Happy birthday, High-Tc. I miss you.

Twenty-five years ago, the first so-called high temperature superconductor was discovered — unexpectedly, almost out of nowhere. Suddenly, the interest of physicists all over the world in the phenomenon of superconductivity was re-ignited (click here for a comprehensive and authoritative review of this subject).
Superconductivity, first discovered 75 years earlier, was considered in 1986 well understood, witness the physics Nobel Prize awarded in 1972 to Bardeen, Cooper and Schrieffer (BCS) for their microscopic theory of the phenomenon.

And yet, in 1987, one year after discovering superconductivity in the LaBaCuO compound, the two scientists responsible for it were awarded another physics Nobel prize; in the quarter of a century since their discovery (followed in rapid sequence by those of many other, related superconducting compounds), over 100,000 scientific articles have been published by experimental and theoretical physicists on this “new class” of superconductors, making this at some point arguably the most important current research topic in physics.
Why was such a discovery deemed so ground-breaking ? After all, many superconducting materials were known already; what could be so special about the new ones ?
Simply put, they just seemed to defy most of what was (believed to be) known about superconductivity. Their discovery also brought back to the realm of possible, what was at the time regarded as fundamentally unattainable, a “pipe dream”, namely superconductivity at room temperature.

What are superconductors, anyway ?
Superconductivity is the property of some substances of conducting electric current essentially indefinitely, without dissipation, and without the need for a battery supplying the voltage difference needed to establish a current in ordinary electric conductors. It is perhaps undoubtedly the most spectacular and stunning manifestation of quantum mechanics on a macroscopic scale.
Why are superconductors not widely adopted in everyday life ? Where are superconducting gadgets, engines, magnetically levitated trains, and the plethora of other dazzling, revolutionizing technological applications that superconductors would be expected to put at our fingertips ?
Until 1986, the answer to this question would have been that superconductivity can in principle only occur at extremely low temperatures [0], difficult to attain, and prohibitively expensive to maintain on a large scale, making the widespread use of superconductors unfeasible in practice. The reason for such a low operating temperature of pre-1986 superconductors, lies in the very microscopic mechanism underlying superconductivity, as described by the BCS theory [1].

The discovery of LaBaCuO, with a superconducting transition temperature of -243 C (already above what was believed to be the highest theoretically achievable) paved the way to that of other compounds with much higher transition temperatures (up to around -140 C [-220 F]), indisputably showing that superconductivity could occur more frequently than once believed, and that there was a lot about this phenomenon that was still poorly understood, for the existing paradigm (namely the BCS theory) simply could not account for such high transition temperatures.
But, if the operating temperature of a superconductor could be over one hundred degrees Celsius higher than previously believed possible… why not three hundred [2] ?

Five great years, and then…
August 1987 is when I started out my doctoral program in physics at Florida State University. High-temperature superconductivity was the “talk of the town” in those days. It was one of the very few physics subjects about which one could hear on television, as there was the sense that we could be witnessing the dawn of a new era.
I may not have known yet whether I would be a theorist or an experimentalist, but there was no doubt in my mind regarding the subject on which I would be working. Unraveling the mystery of high temperature superconductivity — no other problem could capture my imagination like this one. And with me, an entire generation of aspiring condensed matter physicists in North America saw nothing else on which to spend five, ten years (the few lucky ones a lifetime) of research work. There was something undeniably exciting about working on the same problem that was keeping busy the sharpest minds in the world, having Nobel laureates sit in the audience at practically every major conference, being a lowly graduate student and being asked by professors for preprints of work presented in ten minutes at the annual big meeting.
Those were great days to be a physics graduate student, and the United States and high temperature superconductivity (or, as we call it, High-Tc ) were the places to be. Funding was plentiful; just about anyone with a reasonable research project held a research grant. Most physics graduate students working on High-Tc were supported on research assistantships; money to travel to conferences was a given, and few worried about finding academic jobs eventually, let alone postdoctoral appointments after graduate school.
Five years later, though, it was an entirely different story. As the time finally came to wrap things up, defend my doctoral dissertation and move on to the next stage, after five very intense and gratifying, but also emotionally draining and disenchanting years, my own enthusiasm toward high temperature superconductivity had largely faded. I did not know on what I would be working next, all I knew was that I needed a change. And this time too, I was not the only one. An exodus was about to take place.

