UC Irvine UC Irvine Previously Published Works

Title Epilogue: Superconducting materials past, present and future

Permalink https://escholarship.org/uc/item/9b9844b3

Authors Chu, CW Canfield, PC Dynes, RC et al.

Publication Date 2015-07-15

DOI 10.1016/j.physc.2015.03.003

License https://creativecommons.org/licenses/by/4.0/ 4.0

Peer reviewed

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From our in high temperature superconductivity, if proven. At the same knowledge, experience and confidence, we projected that no time, it reinforces the belief of practitioners in superconductiv- superconductor with a Tc greater than 30K would be found. ity that the field is vast and bright, and that more excitements A variety of new classes of materials were subsequently are yet to come. found by experimental chemists, physicists and material scien- Issac Asimov once said the content of the future is what tists beginning with the cuprates and followed by Fe based, B people are most insecure about. Indeed, we may not yet know based and other compounds. They couldn′t have been predicted enough about the future superconductors, especially the room based on our (we thought thorough) understanding. We tried to temperature superconductors, to ask the right questions to find force-fit these materials into our existing understanding (BCS them. However, we have learned from history that as long as and electron-phonon) and it just didn‘t work. A more generic physics does not say it will not happen, it will happen super- model of pairing which includes the electron-phonon interac- conductors at higher temperature will be discovered and new tion but not exclusively will come. There have been many al- heroes will be found. ready described but the sense of triumph after BCS and the electron-phonon interaction is still awaiting. Many of us (me included) keep searching. I look forward to that event when it Paul Canfield happens. After roughly 30 years in this field, I think that the best form to summarize my thoughts about new superconducting materi- Zachary Fisk als is the limerick. Basically, if you cannot find humor in our fumbling about, then I fear the only alternative is insanity. One take on superconductivity is that it shows up to solve some problem in a materials electronics: good metals with- History: out problems such as Ag and Au are in general not supercon- Back in the day, superconductivity was rare. ductors. The cuprates and pnictides descend from compounds About elements and simplicity you′d care. whose prototype compositions make valence sense and whose Now most material classes, charged layer stacking facilitates introducing carriers. This sit- Including oxides and spin glasses, uation is significantly different from what one finds with most Go super without any fanfare. metallic materials where valence is not a particularly useful concept. Conventional superconductors: In the vicinity of this valence border the materials show a va- For traditional Tc’s to reach high, riety of low temperature behaviors that suggest, loosely speak- Look for banding that′s σ or π. ing, problems the materials are having taking care of the de- Use elements light, grees of freedom coming from the free carriers. These carriers And bonding that′s tight, are interacting with a background valence bond structure which And hope critical currents won′t die. seems closer to localized chemistry than free electron physics. From this angle one sees the occurrence of superconductivity High Tc superconductors: as diagnostic of a tension between bonds and bands. So copper and iron are done, Now, cobalt or nickel′d be fun. Bertram Batlogg While the search must be agile, Look for moments quite fragile, Phase-diagram-superconductors And dimensions of two rather than one. While it has become convenient to classify superconduc- tors by the order parameter symmetry or the pairing mecha- Bob Dynes nism as conventional or unconventional, an alternative view of Reflections....and Projections capturing the fascination of superconductors is to realize that many are phase-diagram-superconductors. As an external pa- I grew up scientifically just after the BCS theory of super- rameter is tuned, the superconducting ground state forms next conductivity. In that famous work and in the period following to a different electronic ground state: Mott-Hubbard insulator it, the quantitative verification of the role of the time-retarded, in layered cuprates or spin-density-wave in iron-chalcogenides/ electron-phonon interaction as the mechanism responsible for pnictides. One of the earliest examples of other electronic insta- the pairing of the electrons leading to the condensation into the bilities is found in the bismuthate group with perovskite-related superconducting state was a theoretical and experimental tri- structures and transition temperatures above 30K. Here, a mod- umph. First-principles interactions described the variation of ulation of the charge on the Bi site forms, related to the valence 2 skipping tendency of Bi. A common tuning parameter is the the lattice constant. It would be beautiful if higher resolution variation of the electron count by aliovalent substitution or by STM experiments could determine whether the predicted bond intercalation, and straining the samples by external or chemical contraction actually occurs in charge ordered segments seen in pressure can