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Jamil Tahir-Kheli

Jamil T

Jamil Tahir-Kheli

Jamil Tahir-Kheli

Senior Staff Scientist, Caltech

By David Zierler, Director of the Caltech Heritage Project

August 9, 2023

DAVID ZIERLER: This is David Zierler, Director of the Caltech Heritage Project. It is Wednesday, August 9th, 2023. I am delighted to be here with Dr. Jamil Tahir-Kheli. Jamil, it's wonderful to be with you. Thank you so much for having me.


ZIERLER: To start, would you please tell me your title and affiliation here at Caltech?

TAHIR-KHELI: I am a Staff Scientist at Caltech. I have been here since 2001.

ZIERLER: There's an origin story here; you're also a Caltech alumnus. You got your PhD at Caltech in 1992?


ZIERLER: Tell me how you got to Caltech. What were you doing before? What was your point of contact?

TAHIR-KHELI: There was no point of contact. I went to Oxford University to study math when I was 15. I was very young. I went over there.

ZIERLER: Did you grow up in England?

TAHIR-KHELI: I did not. I grew up in West Virginia. After three years of being at Oxford, I got tired of the weather—

ZIERLER: [laughs]

TAHIR-KHELI: —and so I looked at a map and said, "Well, this looks like a pretty good place." And, the sun was shining. And that was basically it.

ZIERLER: That's all you needed.

TAHIR-KHELI: It was not that deep. And, when I talked to my tutors at the College, they said, "Oh, well, Dick Feynman is there," and all this other good stuff. I had studied some quantum mechanics, so I had a little bit of physics knowledge, but not a lot of knowledge.

ZIERLER: Your focus at Oxford was mathematics?

TAHIR-KHELI: That's correct.

ZIERLER: You came here to do math, or did you switch to physics?

TAHIR-KHELI: No, I came here to do physics. In my final year doing math, I realized I had proven enough theorems, and now it was time to understand how the world actually worked, so I started learning a lot of physics, before I left Oxford.

ZIERLER: Coming to Caltech, the focus was on theory, I assume, coming from math?


ZIERLER: What kind of theory did you work on as a grad student here?

TAHIR-KHELI: I initially thought I was going to do particle physics, and I interacted a bit with the particle theorists, and I realized, thankfully, that I didn't think there was a future in particle theory at that point. So, it was time to move on. Fortunately, the high-temperature cuprate superconductors were discovered, and that was the exciting thing to look at, so it was perfect for me. The timing was perfect for me to start thinking about that.

ZIERLER: What was happening in superconductivity around that time?

TAHIR-KHELI: Until late 1986, there were incremental improvements in superconductors, until Bednorz and Müller made lanthanum barium copper oxide. That opened the floodgates.

ZIERLER: This is the Woodstock conference?

TAHIR-KHELI: The Woodstock Conference—I did not attend it, by the way—was in 1987, I believe. Bednorz and Müller found a 35 Kelvin superconductor. Prior to that, the maximum was about 23 Kelvin. That's only 12 Kelvin, but that was—an eternity. Also, there was fancy theory that proved 35 Kelvin was impossible. The Bednorz and Muller claim was reproduced by late 1986. Then in early 1987, a paper came out of the University of Alabama and Houston groups that found the yttrium-barium-copper oxide superconductor with a transition temperature, Tc, above liquid nitrogen temperature. That basically got everyone's attention.

ZIERLER: You were in PMA, as a grad student?


ZIERLER: Where was superconductivity research happening at Caltech? Was it in EAS, or were there physicists in PMA who were doing it? Because you have the physics and the applied physics distinction. You started in PMA. You were in Physics. You wanted to do superconductivity. This was very exciting. How then did you end up with Bill Goddard in Chemistry being your thesis advisor?

TAHIR-KHELI: The superconducting state fascinated me because it's a macroscopic quantum state and nature doesn't give you very many of them. We see quantum mechanics operating on the scale of atoms. A superconductor is like an atom, but it's big. You can make miles and miles of wire of the stuff. And, it can carry current, so that's what makes superconductors so amazing. Once you realize that it's quantum mechanics writ large, it's a very fascinating thing. Superconductivity has captivated every major physicist on the planet. Einstein had a wrong theory of it. Niels Bohr had a wrong theory of it. Heisenberg had a wrong theory of it. Felix Bloch had a wrong theory of it. Essentially, superconductivity is the graveyard of Nobel Laureates. There is a 1929 letter in the Caltech Archives from Oppenheimer, who was in Zurich at the time, to Millikan that says roughly, "We have been working on superconductivity here with no success."