What happened ?
The problem of High-Tc is a tough one to crack.
The experimental evidence is intricate and confusing; each one of the many competing theories can only account for part of it. The field has always been a fiercely competitive one, involving a lot of big players with even bigger egos. This is normal, of course — interesting and worthwhile problems attract hard working, passionate and competitive people. To a point, competition fosters better science, but when egos get in the way of calm and rational scientific exchange, things turn ugly. And, ugly they got.
Anyone attending a meeting, or a focused session at a conference, had better be prepared to hear words like “charlatan”, “dishonest”, “incompetent” and so on, yelled by attendees to one another. Months of bitter arguments with referees became a requirement, in order to have even the most harmless paper published. Feuds among prominent researchers started breaking out of the confines of specialized journals, spilling over to the pages of Physics Today. Many a young physicist (like me at the time) who had chosen High-Tc as our field of inquiry, could not escape the realization that making a contribution to that subject, in such a fiercely adversarial climate, was impossible — how loud one screamed, and how high the podium (as usual, of course) were at least as important as the quality of the work done, in determining whether one would be paid any attention or not.

Follow the money
There was another reason why things turned so ugly — everyone was fighting for a piece of a pie that almost overnight shrank dramatically. In the early 90s, the US economy was mired in the depth of a recession, and science funding was hit hard. Deep cuts were made across the board, the abundance enjoyed until a few years earlier all of a sudden became a thing of the past. As we all frantically tried to adjust to the new era, the one thing that was immediately clear was that, out of the initial crowd of researchers who had first entered the fray of High-Tc, only a few lucky ones could expect to receive continued support — the others would in the best case have to switch to some other problem, in the worst quit science altogether.

And that is not even the worst part…
The field of High-Tc had been for a long time the target of jealousy and resentment from others within the broader condensed matter physics community, who felt that too large a share of the funding had supported research in High-Tc, at the expense of other, equally worthwhile lines of research (their own). Sensing that the tide was turning, a widespread, concerted effort aimed at discrediting High-Tc became visible, on the part of many condensed matter physicists (bearing many resemblances to that which took place at the same time against the Superconducting Supercollider, that time involving all those outside high energy physics, and to that which will soon target String Theory — kidding…). The whole research effort in High-Tc was labeled as a “failure”. Why ?
Well, because we had not solved the problem [3]. Divided as it was, the High-Tc community was unable to provide the coherent, forceful response that such ridiculous charges would have warranted. As a result, the backlash against High-Tc was largely successful. For the next decade, funding to do research on this subject was not available, for all practical purposes.

You work on what ?
Having been actively engaged in research on High-Tc became almost a mark of infamy. I remember looking for my first faculty job in 1995, being lectured on the phone by the chair of a search committee where I had unsuccessfully applied for a condensed matter theory assistant professorship, about the fact that “society does not need quantum mechanics” (yes, this is a prominent condensed matter physicist — go figure), and that I should “quickly switch to a more useful subject”, if I wanted to retain a shred of hope to land a science job, one day.
In 1997, when I interviewed for another faculty job (again in theoretical condensed matter physics), I was openly told that the search committee had basically wholesale discarded every application from candidates whose area of research was High-Tc, and that an exception was made in my case only because I had done five years of postdoctoral work on “something else”.
One year later, when I was to submit my first grant proposal as an assistant professor to the National Science Foundation, senior colleagues and program directors advised me to pick “any subject but High-Tc“. These are just anecdotes, of course, and personal recollections. I could be wrong. Maybe one of my numerous readers will take exception with what I am writing, but I felt that High-Tc was to be regarded as a “contaminated area”, of which sane persons would stay clear.

