PHYSICA ® ELSEVIER Physica C 341-348 (2000) 37~ ,0

www, elsevier.nl/Iocate/physc

The Search for N e w Materials

T. H. Geballe

Department of Applied Physics, Stanford University

I am delighted to be here, alive and participating in this symposium. I am very grateful to Paul Chu, Mac Beasley, Massimo Marezio and the organizing committee for conceiving of a symposium that provides opportunities to dwell on high risk, lively, and sometimes rewarding adventures that follow from searching for new materials. I thank the previous speakers tonight for giving such nice examples of the diverse and challenging new science that can emerge. The title of the symposium in the context used here should be taken as a shorthand way of meaning searching for new, likely unexpected properties in the materials that may or may not have been newly synthesized. This definition captures the essence of the endeavor.

I. EARLY HISTORY

The early history of the endeavor is the beginning of low temperature physics itself. By the end of the 19th century, Wroblewski in Poland and Dewar and Fleming in England were able to measure the temperature dependence of the electrical conductivity of a variety of metals down to liquid air temperatures. Extrapolation showed there to be a "vanishing point" where the resistance of "pure" metals should cease. In order to follow this idea Dewar made one of the major enabling inventions of all low temperature physics, the vacuum-jacketed insulating vessel which bears his name, and extended the measurements to liquid hydrogen temperatures. He discovered instead of vanishing, the resistance remained a finite sample- dependent residual resistance. Dewar in the l lth Edition of the Encyclopedia Britannica nicely describes low temperature history up to 1911.

After succeeding in liquefying helium Kamerlingh Onnes was able to search another lower decade in temperature, choosing to start with the purest metal available, Hg. The account, or perhaps I should say the legend (I have heard many versions), of how the sudden disappearance of resistance turned from what

was first taken to be shorted wires into one of the major discoveries of the 20th century, has been told many times. That dramatic unexpected discovery has certainly been a major impetus for inspiring others to continue searching. While it is unreasonable to hope to make a comparable discovery, there have been plenty of pleasant surprises. A major one, of course, is the discovery of cuprate superconductivity by Bednorz and Mueller, with the unexpectedly high Tc's above liquid nitrogen temperature by Chu, Wu, and co-workers.

Hans Meissner must be considered the pioneer searcher for superconductors. That he worked in Berlin at a time when liquid helium was expensive and difficult to obtain suggests that he was highly motivated. (As an historical footnote, Meissner did his thesis with Max Planck on black-body radiation.) Even before he did the famous experiments with Ochsenfeld, which demonstrated the reversible thermodynamics of the superconducting state, Meissner had found a surprising variety of unexpected superconductors. In 1929 he found that CuS was superconducting. In the next few years superconductivity was discovered in transition metals, including thorium, and in the first "high temperature" superconducting compound, NbC, as well.

This paper is taken from the notes prepared for my talk at the symposium. I have been limited by space, time, and my own bias. I regret not being able to discuss equally fascinating discoveries in other compounds, doped semiconductors, organic superconductors, doped buckyballs, the inorganic polymer (SN)x (mentioned by Rick Greene) and much more.

0921-4534/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII S0921-4534(00)00384-1

38 T.H. Geballe/Physica C 341-348 (2000) 37-40

II. MODERN HISTORY

Modern is a relat ive term which, f rom my perspective, includes the last half of the 20th century. In 1950 superconductivity was, in spite of Meissner's efforts, a rare and a poorly understood phenomenon. Almost all the leading theoretical physicists had made attempts but were unable to produce satisfactory theories.

John Hulm and Bernd Matthias, then at the University of Chicago, instituted a new search. According to their accounts they were motivated by Enr ico Fermi to search broadly th roughout Mendeleev's Periodic System for new superconductors with the hope of uncovering clues of the underlying physics. It is no stretch of the imagination to think that Fermi was puzzled over why poor metal CuS was superconducting whereas good metal Cu was not. Almost immediately Hulm and Matthias found many new borides and nitrides. Hulm and his student Hardy discovered superconducting V3Si, which formed in a new cubic structure designated (mistakenly) as A15. Matthias, guided by the Periodic Table, recognized the possible existence of other AI5 compounds and synthesized the new compound, Nb3Sn, and others of the same class.

The Periodic Table has remarkable predictive powers over a wide range of energies from microvolts to kilovolts. It has been the most valuable guide in the search for new materials. In 1952 I joined Bell Labs and at first took advantage of the newly available pure Ge and Si single crystals to study transport properties. There were a number of reasons which enticed me to change direction and join with Matthias in searching for new superconductors, not the least was his infectious enthusiasm and energy. I was aware of the role the Periodic Table played in guiding Welker to the discovery of III-V semiconductvity. Also, the time was propitious. Liquid helium had become routinely available using the easily operated Collins liquefier. Crystallographers were interested and had developed efficient methods for characterizing complex materials. Superconductivity is particularly amenable to searches because trace amounts of an unexpected or a new superconducting phase can give weak but easily detected signals. However, playing with weak signals is like playing with fire. If the signal is for real it can heat up the community (as we know from the days of 1986-7), but if it is spurious it can burn. There are,

unfortunately, more examples of the latter than the former.