Everyone who was anyone worked on it. The situation was so bad that in the 1930s, Felix Bloch had a cynical unpublished theorem that stated, "Every theory of superconductivity can be disproven!" The size of the theorist graveyard in superconductivity has only increased with the discovery of cuprate superconductors. But the point is, because it is a macroscopic quantum state, it's not like anything we are familiar with. As you learn quantum mechanics you realize, "This stuff doesn't make any sense," and that's where you want to be. Just prior to the discovery of cuprate superconductivity, the field was not very hot in academia. It had been figured out, there was a theory, a Nobel Prize handed out, and they were making incremental improvements in the transition temperature but there was nothing conceptually new. Cuprates brought superconductors back to everyone's attention. I wanted to work on that.

ZIERLER: Was Goddard involved in this already?

TAHIR-KHELI: By the time I went to speak to Goddard to become his graduate student, he had already gotten a New York Times article on his theory of superconductivity. He had a theory of superconductivity that did not work out well for him at all. Bill made an estimate of what the highest transition temperature would be, in light of his theory, and he wasn't aware of a whole lot of prior theoretical development that showed that the way he calculated Tc was not credible. It was probably an overestimate of what you could get. He got hammered for that, rightly so. He got a lot of flak from the physics community in the New York Times. In the end, his theory did not turn out to be correct. But, guess what? Nobody else's did either. That's not a crime.

ZIERLER: Maybe he took it a little harder because he was a chemist, as opposed to a physicist?

TAHIR-KHELI: I think that unfortunately, there's a no-man's land between physics and chemistry. As far as I'm concerned, a lot of interesting stuff is in the no-man's land. It really is. But both sides have created their camps and way of thinking of things. Bill was trying to go into that no-man's land, and the truth of the matter is, you get shot at from both sides.

ZIERLER: Yeah. We should specify also, Goddard is a quantum chemist, so this is not a brand-new concept for him.

TAHIR-KHELI: That's right. He did what I thought were very clever ab initio calculations, and he noticed that the cuprate electronic structure that the physicists were assuming was not correct. But it turned out that what Bill got out of his calculations did not turn out to be correct either. The physicist's picture sort of survived, but that's part of the controversy. The physicists haven't solved the problem, so no one really quite knows.

ZIERLER: When you become his student, is this all in the recent past? Is this happening in real time?

TAHIR-KHELI: I must have been—under a year after his New York Times article and the backlash. Must have been. I don't quite remember exactly, but it was definitely afterwards.

ZIERLER: But him accepting you indicated he was not giving up on this. He was still interested?

TAHIR-KHELI: Yes. That's what I respected about him. Bill's crazy, but he's crazy in a good way. He's open to any idea. I have now spent a lot of time here at Caltech, and it's remarkable how few people are open to new ideas. He's open to new ideas and is somebody who thinks differently. So the fact that my background was not chemistry or not even traditional physics, I don't think that bothered him.

ZIERLER: When you came to him asking to be his student, what was the presentation? What were you offering?

TAHIR-KHELI: Nothing. Nothing! I said I was interested in this thing, and he said—I don't really remember. He didn't say that much. He just sort of said, "Okay." I don't even recall—did he ask about my background? He must have, right? I would have. I don't know. I just remember him sitting in his chair, doing the Bill Goddard thing, rocking back and forth, and listening. And so—

ZIERLER: Maybe he just saw you as a refugee from physics, and he took pity on you.

TAHIR-KHELI: Who knows?

ZIERLER: Are you in contact with Carver at all, as a graduate student?


ZIERLER: Interesting.

TAHIR-KHELI: No. I never met Carver as a graduate student.

ZIERLER: Was he involved in superconductivity?


ZIERLER: He wasn't there yet.

TAHIR-KHELI: I knew of Carver because a lot of my friends as a graduate student were computer science students, and by fluke, I met one from Carver's group. Because I knew one, I ended up getting introduced to most of them at that time. But I don't believe I personally met Carver at that time.

ZIERLER: Tell me about developing your thesis research as a Goddard student.

TAHIR-KHELI: Bill's electronic structure did not survive for two reasons. One, it predicted that the superconducting electron pairs in the cuprates would be spin triplets; they'd be magnetic. They turned out to be spin singlets, which means they're not magnetic. So they're like the traditional superconductors that have been around for 100-plus years—they're spin singlets—but Bill had predicted a triplet. They found Josephson tunneling, which is superconducting tunneling, between an old superconductor, lead, and these newer superconductors. It's highly unlikely you'd get a superconducting tunneling if there were two different types of electron pairing. So that was a problem for his theory. The second one was his theory predicted—and this is getting down in the weeds—you can measure the magnetic field at a nucleus through nuclear magnetic resonance, NMR. Bill's orbital, what he thought was the electronic structure, would have predicted that the frequency shift—there's a frequency at which you can get a nucleus to talk to you—for the yttrium nucleus would have been higher than the standard value. It turned out to be lower. It's hard to argue that away. I think those two things killed his theory.

I had spent my graduate school years reading all the cuprates experiments, because there was an infinite number of them. Literally every day—you didn't have arXiv at that time, or anything; people were just mailing preprints, because the publication system was too slow. Bill would get several preprints a day, and he would just pass them out to whoever—

ZIERLER: And you're a sponge. You're just absorbing as much as you can.