A mistake
I do not think that the situation has much improved over the past fifteen years. High temperature superconductivity is still not regarded as a viable subject of research by most physicists, especially early career ones — too risky, too controversial, too little funding.
Personally, I think that it is a shame.
The pursuit of the understanding of the phenomenon of high temperature superconductivity remains a worthwhile scientific and technological goal, one that should be brought back to the forefront of active research in condensed matter physics. I have been working in this area of physics for over twenty years now, and I am still to encounter another problem as intellectually stimulating, or of comparable richness, subtlety, diversity. It provided a superb training ground for many students and postdocs, including those of us who at some point elected to work on something different, or even those who left research altogether.
The contention that “High-Tc research never went anywhere” is disingenuous and/or ill-informed, even though one hears it often, even at condensed matter physics conferences, made by individuals who should know better. The truth is, a lot of progress was made, surely much more than in other areas that benefited from even greater funding and/or wherein even more “hot air” was produced.
It is important to keep in mind that, by the time high temperature superconductors were discovered, condensed matter physics was considered a mature field, with essentially nothing fundamental left to be understood. High-Tc brutally proved the opposite. Existing tools and concepts turned out to be inadequate; new ideas, approaches and investigative techniques developed in the context of High-Tc research, have been successfully exported to other areas of physics [4].

True, the problem has not yet been solved. We still do not know why some materials are superconducting at higher temperature than the “conventional” ones (i.e., those known before 1986). The problem is hard. So what ? Did we get into the business of scientific research because we thought it was a piece of cake ?
Since when do physicists give up working on something because it is “too difficult” ? If after two decades of intense work the solution of a problem is not in sight, then odds are pretty darn good that there is something very important to understand, and that makes the problem worth pursuing, not abandoning.
These are not my words. I am paraphrasing Nobel laureate Philip W. Anderson, in his keynote speech at the conference on Recent Progress in Many-Body Theories in Santa Fe, in Summer 2004, which I attended. As I listened to him, I remember wishing that he had said that ten years earlier.

Notes

[0] In 1986, when Bednorz and Mueller made their historic discovery, -250 C [-418 F] was the highest temperature at which superconductivity had been observed in the laboratory.

[1] According to the BCS theory of superconductivity, crucial to the appearance of the superconducting phase at low temperature, in a crystalline solid, is the formation of pairs of electrons. Because electrons are negatively electrically charged, they repel each other in vacuo, and thus cannot bind in a pair. In a crystal, however, there exists a “glue” that can keep the two electrons together; it is a subtle effect whereby, in essence, two electrons are attracted to one another indirectly, as a result of a distortion of the underlying crystalline lattice of positively charged ions, a distortion that the electrons themselves create in their motion through the lattice. Because the energy scale involved in this mechanism (i.e., the binding energy of the pair) is very small compared to the characteristic energy of electrons in an ordinary crystal, the temperature at which it becomes relevant is very low.

[2] Granted, -140 C [-220 F] is still impractically low, but the fact remains that in half a decade, the distance between where we were in 1986 and room temperature was halved. It is not difficult to see why optimism was so high, back in those days. Today, room temperature superconductivity is nowhere in sight, and there are of course many who are skeptical about its achievement any time soon (me among them). However, I do not believe that any condensed matter physicist nowadays would claim that there are fundamental reasons why it ought not be possible.

[3] Of course it is silly ! By that token, one could just as well brand as “failure” science as a whole. But that was not the point, the attack had to be forceful, not credible.

[4] On the theoretical side, one need only think of the advances made in the development of general numerical methods for quantum many-body systems (e.g., Density Matrix Renormalization Group), most of which were motivated by the need of gaining insight into the physics of strongly correlated electronic models (such as the Hubbard model) believed to capture the essential physical behaviour of high temperature superconductors. These powerful techniques are now regarded as valuable instruments in the toolbox of a theoretical physicist. They would not exist, if not for High-Tc.

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2 Responses to “Happy birthday, High-Tc. I miss you.”

  1. Luis Says:

    Excellent post. The link to the Anderson x Pines-Scalapino debate in the Letters section of Physics Today in 1994 is quite revealing of the warfare mood in those days.

    By the way, what’s your take on the “d-wave + spin-fluctuations” debate? Which side has gained more ground in these last 17 years?

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