also be a powerful method to map out the phase lightly doped cuprates. diagram. The challenge then is to identify the underlying inter- At the other end of the phase diagram, are metallic-like actions and how the superconducting state emerges. superconducting samples really metallic in the conventional Whenever a new superconductor is synthesized, a first char- sense? Or do localized pseudo-gap states persist up to the acterization may include measuring the electronic density of upper edge of the superconducting dome? This question is states at the Fermi level, the Sommerfeld γ. Consulting the both of academic and practical interest because localized states γ − Tc “map” with its numerous entries of all superconduc- take away some of the superconducting condensation energy, tors is helpful in suggesting if the superconductor is extraor- which impacts negatively vortex pinning. Their elimination dinary in that the transition temperature is unusually high or would greatly improve high temperature performance of High low compared to others with similar density of states. Promi- Tc wires. It might enable superconducting magnets providing nent outliers on the high- Tc side include bismuthates, cuprates, fields of the order of 10 T at liquid nitrogen temperature, not a and also MgB2, suggesting a particularly high characteristic en- small achievement. ergy of the pairing glue. On the other extreme one finds the Last but not least, pervading the entire phase diagram, is the heavy-fermion superconductors. Interestingly, Fe-based super- question of the significance of the two distinct energy scales conductors with their particular relationship between Tc and γ, which I originally noted by comparing the results of Point Con- are unremarkably located on the γ − Tc “map”, near the trend tact in the Sharvin limit and Tunneling experiments. The first line of ,e.g., ternary boron-carbides and Nb3Ge. Thus, con- one, the coherence energy scale ∆c, follows the same doping sulting this map might reveal unusual parameter combinations dependence as the critical temperature does, while the second relevant for superconductivity. one ∆p increases continuously as doping is reduced. It is this It will be interesting to see if future high-Tc superconductors, second one that was first thought to be the superconducting gap hopefully with application potential, will also be of the “phase a la BCS. Several beautiful experiments STM have in the mean diagram superconductor” type. time shown that it is highly inhomogeneous (while the DOS is homogeneous on the scale of ∆c) and there is convergingagree- Guy Deutscher ment that it is not in fact the superconductinggap. What is still missing is a determination of the angular dependence of ∆c. Is This special issue in honor of Ted Geballe on the occasion of it well defined in the anti-nodal regions of the Fermi surface, or his 95th anniversary brings back many happy memories of our only in the nodal ones? More detailed interference experiments never ending although geographically distant relationship. One between Bogoliubov-de Gennes quasi-particles, as performed of them is from the 1974 meeting on High Temperature Super- by the group of Davis, might give us an answer to this question. conductivity held in Israel at the Kibbutz Kefar Gileadi, near the Lebanese border, where Ted managed to transform a threat- Ted Geballe ening confrontation between Bernd Matthias and Alan Heeger into a peaceful and rewarding scientific discussion on reduced I thank the editors for this opportunity to share insights that dimensionality and High Tc. A theme that is still with us 40 I have acquired over lengthy years of research at Berkeley, Bell years later. Labs and Stanford. They come from a healthy mix of physics, One can only marvel at the rich playground that has opened chemistry, material science, technology and chance conversa- up following the discovery of the High Tc cuprates by Bed- tions with colleagues. norz and M¨uller. The decisive step they took in their search for Why did it take so long to discover Type II superconductiv- higher temperature superconductivity was to move from metals ity? Was it because people of my generation were taught that and alloys to nearly insulating compounds having strong corre- the high Tc’s [i.e. above 10K] were only found in “hard”, i.e. lation effects. Cuprates turned out to be the right choice. Upon brittle superconductors and were explained by Mendelsohn′s doping the parent antiferromagnetic compound a phase dia- sponge theory? After the World War II advances in metallurgy, gram develops, running from the insulating phase to the normal made in the Manhattan Project, were declassified and “hard- metallic phase, with the now famous superconducting dome in ness” was identified as extrinsic, mainly due to unrecognized between. This phase diagram has been already studied in depth, traces of oxygen. Mendelsohn′s theory assumed a spongeof su- but I feel that the two edges of the dome deserve more attention perconducting filaments with cross- sections less than the pen- than they have so far received. etration depth, thus explaining why they had higher Tc’s but Does the electronic structure of lightly doped, still insulating were useless for carrying superconducting currents. If the Lei- and anti-ferromagnetic samples, contain seeds of the supercon- den studies of PbBi alloys back in 1930 had included