TAHIR-KHELI: Yeah. My thesis was more mathematical in that I found a model that you could solve in one dimension exactly, and you could make a good approximation in two dimensions. A lot of people thought that the cuprates were intrinsically two-dimensional things, because they had these copper-oxygen two-dimensional layers. People thought that the layers didn't really communicate, and the stuff in between wasn't all that important. Later I discovered that other people had solved the same model in 1D, so I don't think I was the first. I think other people had it, too. They got at it from a different way than I did, but that didn't matter. There was a mathematical picture called a Hubbard model, which is a simplistic model for how the electrons may hop around in these superconducting materials. It goes by the name of the t-J model and there are generalizations. But the theme is that there's a simple toy model for understanding the cuprate superconductors using electron states with their orbital character primarily inside the CuO2 planes common to all cuprates. I was working on that. I did my thesis work, and it dawned on me by the end of it that I didn't think any of these Hubbard models had any hope of solving superconductivity. I didn't believe a lick of it. And so, when I graduated, that, I knew.

ZIERLER: When you say, Jamil, "solving" superconductivity—

TAHIR-KHELI: The mechanism. How does it work? There was an infinite number of experiments, and new experiments were coming out every day, and they were getting better and better at making materials, and bigger single crystals. So, the measurements were getting more and more accurate and more and more elaborate. But nobody understood how these things worked. And they still don't understand how they work. But for me, the thing that came out of the PhD was the realization that none of the standard approaches were going to work. I didn't believe a word of them. But everybody else believed, and so the field went on, for the next 30-odd years and it has not worked out well for them. But at that point, I knew that you had to go in a different direction.

ZIERLER: The question of solving superconductivity, is that what room temperature superconductivity means? Was anybody talking about room temperature superconductivity back then?

TAHIR-KHELI: They were talking about it, because once you got over liquid nitrogen, of course the race is on, to see if you can get to room temperature superconductivity.

ZIERLER: Why is liquid nitrogen so important here?

TAHIR-KHELI: Because it's cheap. It's about the cost of buying milk. So, it's a perfect thing for cryogenics. Before that, you'd have to use liquid helium, which at that time wasn't that scarce but was still expensive. In the last 30 years, cryogenics has gotten a lot cheaper, and that has changed the game. I don't think liquid nitrogen is as relevant as it used to be, because of the modern cryocoolers, but at the time, liquid nitrogen was the thing. There was a lot of hype about room temperature superconductivity after the discovery of cuprates. With LK-99, it has all come back again. Obviously, we did not get to a room temperature superconductor with the cuprates, and I question whether LK-99 is a superconductor at all. A lot of the hype for room temperature superconductors I think is overblown. So, let me ask you, David—your question is really about—

ZIERLER: How you got involved in the field, sort of the origin story. Now, let me add to that. You defend, you spend the time away from Caltech, and then you return in the early 2000s. What were you doing in the intervening years?

TAHIR-KHELI: Because I didn't believe any of the theories that were out there in superconductivity, that's not going help you get a postdoc position. There's no way.

ZIERLER: You have to be a team member, is what you're saying, basically.

TAHIR-KHELI: Well, the only funding out there—and there was funding at that point—was to do those things, and if you don't really believe it, you're not going to sell yourself very well. I poked around a little bit at that, but Bill Goddard was considered an outsider in the physics community because of the New York Times debacle, and so that wasn't going to make me a very popular hire anyway. It didn't make sense to stay in the game. So, I went and got a job doing some finance consulting.

ZIERLER: Oh, wow.


ZIERLER: Did you stay current with the literature?

TAHIR-KHELI: Yes, I did. A good thing about Bill Goddard is he never stopped me from walking back into the lab and chatting with people or anything like that, and so I could stay current. I realized finance wasn't it for me, because I still wanted to solve something fundamental. I wanted to know how things worked. And, this superconductor is staring you in the face. Finance is interesting but it's a different sort of thing. While I was doing finance, I kept thinking about superconductivity, and realized there was a different way of looking at it. I hooked up with a former officemate of mine, Jason Perry, who got his PhD with Bill Goddard, and was an ab initio chemist. His PhD was in chemistry. Mine was in physics. He and I hooked up and we started chatting, and thanks to the collaboration with him, where he added a lot of insight, we found a way to think about superconductors in a different way. So, I quit the finance job. We started a little consulting company, and we made money doing scientific consulting gigs. We would spend our spare time trying to work out superconductivity. What we did is we figured out what went wrong with Bill Goddard's calculation that ended up in the New York Times and didn't work out. The problem with Bill's calculation was, back then, he couldn't put in real atoms outside of the copper-oxygen plane, so what he did is he just put in point charges to make the computation feasible. It was a perfectly sensible thing to do.

ZIERLER: What does that mean, "point charges"?