My first collaboration with Matthias in 1954 was in studying Nb3Sn, as John Rowell has already mentioned. Even though Nb3Sn was found to have the highest known Tc, it didn't attract much interest until seven years after Kunzler, Wernick, and Buehler discovered the technologically useful Jc behavior. In fact, worldwide there had been little effort in searching for superconductors outside of the groups at Bell and Westinghouse, other industrial laboratories, and a few individuals such as Alekseyevsky.

In 1957 Matthias generalized the occurrence of superconductivity using simple "rules" which required no more than position in Mendeleev's Periodic Table as input. As a first approximation, Tc was considered to depend only on the average number of valence electrons per atom. In the non-transition metal part of the Periodic Table this meant all metals or metallic compounds with 2 (i.e. Hg) to 5 (Bi, quench-condensed by Buckel and Hilsch) electrons per atom. Covalent semiconductors and semimetals (e.g. Ge, Sb), if they could be collapsed into a more highly coordinated metallic phase by pressure or by quenching from the liquid or gas phase, were included, as was well-known in the gray-tin/white-tin situation. A major conclusion was that superconductivity should be considered to be the common ground state of metals rather than a rare phenomenon. The alkali metals were and still are somewhat controversial. Theoretical predictions of superconductivity in the special case of metallic hydrogen remain an experimental challenge.

The most useful Matthias rule is that T c in the transition metals is a universal function of the electron- to-atom ratio, with a minimum near the half-filled shell. It is better understood as a function of band filling, particularly in light of the BCS theory, band-structure calculations, and the Collver-Hammond studies of amorphous transition metals. In addition to being so simple to use (requiring only the Periodic Table as input), the empiricism was a valuable guide both when it worked and when it didn't work. Failures frequently turned out to be consequences of new physics or of unsuspected materials science. Successes helped to systematize transition-metal superconductivity. For example, alloying elements to the right of Ti in the Periodic Table would result in Te initially increasing at a rate proportional to how far to the right of column IV, to which Ti belongs, was the column of the added

ZH. Geballe/Physica C 341-348 (2000) 37-40 39

element. The rule was found to have merit throughout the 3d, 4d, and 5d bands by Hulm and Blaugher and provided the guidance needed for the technology of today's NbTi superconducting magnets.

Exceptions to the rule sometimes signaled the ordering of competitive ground states-ordering that was obvious in cases such as the 3d ferromagnetic metals, but not obvious in others. Dramatic decreases in T c caused by the presence of dilute concentrations of some elements led to the concepts of the virtual magnetic bound state and magnetic pair breaking. Some of the interesting magnetic and superconducting compounds that have been and still are forthcoming have been mentioned earlier in the symposium by Brian Maple, Oystein Fischer, and Zack Fisk.

The absence of superconductivity in elemental Mo was a puzzle which had stimulated searches down to a few hundredths of a degree. The puzzle was solved when Tc near 1 K was found in samples from which the few hundreths of a percent of Fe in nominally pure Mo was removed. On the other hand, a trace of Fe dissolved in Ti was found to increase Tc ten times faster than empirically expected from Matthias' rule. These results were found at a time when there was much unusual behavior in transition-metal superconductors that enticed us to look for non-phonon mechanisms of pairing. However, further studies in collaboration with the group of Ernst Raub showed the anomalous behavior of Fe in Ti was due to materials science, not new physics. The Fe induced small amounts ofbcc Ti (too small to be detected by x-ray diffraction) to be precipitated in grain boundaries of the hexagonal Ti host. The bcc Fe was found to be an order of magnitude more concentrated in the bcc phase than its nominal composition, and thus the behavior was exactly as predicted by the rule. Rhodium, in which we were interested because it was not, and still is not, clear whether it will become superconduct ing , ferromagnetic, or something else in its ground state, provided another closely related case. Our strategy was to find a superconduct ing alloy and to use the concentration dependence of Tc to extrapolate to pure Rh. We found that the presence of less than one percent of La induced a sharp superconducting shielding signal in the Rh, which remained but became weaker as the concentra t ion o f La was further reduced. The superconducting compound LaRh 5 was found to be responsible for the superconducting signals. Small additions of Fe had no effect in the one-percent sample

where the LaRh 5 filaments were continuous, but destroyed the signals in the less-than-one percent samples where they were not. In the more dilute case the superconduc t ing shielding was evident ly maintained by a proximity effect, which was the closest we could come to making Rh superconducting, and which was destroyed by the pair breaking of the dissolved Fe. As a result of these experiments we developed the practical, but not necessarily conclusive, diagnostic procedure of measuring unknown bulk samples also as powders assumed to be fine enough to destroy f i lamentary shielding paths to help in distinguishing bulk from filamentary superconductivity.