measure- ducting phase? This is what the late Pierre Gilles de Gennes ments of Jc(H) the sponge theory would never have seen day- and I have suggested. We predicted that in this regime carri- light, but amazingly it was taught and accepted for more than ers of opposite spins could form bound pairs by contraction of 25 years. Cu-O bonds around oxygen atoms, triggering the formation of Advances in superconductivity come from both fundamental charged ordered segments having a periodicity of four times and applied research. The need to shield the three level mi- 3 crowave amplifiers [masers] to be used in the transcontinental impurity phase but I had long learned to listen to John and stud- microwave link is an example of the latter. Rudi Kompfner ied the Russian work carefully. Their extensive measurements asked me if Nb3Sn, which we had just discovered, might be [A. Chernik and S. N. Lykov, Sov. Phys. Solid State 23, 817 suitable. This open mindedness contrasts with the negative re- 1981; V. I. Kaidanov and Y. I. Ravich, Sov. Phys. Usp. 28, 31 action of the experts when Bernd Matthias reported the discov- (1985)] suggested new physics. These led to investigations in ery of Nb3Sn in a talk at the international low temperature con- Ian Fishers group that showed thallium ions with skipped valen- ference in Europe. Rudi asked me to test the idea. cies [6S 0 and 6S 2] configurations are degenerate and accompa- But we were fully engaged in what seemed to be a much nying charge Kondo resistance gave strong evidence for a neg- more fundamental problem − looking for a new mechanism to ative U superconducting pairing mechanism [Phys Rev Lett 94 account for unusual superconducting behavior we had found in 157002 2005; Phys. Rev. B 74, 134512 (2006); a history of transition metal compounds that appeared to be at odds with negative U is given in arxiv.org/pdf/1406.3759]]. BCS phonon-mediated pairing. A key input to that theory, the When compared with BCS superconductors that have the ′ “isotope effect”, i.e. the square root dependence of Tc upon same superfluid density, the Tcs of PbTe(Tl) are two orders of ′ isotopic mass, that comes from the mass dependence of the magnitude greater. Much higher Tcs are possible if the density phonon spectrum. We noticed that it had been established only of negative U pairing centers could be increased. Experiments with non-transition metal elements. We obtained samples of Ru attempting this have so far not been successful . 99 and Ru 104 from Oak Ridge, and found no dependence of Synthesis and characterization capabilities have been Tc upon mass. For the next few weeks of checking I walked steadily improving throughout science and technology [think around in a state of euphoria. It ended when I walked into Moore’s law]. It is easy to predict that this will continue and Phil Andersons office and found that our result was of funda- open vast new regions of non-equilibrium phase space . It is mental importance but not because it demonstrated the need a matter of faith to predict that some will contain surprises for a new mechanism, but rather the opposite. He pointed out that rank with Bednorz and Muellers discovery of supercon- that the electron-phonon interaction in narrow band supercon- ducting cuprates. I am optimistic. I believe that some of the ductors is localized in space and retarded in time, and for Ru USOs (unidentified superconducting objects) mentioned in ear- the retardation is mass dependent and approximately cancels lier chapter may be the tip of the iceberg. To paraphrase Van- the mass dependence of the phonon spectrum, [Phys. Rev.125, nevar Bush, our field remains a never-ending frontier. 1263 (1962)]. I am pleased about our contribution even though it didn’t lead to to a new pairing mechanism it gave strong Zhongxian Zhao support to a deeper understanding of BCS. The localized na- ture of the interaction also offered a rationale for me to under- Route towards high critical temperature superconductors stand the chemical correlation that had puzzled me−why did Nb−containing compounds in any given family of intermetallic Prof . Ted Geballe was the first American I met in my life. It superconductors always have the highest Tc’s? was 1975 when I was a visiting scientist in University of Cam- I suggested to Rudi making Nb3Sn samples needed for test- bridge. Dr. Jan Evetts introduced me to him, calling me as a ing could best be done in the Metallurgy Dept. The clever native Chinese from Beijing. Ted told me that he was preparing “wind and then react” method that was developed [Phys. Rev. to visit Beijing. I felt very lucky for meeting Ted because half Lett. 6, 89 (1961)] succeeded in making wires. The discovery a year later I decided to engage in exploring high critical tem- that they carried huge currents in high fields was totally unex- perature superconductors and he is the well-known authority in pected and had major impacts in solid state and high energy the field. Putting it in a Chinese way, I felt this must be a fate. physics, in medical diagnostics, and in enabling many other We met many times over the past forty years, with a main topic technologies. Abrikosovs here-to-fore unappreciated theory in our conversations on how to search for new superconductors. was “discovered” and type II superconductivity became main- In the early days of my research, I came to form two line. main viewpoints on