TAHIR-KHELI: Oxygen typically has a minus-two charge. If you don't give it any orbital character, an electron cannot hop on or off of it. But you can still have the fact that it's a minus-two charge, so that's a good way to approximately simulate what's going on in the plane. Since most people, including Bill, believed the action was in the plane, it was a good approximation. With the computational resources available at the time, that's about all he could do.

ZIERLER: You basically did what Bill presumably would have done if he had access to these computers?

TAHIR-KHELI: I don't know the answer to that, because there were multiple steps along the way. It wasn't that we just added—what happened is I was very worried about the nuclear magnetic resonance data, and I couldn't make any sense of it. The only way I could make some sense of it at the time was if there was electronic character that was out of the plane, not just residing in the plane. To me it looked like the NMR was suggesting that you should look out of the plane. The NMR is still not explained today, so I don't think my original picture of the NMR that got me thinking of out of the plane was correct. But the NMR did point to things that you should be looking at.

The ab initio density functional method that the physicists used at the time for computing band structures was called the local density approximation, LDA. The chemists never used LDA, because it's terrible for chemical bonding. It just doesn't work. Part of the reason is that in LDA, an electron slightly repels itself and it shouldn't. It's not entirely understood as far as I know how an electron knows not to repel itself, but the electron does not repel itself. For example, you and I will repel each other if we're electrons, but since you and I are supposedly spread out as waves in quantum mechanics, I do not repel the other part of me over there, and you do the same for your parts. But you and I repel each other. Since in LDA the electron repels itself, it tends to spread the electron out more than it should. The chemists had devised different density functional methods that were not as rigorously theoretically based as the physicists' ones. The physicists could make very credible arguments for why their local density approximation or improvements to it were based on sound physical principles. In reality, nobody can solve anything of any chemical relevance exactly using the Schrodinger equation. So, you're basically engineering, and you find empirical things that work. There was a functional that had come out in 1992 by Axel Becke called Becke three LYP—Lee-Yang-Parr. The trick in the B3LYP functional was it did a better job of eliminating the self-repulsion of the electron than LDA. What that does is it allows the electron to get tighter, cuddle up with itself more. Now, the parent compound of cuprate superconductors is an insulator that is an antiferromagnet. In an antiferromagnet, the electrons are localized with the magnetic moments of neighboring electrons opposite to each other. To make a cuprate superconductor, you start taking electrons away. It's called hole doping. Eventually the cuprate stops being an antiferromagnet and turns into a superconductor. In the superconducting phase, the electrons are delocalized, and you get what is called a band structure. Now, when the LDA functional, which is what the physicists liked, was applied to the antiferromagnetic phase, because the electron has a self-repulsion error in it, the electrons spread out, and it refused to localize and form the antiferromagnet. So, LDA got the parent compound of cuprate superconductors wrong! Just dead wrong! That's not a good start. Not an auspicious start.

What the physics community did is they said, "Well, look, these cuprates are doped anyway, and there's some semblance of a band structure in the superconducting phase, so maybe the LDA result is okay over there." That was their starting guess. Now, the result of LDA was that the electronic states relevant for superconductivity were in orbitals with character primarily in the CuO2 planes. Since those calculations came out very early, that was the zeitgeist at the time, that all you had to really worry about was these planes. Now Jason Perry realized that what you really want to do is a band structure calculation for the undoped parent compound using B3LYP. If we got an antiferromagnetic insulator, then if we doped it, maybe we'd actually see what's real, because we would have at least established the undoped compound correctly. I should point out that the physicists do not use these hybrid functionals such as B3LYP. They are opposed because they're empirical, rather than based on rigorous theory. I think they need to believe that density functional theory is not just an engineering hack, which it basically is. Another reason for their opposition to B3LYP is due to the way that they do their calculations. It's almost impossible to code in a hybrid functional the way they do it. When you do a quantum mechanical calculation, you have to have a basis set, or the space of functions over which the electron can evolve. In theory, the result of a calculation is independent of the choice of basis set. In reality, computer memory and speed is finite. Hence, finite basis sets are used and the result is dependent on the choice of basis set. The physicists use—and it's a very sensible thing to do, and they've been doing it since the 1930s—plane waves for their basis functions. If you want to code a hybrid functional like B3LYP using a plane wave basis set, it's extremely difficult to code, and if you do, it's thousands to million times slower than LDA. Essentially, you can never compute with it. My guess is, the physicists may have been interested in using hybrid functionals like B3LYP, but it's not practical with their codes, so therefore, you shouldn't be interested in it. It's not good.

