. Boulet, E. Colineau and G. H. Lander, direction of the field. The next time that 7 Y. Maeno, T. M. Rice and M. Sigrist, Nature, 2002, 420, 297. happens you are most welcome to join in. Phys. Today, 2001, 54, 42. 16 See, for example: Magnetism in Heavy If you are a theorist, I will happily have 8 J. Nagamatsu, N. Nakagawa, Fermion Systems,H.B.Radousky, my students try to make any compound Y. Z. Murakana and N. Akimitsu, ed., Series in Modern Condensed Nature, 2001, 410, 63. Matter Physics - Vol. 11,World that you suggest might be superconduct- 9 See, for example, the special issue of Scientific, Singapore, 2000; B. D. Gaulin, ing as long as it is not radioactive or Physica C, 2003, 385, 1–305. M. Mao, C. R. Wiebe, Y. Qiu, (very) poisonous. So please send me your 10 C. Pfleiderer, M. Uhlarz, S. M. Hayden, S. M. Shapiro, C. Broholm, S.-H. Lee and J. D. Garrett, Phys. Rev. B, 2002, 66, suggestions. R.Vollmer,H.V.Loehneysen, N. R. Bernhoeft and G. G. Lonzarich, 174520. Nature, 2001, 412, 58. 17 A. P. Ramirez, Annu. Rev. Mater. Sci., Notes and references 11 S. S. Saxena, P. Agarwal, K. Ahllan, 1994, 24, 453–480. F. M. Grosche, R. K. W. Haselwimmer, 18 M. Hanawa, Y. Muraoka, T. Tayama, 1 G. Vialdi, Superconductivity: The Next M. J. Steiner, E. Pugh, I. R. Walker, T. Sakakibara, J. Yamaura and Z. Hiroi, Revolution, Cambridge University Press, S. R. Julian, P. Monthoux, Phys. Rev. Lett., 2001, 87, 187001. Cambridge, UK, 1993. G. G. Lonzarich, A. Huxley, I. Sheikin, 19 S. Yonezawa, Y. Muraoka, Y. Matsushita 2 J. Hauck and K. Mitka, Supercond. Sci. D. Braithwaite and J. Flouquet, Nature, and Z. Hiroi, J. Phys.: Condens. Matter, Technol., 1998, 11, 614. 2000, 406, 587. 2004, 16, L9. 3 J. Etourneau, in Solid State Chemistry: 12 See for example S. Sachdev, Quantum 20 E. Gregoryanz, V. V. Struzhkin, Compounds, A. K. Cheetham and P. Day, Critical Transitions,Cambridge R. J. Hemley, M. I. Eremets, H. Mao eds., Clarendon Press, Oxford, 1992, University Press, Cambridge, UK, 1999. and Y. A. Timofeev, Phys. Rev. B, 2002, pp. 60–111. 13 T. He, Q. Huang, A. P. Ramirez, Y. Wang, 65, 064504. 4 Concise Encyclopedia of Magnetic and K. A. Regan, N. Rogado, M. A. Hayward, 21 K. Shimizu, K. Suhara, M. Ikumo, Superconducting Materials, J. Evetts, ed., M. K. Haas, J. J. Slusky, K. Inumara, M. Eremets and K. Amaya, Nature, Pergamon Press, New York, 1992, H.W.Zandbergen,N.P.Ongand 1998, 393, 767. pp. 533–549. R. J. Cava, Nature, 2001, 411, 54. 22 E. Dagotto, Science, 2005, 309, 257. Downloaded on 05 January 2011 Published on http://pubs.rsc.org | doi:10.1039/B512643F

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