exploring high temperature superconduc- Of course many advances in superconducting science and tors based on literature search and deep thinking. The first technology come from theory. Until graduate student Brian viewpoint is that the superconducting critical temperature is Josephson formulated his theory of coherent pair tunneling possible to exceed 40 K based on the electron-phonon inter- across weakly linked superconductors there was no justification action mechanism. The second viewpoint is that there could for assuming that zero bias current flow was due to anything be other mechanisms to realize superconductivity in addition to other than shorts. That changed when phase interference effects the electron-phonon interaction (Zhongxian Zhao, Exploration in small magnetic fields were observed by Rowell and Ander- of High Critical Temperature Superconductors, Physics (in Chi- son and gave convincingproof of Josephson’s theory. The enor- nese) 6 (1977) 211). I still hold on these two views today. mous impact that Josephson tunneling by paired electrons has Strong electron-phonon interaction leads to lattice instability; had and continues to have is well known. the stronger the electron-phononinteraction, the higher the crit- One day out of the blue John Bardeen phoned to ask what I ical temperature, as long as the lattice is not undergoinga phase thought of the Russian reports of superconductivity in the 4-6 transition. When J. Bednorz and K.A. M¨uller reported possi- semiconductor PbTe when 1% if the Pb was replaced by Tl. ble high temperature superconductivity in copper-oxide com- My first thought was that the signals came from an undetected pounds in 1986, I quickly realized its importance because it res- 4 onated with my viewpoints: dynamic Jahn-Teller effect can lead Rick Greene to lattice instability but without phase transition, thus making it possible to realize high temperature superconductivity. Now I have known Ted Geballe since 1967 when I became the first we know that the superconductivity of copper-oxides cannot be postdoc in his new laboratory at Stanford University, to which accounted for by only considering the electron-phonon interac- he had just moved after a distinguished 15 years of research at tion. The exploration of high temperature superconductivity in Bell Laboratories in New Jersey. I have now enjoyed almost 50 metallic hydrogen, a prediction based on the electron-phonon years of interaction with Ted, both scientific and social. He has coupling mechanism, is promising. With an aid of some cataly- had a tremendous influence in my life— he is one of the finest sis, I think it is possible to realize superconducting metallic hy- scientists and human beings that I have ever known. Little didI drogen in some hydrogen-rich compounds under a pressure of know how my scientific career would be changedwhen I started a few hundred GPa. In general, intuition and empirical under- working with Ted in the summer of 1967. In fact, it was sur- standing can be helpful in finding new superconductors. New prising to me that Ted hired me since I knew nothing about, superconductors can be explored and discovered in materials nor had any experience with, the research that he wanted to do with multiple interactions (such as charge order and spin order at Stanford on superconducting and other materials with novel and tetragonal layered structures). electronic properties. My PhD training had been on the optical properties of magnetic insulators. So here was Ted with a com- The experimental data on the iron-based superconductors are pletely empty lab in an old World War II building with no air not systematic enough to extract general and key information conditioning, with a new postdoc who knew nothing about ex- for understanding the superconductivity mechanism. In par- perimental materials research. After a few months at Stanford ticular, superconductors based on FeAs- and FeSe- building he may have regretted leaving the well-funded and scientifically blocks show quite different behaviors that lead to different the- stimulating Bell Labs, but he never let this be known to me or oretical explanations. Such a difference may originate partly his newly recruited graduate students. from the sample quality issue of the FeSe-based superconduc- The breadth of Teds creativity was amazing to me! During tors or partly from the lack of many classes of the FeSe-based my time as his postdoc (1967-70) we worked on many new superconductors present. Recent results on the phase diagram things: granular superconductors, intercalated transition metal of new 1111 system (Li1−xFex)OHFeSe (X.F. Lu et al, Nat dichalcogenide superconductors (reviewed by R. A. Klemm in Mater, doi:10.1038/nmat4155 (2014); X.L. Dong et al, J Am this issue), tungsten bronze oxide superconductors, Kondo ef- Chem Soc 137 (2015) 66) indicate that the basic characteris- fect materials (e.g., SmB6, which has become a hot topic re- tics of FeAs-based and FeSe-based superconductors could be cently because of its low temperature topological properties), the same. These imply the same superconductivity mecha- A15 and intercalated graphite superconductors, and a few oth- nism for the FeAs- and FeSe-based superconductors. They ers that I have forgotten. Some of these and many of the other may even share some common features with the copper-oxide superconducting materials that Ted has