The chemists, on the other hand, work with molecules where electrons are in chemical bonds, so their basis sets are localized. That's just how they start. It's a fundamental difference between chemists and physicists. Theoretically, it should make no difference; you should get exactly the same answer. But practically, since you can never do an infinitely large basis set, they can lead to entirely different results. If you're doing chemistry, and molecular stuff, you want to start with a localized basis set—the plane waves will make it computationally not feasible—whereas if you're doing a band structure, then the sensible thing is to do a plane wave basis set. The problem is you're stuck with LDA, whereas this B3LYP is doing something good in chemistry. What we wanted was a B3LYP band structure calculation. It turns out that a group from University of Torino in Italy had developed a piece of software called CRYSTAL. They figured out how to use the chemists' basis sets and do a band structure calculation. We got a copy of their source code, and we altered it so we could do our calculation. When we did this calculation, we found that the undoped superconductor was an antiferromagnet, and we got the right band gap of two volts. You could also calculate how strong the magnetic coupling between the localized electrons was, and we basically got that, accurate to probably 10 or 20 percent. So, we felt really good about ourselves. It was the first time anyone had gotten the undoped parent compound of a cuprate superconductor right using simple density functional methods, and it was because we used a different class of density functionals. The most important new result from this computation is that it hinted that electronic character outside the CuO2 planes may be relevant for understanding cuprates, in contradiction to the LDA conclusion.

ZIERLER: Did you publish this?

TAHIR-KHELI: Yes, that's published. It got no traction in the physics community, and the fundamental reason is because by then the paradigm was that you just look at the planes. The 1990s was not a pleasant time in the high-temperature superconducting field. It was blood sport, I think. There were huge fights going back and forth. Not a pleasant world at all. But they had sort of converged on their picture of how they looked at things. We got published in Phys. Rev. B. I remember a referee saying, "I can't find anything wrong with the calculation. It's pointless anyway, but I'll accept it." I don't think it's highly cited by the physics community. I think the physics community didn't care that it pointed to a flaw in their LDA and simultaneously suggested their fundamental starting point for developing theories may be wrong.

ZIERLER: This was too inconvenient for them?

TAHIR-KHELI: I don't know if it was inconvenient; it was just not the way they were thinking. The paper appeared in 2000, long after their paradigms were in place. It was hard-fought ground for us to even get this calculation done.

ZIERLER: You're still doing this basically as a hobby? You're still at the finance firm?

TAHIR-KHELI: At that point, Jason and I are consulting, but at that point, we barely managed to do this calculation. We could not afford the computing power. But I had a relative who was a chemical engineer working at Intel. The Pentium Pro, or PPro, chip had come out. I called him, and I said, "Do you have any spare CPUs sitting around in your lab?" He said, "Let me see what I can do." A couple of weeks later, he called me and said, "There's four Pentium Pros in the mail, coming to you."

ZIERLER: Whoa. [laughs]

TAHIR-KHELI: I think those were expensive CPUs back then. Very expensive. The four arrived in the mail. Jason and I built computers around them. I think we got the chips computing together, so we kind of built our own little board and put the whole thing together, and got just enough compute power that we could do that first calculation and get the undoped antiferromagnet. That was when we knew we had something new. But we couldn't quite do it well enough with our computing resources. What happened is we came back to Bill Goddard and asked him if we could use his computers. Bill had resources, but you have to pay the standard cost of doing business in academia—authorship on the paper and control and all that. It's a feudal system. So, we got that paper published. Then the question became, what happens if you actually dope it to something that's representative of the superconducting state? When we did that calculation, which was an even larger calculation—but we managed to massage the code and get it done—we found that the hole states created by doping had character out of the CuO2 plane, not in the plane. It just didn't happen. Completely contradicting the physics community. We published that result. That paper, nobody wants to touch. But at that point, we were dead convinced, we knew, that basically the whole field had gone down the wrong path.

Back in 1992 when I graduated, my sense was that all those Hubbard models, what they were doing was not going to work; I felt vindicated. We're now at 2002. But nobody wants to hear it, because the field has just become whatever it is. It had become too big and entrenched. I think everybody had a theory—as Carver puts it, everybody broke their pick on this one—but at that point, everybody had a theory and it was all viable, and so there were just loud fights with each other, because I think everybody thought they were going to Stockholm. Meanwhile, we don't have a theory, but we know that everybody else is going down the wrong path. That's where it stood. What happened is, I came back—Bill had a project with Seiko-Epson. He asked me if I'd come back and work with him. So, I came back.

ZIERLER: As a staff position or on contract?

TAHIR-KHELI: On staff. I came back here in 2001. Jason went off in a different direction. He was part of the team that found the cure for hepatitis C.

ZIERLER: Oh, wow.

TAHIR-KHELI: He went into medicinal chemistry.

ZIERLER: Jason is also a Goddard student?

TAHIR-KHELI: He's a Goddard student, yeah. But I thought that now that—"I know the orbital. Well, how hard can this be? In a year or so, I'll have this problem figured out. So, perfect. I'll come back to the Goddard group and figure this thing out. And then, I can move on in life."

ZIERLER: This is NSF funding?

TAHIR-KHELI: No, it was not funded at all. I came back to Bill's group and got a paycheck for other work. Superconductivity was done during spare time.