The inability to find any good evidence for phonon- induced superconductivity in transition metals (for reasons, which we now know, were materials-science- related) led to studying the isotope effect in isotopically enriched Ru, which had just become available. The square-root dependence of Tc upon mass found in the non-transition metals was well understood to be a consequence of the dependence of phonon frequency upon mass. Measurements of the purified Ru isotopes showed, within experimental accuracy, zero dependence of T c which we initially took to be strong evidence for another pairing mechanism. However, as Phil Anderson has already discussed, he and his student Pierre Morrel were at the same time s tudying retardation effects which reduce the coulomb repulsion and favor superconductivity. They found the mass dependence of the retardation opposes the frequency dependence in a way which accounts for the results. In succeeding years the materials-science problems with transition-metal interfaces were overcome as John Rowell has mentioned, and the phonon mechanism was demonstrated directly using tunnel junctions.

During the period immediately following BCS, the predictions were made and verified. Most practitioners were under the delusion that all of superconductivity was becoming a well understood field. But at least one cloud was visible. Studies were largely limited to non- transition metals with sp conduction bands. This was because these "soft" metals (so-called because they were mechanically soft, and also because their critical field curves were soft) were easily amenable to purification and experimentation. This is quite understandable and was well appreciated partly because of the enormous advances made when pure semiconductors and metals became available.

40 T.H. Geballe/Physica C 341-348 (2000) 37-40

There is a price to be paid, however, when research is restricted only to "pure," well characterized samples. By definition, progress is likely to be confined to the understanding of that restricted class. The search for new superconductors and new clues does not require purity. In fact, superconducting studies in impure samples are of value simply in unraveling phase equilibria. The field broadened dramatically after the discovery of the high field, high current capability of Nb3Sn by Kunzler, Wernick, and Buehler in 1961, and studies of transition-metal superconductivity became mainstream. The Ginsburg-Landau Abrikosov-Gorkov (GLAG) theory clarified behavior and Anderson made "dirty" into a respectable adjective in contrast to the earlier use of "schmutz," used to characterize work with Nb3Sn.

III . CONFERENCES-PAST AND FUTURE

A few mavericks continued searching and finding results that didn't fall into place. The time was ripe for a more concerted effort. In 1972 the first conference on "Superconductivity in d- and f-Band Metals" nucleated spontaneously at the University of Rochester under the leadership of Dave Douglass. It took only a few weeks from the time that conference was conceived until it took place. Since then, as the conference has grown, the time needed to organize the meetings has also grown, unfortunately, at a rate even faster than Tc.

The first three conferences were concerned with many of the still-current topics. Frank DiSalvo already discussed the layered transition-metal dichalcogenides and charge-density waves earlier tonight. The increases in Tc observedupon the intercalation of organic molecules between the TaS2 led to the Lawrence- Doniach-Josephson coupling model. While the increases were first interpreted as a result of enhanced pairing, further work showed they were caused by the destruction of the competitive charge-density wave upon intercalation. By 1982, the first meeting outside the U.S. was held in Karlsruhe, and two newly discovered classes of superconductors were included, namely, organic compounds and oxides with strongly

hybridized s-p bands. Neither of these involved d and f bands but they were of great interest to the attendees. The conference name was changed to the more appropriate "Materials and Mechanisms of Superconduct ivi ty" for the succeeding fourth conference at Iowa State, which became more international with Japanese participation. That name lasted for only one conference because of Bednorz and Mueller's search, which led to the discovery of superconductivity in the layered cuprates and refocused the conference, which became truly international. "High Temperature Superconductors" was explicitly incorporated into the title and it has remained in the subsequent conferences at Interlaken, Stanford, Kanazawa, Grenoble, Beijing, and the present one here at Houston.

New pairing interactions already found in the heavy-ferm.ion superconductors and in the cuprates suggest more will be found in the future as more complex structures come under investigation. I am intrigued by the evidence that TI forms a negative-U center in semiconducting PbTe, and, in concentrations of the order of one percent, induces superconductivity. In a paper to be given later in this conference, Boris Moyzhes and I consider the possibility that negative- U centers in the charge-reservoir layers of the cuprates may be additional pairing centers outside of the CuO2 planes. We suggest that the doubling of Te, which is found when the distance between optimally doped CuO2 layers in the (LaSr)2CuO4 family is increased from 6 to 9 angstroms by the insertion of an insulating Hg layer, is due to Hg negative-U centers.

There are powerful new combinatorial methods becoming available which will enable the investigation of an enormous number of new systems and provide more clues for new physics. Advances in film growth are making it possible to study interfaces, stacking sequences, metastable systems, and artificial heterostructures in controlled ways. These powerful new techniques and the ongoing worldwide research efforts make me optimistic that at the next conference in Brazil some ingenious and perhaps lucky investigator will again make it appropriate that the Conference name be changed once more.