worked on during his superconductors such as high Tc, tetragonal layered structure, long career are mentioned in this special issue of Physica C. and similar transport and magnetic properties. Such quasi- In addition to discovering many new and interesting materials, single-layer FeSe-based (Li1-xFex)OHFeSe superconductors Ted initiated the development of a new relaxation method for offer new opportunities for superconductivity research, espe- measuring the specific heat of small samples. Everyone in Teds cially with high-quality single crystal samples. group worked on various aspects of this idea which culminated There are still plenty of rooms for exploring superconduc- in a famous and highly cited publication (R. Bachmann et al., tivity in cuprates. For example, the development of new tech- Rev. Sci. Inst. 43, 205 (1972)). If you look at the author list on niques to precisely tune magnetic order and fluctuations, with this paper you will see an alphabetical listing of all the people an aim to control both the carrier concentration and magnetic who were workingfor Ted duringthis time, and indicativeof his order in ultrathin films of the copper-oxide parent compounds humble nature, Ted did not put his name at the end as the senior in order to achieve superconductivity, is promising in under- author. Improvementsof the relaxation method for specific heat standing the superconductivity mechanism and searching for measurements were developed by subsequent Geballe gradu- new superconductors. Overall, there are a lot of opportunities ate students—e.g., Greg Stewart who has written about Ted in in novel superconductor exploration based on the cuprate and this Physica C issue—and this method is now used, in modified iron-based superconductors. form, in the Quantum Design PPMS systems found in many labs around the world. So, THANKS, TED for inspiring me Interface superconductivity has been studied for many years. in my career of research on superconducting materials and in With the advancement of related material preparation and ex- many other aspects of my life. perimental techniques, this topic has ushered in a new oppor- So what can I say in a few paragraphsabout the present status tunity. The discovery of superconductors always comes as sur- and future of superconducting materials research? The present prises. I believe there will be many surprises to come in the status is well covered by the many articles in this Physica C is- future, including the discovery of room temperature supercon- sue. There are crucial questions that remain unanswered about ductors. In the meantime, the complete solution of the high the normal state physics and the pairing mechanism in high- temperature superconductivity mechanism should come with Tc superconductors, primarily for the cuprates. The solution the emergence of new solid state physics. to the mystery of the cuprates is perhaps the most outstand- 5 ing problem of condensed matter physics. Much new physics magnetic when undoped, and they are oxides! The iron-based has been learned and many new or improved experimental tech- superconductors have iron as one of their constituents and they niques have been developedover the past 28 years of cuprate re- are antiferromagneticat low doping (or pressure). The cuprates, search and I expect that there will continue to be slow progress organics and iron-based materials not only have antiferromag- in our understanding of strongly correlated superconductors. I netism in their phase diagram, but they are poor metals in their have my own opinions about what may be important for solv- normal state. ing the problem of the cuprates, but I cannot justify them here So, what should be the new rules for finding new supercon- so I will pass on this part of my assignment from the editors. ducting materials? I would propose: One thing I will say is that I believe that an understanding of 1) magnetism is good, i.e., having a phase diagram where the electron-doped cuprates will be crucial for understanding magnetism can be suppressed by doping or pressure; the pairing mechanism for all of the cuprates. Recent research 2) layered structures are good; on the hole-doped cuprates has focused on the many interest- 3) controlled interfaces between metals and insulators (or ing and perplexing properties of the pseudogap state. How- semiconductors) are good; ever, there appears to be no comparable pseudogap state in the 4) having light elements in the structure is good, i.e., high electron-doped cuprates! This suggests to me that most of the energy optical phonons are good if mobile electrons can couple interesting physics going on in the pseudogap part of the phase to them. diagram is not directly relevant for the mechanism of supercon- To reach higher Tc it will be necessary to have a large J (ex- ductivity in the cuprates. Based on the research done on the change energy) in superconductors driven by a magnetic mech- electron-doped cuprates I believe that antiferromagnetic inter- anism. But, lower Tc new materials should not be ignored since actions of some kind (e.g. spin fluctuations or quantum critical modest structural or chemical changes or high pressure could point fluctuations) are the driving mechanism for the supercon- lead to much higher Tc. Superconductors discovered at high ductivity in all of the cuprates. Hopefully, the theorists will pressure are important because