ZIERLER: Your purview was two years. You're not thinking this is a career.

TAHIR-KHELI: I thought I could manage the paying work, and now that I knew the orbital—I mean, everybody threw the baby out with the bath water! Well, if you've got the right electronic structure, it can't be that hard. Well, that turned out not to be true! But at the time, I came here to do that. So, no, I was never funded. To my knowledge, Bill Goddard had no cuprate superconductivity funding by then. He worked on some calculations related to the physicists' approaches, but I think that slowly petered out. I'm not sure he could ever have gotten funded anyway, because the physicists would have made sure that he was never going to get a dime. I stayed in the Goddard group until 2012.

ZIERLER: That's how long you were with Goddard? Until 2012?

TAHIR-KHELI: Yeah! It took about 10 years to come up with a different picture for the superconductors. The problem is, once you allow that orbital out of the plane, it turns out that you're forced into this position where the superconductor can't be a homogeneous system. The chemistry forces you in that position. And yet, the way we reason about superconductivity is that we assume the material is a good crystal, and then the plane waves that the physicists use for their computations, those are good quantum numbers, and that's how we reason. All the textbooks about solid-state physics, that's what they teach you. So, when this thing hits you in the face that, "Dammit, the chemistry is forcing me into this inhomogeneous world"—there's no translational symmetry. The textbooks weren't helping, and you're really out to sea, trying to find a way to reason with inhomogeneity, but the B3LYP calculation forced it on me. I couldn't find any way to show that the calculation was wrong. As the years went by, B3LYP became the workhorse of quantum chemistry. It was so successful. Every day in the Goddard group, people were running B3LYP calculations on molecules, and I saw how well it worked. If it's working that well, how could it be wrong for the superconductor? But then it forces you into a place where you can't find a way out. That took 10 years, just finding a way to try to reason.

ZIERLER: From when you joined Goddard group in 2001, to 2012.

TAHIR-KHELI: Something like that.

ZIERLER: —are you interacting with Carver at this point?

TAHIR-KHELI: I'm actively interacting with Carver by 2010. One of his former students, Lounette Dyer, introduced me to Carver around 2005 or so telling me that Carver is interested in superconductors. I had dinner with him at the Langham. It was the Huntington Hotel back then. We had a delightful evening talking about all things superconductor. We realized we were on the same wavelength. I was complaining that the physicists aren't listening and they've thrown the baby out with the bath water, and I think there's a different way of looking at cuprates. I'm working off in a corner in the Goddard group, and Bill's vaguely involved but he's not, really. So, I was complaining to Carver, "Nobody's listening," and Carver is giving me his past history with the physicists, not a very pleasant history as I've learned. Somehow we bonded over that. But nothing came of it immediately.

A few years later, probably around 2009 or 2010, I was attending a Thursday physics seminar. The speaker was a superconductivity theorist from Harvard, Subir Sachdev, who gave a talk connecting the mathematics of the t-J model to black hole calculations. The cuprate physicists were still working with models that ignored the out of the plane character that I believed had to exist, so I would always go sit in the back row of these seminars and roll my eyes. They wouldn't talk to me. I couldn't get in the room to talk to them, because the first thing they would ask is, "Who was your advisor?" and I'd say Bill Goddard, and literally the door never opened after that, because of the New York Times debacle. So I couldn't talk to any of them, but I'd go listen to the seminars. The idea of Sachdev's talk, to the extent I understood it, was that there is a mathematical connection between black hole models and the t-J model, and it was going to lead to new approaches for solving the t-J model. Since many theorists still believed cuprates were all about the t-J model, the implication was that cuprate superconductivity would be solved by these new mathematical insights. At this point, we're 20-plus years into cuprate superconductivity and they haven't gotten anywhere.

I was sitting there laughing to myself, because he never mentioned a single experiment. Typical for a theorist in this field, I thought. By this point I had read every damn experiment there was. More experimental probes had been put onto the cuprate superconductors than anything on the planet. If you developed a new experiment, first of all you got funding to develop that experiment by saying you were going to use it to explain superconductivity. Then you went and applied it to the superconductor. I don't think anything else has been more studied than the cuprate superconductors.

During the talk, I noticed Carver sitting in the front row listening intently. I couldn't imagine that he was getting anything out of the talk. I was scheduled to give a small chemistry seminar a few days later about cuprates, so I emailed Carver and promised the talk would be about real experiments. Surprisingly, Carver showed up. The next day he emailed me and suggested we have lunch. That was the beginning of our regular interaction.

ZIERLER: The intensity of interest, what can be explained in terms of fundamental science, that people just want to understand it? And where is the interest in applications, that if we solve this, it's a societal game-changer?