it may be possible to stabilize soon come to some agreement on how this all works and give them at atmospheric pressure via chemistry for bulk samples or the experimentalists a theoretical expression for Tc similar to by interfacial strain in a film. the BCS, McMillan or Allen-Dynes formulas for the electron- Now what about the Matthias rule stay away from theorists? phonon interaction. This was mostly true in his day, but now I would strongly rec- What about the prospects for finding higher temperature su- ommend that experimentalists pay close attention to theorists perconductors? I am quite optimistic that a room temperature and even enlist their help. First of all, it is helpful to have a superconductor will be found wihin the next 30 years, hope- working hypothesis for where to search for higher Tc materi- fully much sooner so that I can see it happen. There appears to als even if it is wrong. The discovery of high-Tc supercon- be no theoretical reason why this is not possible. Moreover, the ductivity in the cuprates is an excellent example of this. The recent discovery of superconductivity at 190K under high pres- recent high-Tc found in compressed H2S suggests that theoret- sure (Drozdoz et al., arXiv: 1412.0460), if confirmed, suggests ical predictions of high-Tc superconductivity can occasionally that even the electron-phonon interaction can lead to higher- be correct, even if not exact in all the details or the predicted Tc superconductors. Research on strongly correlated materials Tc value. Second, computational methods for calculating band like cuprates, organics, heavy fermions and iron-based super- structure and predicting superconducting interactions are vastly conductors indicates that a magnetic pairing of some kind is improved since the days of Matthias. In conclusion, I think that necessary for higher-Tc. This is certainly true for these materi- the future is bright for significant fundamental advances in su- als, but the electron-phonon interaction is back in the game for perconducting materials research. high-Tc. Phonons may also be involved in the Cooper pairing for the 100K superconductivity in single layer FeSe on STO Hideo Hosono (see I. Bozovic and C. Ahn, Nature Phys. 10, 892 (2014) for a commentary on this interfacial superconductor). Toward Room Temperature Superconductors What is the most promising route to higher-Tc? I be- lieve that we must try many different approaches. Many of It is a dream for human beings to realize a room temper- the highest-Tc and most interesting superconducting materi- ature superconductor since the discovery of superconductivity als have been discovered by the empirical method—educated by Heike Kammerling Onnes in 1911. Although the fundamen- guesswork guided by experience, intuition and hunches—with tal theoretical framework was established in 1957 by BCS the- a lot of luck (serendipity). When I started searching for ory, there exists no theory which can quantitatively predict the new superconductors with Ted Geballe in the late 1960s the critical temperature (Tc) even now. Thus, exploration of high Bernd Matthias “rules” were a guiding principle. Roughly Tc superconductors is like a voyage in a big ocean without a stated these rules were: high symmetry is good with cubic best, precious compass, i.e., the researchers have to determine the high density of electronic states is good, stay away from oxy- course believing their materials sense and/or intuition referring gen, stay away from magnetism, stay away from insulators, to what the theorists says. In this sense, exploration of high and stay away from theorists. Now we have learned that these Tc superconductors is a really challenging subject in condensed Matthias rules are mostly incorrect or too limiting. Cuprates matter research. Most people would agree that this is a typical are layered materials of lower symmetry, they are antiferro- “all-or-nothing” research subject. 6 It is a historical fact that a material leading to a breakthrough state chemists. has been discovered in most cases by chance, amidst concen- One notes that when one sees the plot of Tc vs. year that trated (and often uniquely-styled) research efforts, where the there are two modes, a continuous mode based on the conven- researcher kept an open and flexible view of the possible routes tional pairing mechanism and an abrupt mode due to a non- to a new material with interesting properties. This methodol- conventional mechanism. What is the next example of a break- ogy appears to be particularly true for superconductors. Here, through superconductor? A striking result was posted on the please allow me to introduce my experience as a case study, on online Archive on Dec. 2, 2015. The preprint reports the obser- which I now elaborate. In early 2009, the Japanese Govern- vation of zero-resistivity at 190K in H2 − H2S under ∼20GPa ment announced the launch of a new funding program, FIRST. [A.P. Drozdov, M.I. Eremets, I.A. Troyan, arXiv:1412.0460]. My research proposal, “Exploration for novel superconductors If this finding comes from true superconductivity, this seems to and relevant functional materials, and development of super- be a straightforward approach based on the conventional BCS conducting wires for industrial applications” was fortunately framework, i.e. via the formation of metallic hydrogen. accepted as one of 30 projects covering a wide range of sci- Last, I would like to stress the fertility richness of materi- ence and technology areas. It was my expectation that novel als we are engaging in. Exploration of novel superconductors functional materials with high potential could be found through needs a non-conventional approach in various aspects such as this tough but really worthwhile challenge. I organized a re- the material system and synthetic processes such as high pres- search team mainly composed of Japanese solid state chemists sure and field effect. As a result, there should be much higher who have much experience and a clear record of achievement probability than in other materials research to find new func- in superconductors. Since research of superconductors belongs tionalities or to encounter new phenomena during the research. to the domain of condensed matter physics, this team orga- A well-known example is the finding of high performance ther- nization would be a unique feature of this project. It is my moelectric material, NaCo2O4 in the course of a comparative belief that excellent solid state chemists may find serendipity, study of high Tc cuprates with layered cobaltites [I. Terasaki, Y. even if they first fail in their hunt for the targeted new high Sasago, K. Uchinokura, Phys. Rev. B 56 (1997) R12685(R)]. Tc materials, i.e., all or something! Our project has examined In our immediate case, excellent catalytic properties [M.Kitano more than 1,000 materials to explore new superconductors. The et al. Nat. Chem. 4 (2012) 934] for ammonia synthesis percentage of success remained low ( 3%) as we estimated at from N2 and H2 gas at an ambient pressure and decomposi- first. Records of an unsuccessful trial, in a particular systematic tion of NH3 were found in inorganic electride C12A7:e (where and/or unique approach, are so informative that these should C:CaO, A:Al2O3) which exhibits a bulk superconductivity at be regarded as treasures of the community. However, no such 0.2-2.5K [H. Hosono et al. Phil.Trans. Roy. Soc. A 373 (2015) records are available to the community as far as I know. Here 20140450]. I believe an extensive effort of exploration for new I want to propose that we open the record of unsuccessful tri- superconductors would lead to the discovery of new function- als in easy accessible media such as journals, books, and data ality even if a high Tc material could not be targeted, because bases. We are going to publish a list of materials examined in materials have huge potential and only a part of it is exposed this FIRST project in an open-access journal [H.H. Hosono et to readily visible places (the relation between graphene and al., Sci. Tech. Adv. Mater., 2015 (in preparation)]. graphite is a typical example). I believe extensive exploration Discovery of a breakthrough superconductor has triggered of new superconductors would be an all or something−type re- the paradigm shift for new high Tc superconductors. The most search if researchers watch for various aspects of materials. representative example is the shift triggered by the discovery of high Tc cuprates from non-magnetic intermetallic compounds with cubic symmetry to doped Mott insulators in layered tran- Brian Maple sition metal oxides. High Tc cuprates were completely op- posite to the working hypotheses for exploring new materials It is very fitting that this Special Issue on Superconducting proposed by Bernd T. Matthias [W. E. Pickett, Physica B 296 Materials is dedicated to Ted Geballe, one of the pioneers of (2001) 112]. the field of superconductingmaterials. Ted worked closely with In 2010 Igor Mazin [I. I. Mazin, Nature 464 (2010) 183] pro- another pioneer of this field, the late Bernd Matthias, who is posed new working hypotheses to explore new high Tc super- known for having synthesized hundreds of new superconduct- conductors by taking discovery of high Tc cuprates and Ferro ing, ferromagnetic and ferroelectric materials and for his out- pnictides into consideration; spoken views concerning superconducting pairing mechanisms (a) Layered structures are good. and the search for high temperature superconductors. I was as- (b) The carrier density should not be too high (comparedwith sociated with Matthias during most of the period he was a Pro- conventional metals) fessor of Physics at the University of California, San Diego, (c) Transition metals of the fourth period (V, Cr, Mn, Fe, from 1960 to 1980, when he unexpectedly passed away, as a Co,Ni, and Cu) are good. graduate student, postdoctoral research physicist and faculty (d) Magnetism is essential. colleague. Thus, it seemed appropriate to make a few remarks (e) Enlist theorists, at least to compute Fermi surfaces about Matthias and his laboratory at UCSD and a set of em- A corollary to these rules: Materials of interest are likely pirical observations about superconducting materials known as to be complex chemical compounds - work closely with solid- Matthias rules. 