TAHIR-KHELI: After the Woodstock of physics, the field broke into two camps—the engineers who were going to make useful superconducting technologies, and the academicians who were going to figure out how these materials worked. So far, the practical applications that were promised have not appeared. The big problem for the engineering is that the superconducting gap of cuprates has a directionality to it that is called d-wave in the jargon. The d-wave gap of cuprates looks like a four-leaf clover. Its leaves have plus-minus-plus-minus signs as you go around. Between the leaves the gap is zero. This clover is attached to the crystal structure. Hence, misalignment of the grains inside superconducting wire leads to reduced current carrying ability in d-wave superconductors because the gap magnitude is not constant on adjacent grains. Thus, it's very difficult to make a useful polycrystalline wire. If you can't make a wire of the superconductor, it's hard to make a technology. This d-wave gap is believed to be intrinsic, meaning that it is part the "magic" of cuprates and you can't get rid of it.

For the old BCS superconductors, the gap is isotropic, or s-wave. It means the superconducting gap has the same magnitude in all directions. For an s-wave superconductor, it doesn't matter how the grains in a wire are aligned. For technology you would prefer s-wave, but that is not what Nature has given you with the cuprates. Now, even if you have an s-wave gap, you still have to engineer good grain boundaries, but since everyone knew these materials were d-wave, there was no point in putting resources into grain boundary engineering. Improving grain boundaries was given up by the early 2000s.

My guess is the field accepted the inevitability of d-wave and then switched to making superconducting tapes with grains that are aligned to less than 4 degrees of mismatch! It is an astonishing engineering feat to make a kilometer of such a tape. The problem is practical applications such as MRI magnets require tens of kilometers of tape and modern fusion reactor applications need on the order of 10,000 km of tape! And these tapes are expensive and fragile. Whether these tapes succeed or not remains an open question. Therefore, we haven't gotten all the amazing technologies that were promised when cuprates were discovered 37 years ago.

On the other hand, the theorists who devoted their careers to understanding the mechanism of cuprates have not succeeded either. I feel the theorists are bitter because they have nothing to show for over 35 years of effort. In the real world, failure of this magnitude is punished. However, academia is not the real world. Experimentally, the field has become unexplained experiments piled on top of unexplained experiments. There are so many experiments now that I don't think the human brain can keep them all in their head. Most theorists cherry-pick an experiment or two and ignore the rest. Some theorists assume their mathematical model is all that matters and hence absolve themselves of thinking about any experimental data! It makes no sense because the electron doesn't care whether you like NMR or like photoemission. It's the same electron. If you're going to explain superconductivity, you're going to have to explain how all the experiments fit into one coherent picture.

Since I wasn't employed directly to work on superconductors, I actually had the opportunity to read the experiments, and so maybe that was an advantage, in that I could look at every theory paper and say, "Well, you cherry-picked this experiment, but you're missing this," or, "This experiment that you don't mention is not going to help you."

ZIERLER: In joining forces with Carver, 2016, 2017, what was the goal in coming over here?

TAHIR-KHELI: The d-wave gap and the wire synthesis problem brought Carver and I together to experiment for the last 6 years. We believed that the d-wave gap of the cuprates could be converted to a technologically useful s-wave gap without too much loss in the superconducting transition temperature. The idea was to substitute a large amount of Calcium and Cerium atoms at the Yttrium atomic sites in the YBCO cuprate. We did a whole bunch of experiments, and the Tc dropped from 92 Kelvin down to about 72 Kelvin, or a little bit below liquid nitrogen. Most people would have just rejected it out of hand right then. If you were a graduate student who tried this, you would have gone to your advisor and said, "The Tc is now below liquid nitrogen," and the advisor would have said, "Well, go try something else." You only get in the New York Times if you increase Tc.

We now have experimental results that show that the 92 Kelvin d-wave YBCO superconductor has converted to a 72 Kelvin s-wave superconductor after substituting Calcium and Cerium atoms at the Yttrium site. Yes, we lost 20 Kelvin, but it's still cheap with modern cryogenics. Our result suggests a new pathway to synthesizing superconducting wires that could have a huge affect on technology. That's what we're excited about.

ZIERLER: Do you see this specifically geared toward—which is going to be now the main topic of our conversation—the achievement of room temperature superconductivity? Is it in support of it? Is it that itself?

TAHIR-KHELI: Our experiments are not in support of room temperature superconductivity. Let me just say that I believe there are room temperature superconductors out there. I'm very optimistic that they exist. Will a room temperature superconductor when discovered change the world the way the popular press makes it out to be? Yes and no. For a superconductor to be useful, it needs to be able to carry current without resistance. However, every superconductor has a maximum current it can carry before it becomes resistive. This value is called Jc. With modern cryogenics, it is Jc that is far more important than Tc. For example, the limit on the magnetic field in an MRI arises from the maximum Jc of the Niobium-Titanium wire rather than its Tc. Historically, it has required an enormous engineering effort to increase the Jc of a superconductor because, while its Tc barely changes, its Jc is very sensitive to the synthesis recipe. In addition, since Jc increases as the operating temperature decreases below Tc, it is always preferable to operate at the lowest possible temperature possible. For many applications, I suspect the cost of modern cryogenics will make cooling a very attractive way to increase Jc.