7 Matthias had a strong personality and a charismatic manner. tion of Fe into Ti was found to raise the Tc of Ti much more Working with him wasnt always easy, but it was always inter- rapidly than expected from his “rules”. (Actually, the Fe addi- esting! He taught us about the role of the periodic table of the tions stabilized another nonmagnetic phase of Ti with a higher elements in developing novel materials that serve as vehicles for value of Tc.) While the interpretation of this experiment was studying challenging problems in condensed matter/materials incorrect, such a magnetic pairing mechanism as envisaged by physics. One of the most important things we learned from him Matthias is widely believed to be responsible for the unconven- was the excitement of research and the thrill of discovery. We tional superconductivity in heavy fermion, organic, as well as met and helped entertain many of Matthias distinguished vis- high Tc cuprate and iron-based superconductors. With respect itors; for example, John Bardeen, Ted Geballe, Ivar Giaever, to rules (4) and (6), Matthias was actually quite interested in ox- Herbert Fr¨ohlich, John Hulm, and Willy Zachariesen. Matthias ide superconductors, several of which had Tcsashighas 12K traveled extensively between UCSD, Bell Laboratories and Los (e.g., Ba(Bi, Pb)O3, LiTi2O4) for several years before he passed Alamos National Laboratory to work with his students, post- away. It would have been interesting to see how he would have docs and collaborators, many of whom are mentioned in the reacted to the discovery of high Tc superconductivity in the Biographical Memoir written by his long time colleagues and cuprates in 1986. I believe he would have been right in the friends, Ted Geballe and John Hulm, for the National Academy middle of it! Finally, “rule” (7) appears to have been added be- of Sciences. In fact, a number of Matthias former students from cause of Matthias’ public chiding of theorists about the inabil- these times have prepared articles or comments for this Special ity of theory to predict where to find high Tc superconductors. Issue, including C. W. Chu, Zachary Fisk, George Webb and While it is indeed true that Matthias publicly railed against the- myself. orists, he did so to emphasize the fact that theory had not been My Ph.D. thesis research was on the interplay be- useful in predicting where to find high Tc superconductors and tween superconductivity and magnetism, which is a com- for effect. In fact, he interacted with a number of distinguished mon thread that runs through much of my research through- theorists, many of whom were also his friends, such as Bardeen out my career. My entry into this subject was inspired and Fr¨ohlich, mentioned before, as well as Phil Anderson, Al by the pioneering research of Matthias and his coworkers who Clogston, Harry Suhl, Peter Wolff, and others. first addressed the question of whether superconductivity and Progress in superconducting materials research has primar- magnetism could coexist in 1958. This turned out to be a very ily been driven by the development of new and better ma- interesting and fruitful direction of research, which led to the terials. Although this field has been declared to be dead at discovery of many new phenomena that are described in the many junctures, it has proven to be very resilient. Time af- article “Conventional magnetic superconductors” by Wolowiec ter time, new and unexpected types of superconducting ma- et al. in this Special Issue. Subsequently, unconventional su- terials have been discovered that reenergized the field; e.g., perconductivity was discovered in heavy fermion compounds, A15 superconductors, magnetically ordered superconductors, cuprates, and iron pnictides/chalcogenides. The unconventional organic superconductors, cuprate superconductors, magnesium superconductivity often occurs in proximity to and sometimes diboride, iron pnictide and chalchogenide superconductors. On coexists with antiferromagnetic order, where both phenomena the basis of past experience, it seems likely that the field of share the same set of electrons. Many researchers believe superconducting materials will continue to yield new and unex- that the pairing of superconducting electrons in these mate- pected surprises in the future. Perhaps even room temperature rials is mediated by magnetic excitations. It is noteworthy superconductivity! that the materials with the highest Tcs, the cuprates and Fe- pnictides/chalcogenides, contain copper oxide and iron pnic- tide/chalcogenide layers, respectively, that can be doped with charge carriers, which suppresses the antiferromagnetic order In closing, the editors would like to extend their heartfelt and renders these compounds superconducting. thanks to all the authors of this Special Issue for taking time Much has been made of Matthias “rules” which were based out of their busy schedules to prepare their thoughtful contribu- on investigations of a vast number of superconductingelements, tions to this endeavor under very tight deadlines. alloys, and compounds. These “rules” could be used to guide searches for new superconducting materials, particularly those with high values of Tc. A “loosely formulated” statement of Matthias “rules”, based on a version due to Warren Pickett, reads as follows: (1) Transition metals are better than sim- ple metals, (2) valence electron per atom ratios of 5 and 7 are favorable, (3) high symmetry is favorable, especially cu- bic, (4) avoid oxygen, (5) avoid magnetism, (6) avoid insula- tors, and (7) avoid theorists. While rules (1), (2) and (3) are well founded, the justification for “rules” (4) through (6), and, especially (7), is not so clear. With regard to “rule” (5), it is noteworthy that Matthias actually proposed a magnetic pairing mechanism in 1963, based on experiments in which substitu- 8