ZIERLER: And yet—and this will now move to the main topic, what brought us together—LK-99. Just for the back story here, this paper came out. It created quite a stir. I don't have to tell you that. I emailed Carver. I wanted to make sure he was aware of it. He said, "Well, you've got to talk to Jamil." Let's just start with some basics now. The South Korean research group that put out this initial paper, what are their claims? What is LK-99?

TAHIR-KHELI: They're really two papers. One is a three-person paper, and then they posted immediately following it, on arXiv, the six-man paper. The three-man paper is an announcement of a room-temperature superconductor. They claim that the Tc is greater than 400 Kelvin, which is about 127 Celsius, above the temperature of boiling water, which is 100 Celsius. This is hot. They make the claim, and they provide some data. Figure 1 is their data that shows the material is a superconductor. Then the rest of the paper is materials characterization work, showing you what they think the structure is of this material, and then they provide a theoretical mechanism for the superconductivity. The second paper fills in more detail, particularly on the materials synthesis. I really think you should look at them together as a unit, if you want to gauge the credibility.

My first reaction when I looked at the three-man paper was if I had discovered a room temperature superconductor, I would never have put theory in the original paper because the most important thing you need to prove to the reader is that you made a room-temperature superconductor. Your theory could be right or wrong.

ZIERLER: It doesn't matter.

TAHIR-KHELI: It doesn't matter. If the material works, your trip to Stockholm is guaranteed.

ZIERLER: Not only that, but if you made the thing, the theory is what it is now. You've defined the theory.

TAHIR-KHELI: Right, the cuprate superconductors have been around for 37 years, and there are niche engineering applications out there, but nobody has a theory. So my first reaction is that what I would have done is I would have used the space in the paper to do everything I could to make it as convincing as possible that it was a superconductor.

ZIERLER: Had you heard of LK-99 before?

TAHIR-KHELI: No, I was not aware of this compound. Okay, so I look at the title and think, "Wow, this is cool." My first reaction looking at the abstract, is, "Wait a minute, why are they doing theory?" I don't care about the theory. I don't care about what the theory is for how it works until I'm convinced that it's actually a superconductor. If it's not a superconductor, then the theory is really pointless. Hence, I immediately ignored that part of the paper. The next thing I did is look for a figure with data showing the transition from the normal state to the superconducting state. Typically, I would expect to find a plot of the change in resistance with temperature. It's not there in the paper. There's no resistance curve in the paper, or any other plot showing the phase transition from normal to superconductor. What I would have done is solder four leads onto the sample, stick it in a furnace, and measure the resistance from 200 Celsius down. This plot doesn't exist. What it means is that I never see the normal state, so I have no idea what the superconductor is condensing from. I also don't have a baseline resistance right above Tc to use as a gauge when they say that they've got zero resistance because you can never measure zero resistance; there's always noise in your instrument, so you're just going to measure down to the threshold of your instrumentation. So, how much of a drop did you get from the normal to the threshold of your instrumentation? That tells me a lot. If it's only 10%, well, then, I don't really know. If it's a factor of 1,000, well, I'm paying attention now because something is happening in the material that might be superconductivity. I think it's a cheap and easy experiment. I personally would not have published without that, because I would have wanted to know what the normal state is.

The second thing I looked for is the magnetic response of the superconductor in order to estimate how much of the material is superconducting. The way you estimate the superconducting fraction is you measure its magnetization. You start out above Tc, which by the way, they never do. But what you typically do is you start out above Tc, in zero magnetic field, you cool the sample down well below Tc in zero magnetic field, apply the magnetic field, and then slowly warm it up and collect data. That's called the Zero Field Cooling, ZFC. The plot is Figure 1D of the three-man paper. Here, they did a magnetic field of 10 Oersteds, which is 10 Gauss, or 1 millitesla.

When I first looked at the plot, I thought, "That looks like a standard superconducting flux exclusion curve." The problem is, they didn't go above Tc, so it's not clear—the reason you go above Tc is because that wipes out the superconductivity and you get a fresh start. If the thing was already superconducting at 400 Kelvin and you put a field on it, there may have already been supercurrents lurking around in there, and that's going to change your measurement, so it's harder to interpret the results. But let's say somehow that's okay. The first thing is, if Tc is above 400 Kelvin, I wouldn't see the field cooled and zero field cooled dots identical at 400 Kelvin. They should already be separated. I don't see that, so it looks a little off.

The y-axis of the figure says DC magnetization in units of EMU per gram, or electromagnetic units per gram. The value at their lowest temperature of 200 Kelvin of -7.5 x 10-4 emu/g is the one I'm going to look at in order to estimate the superconducting fraction. Since they quote the magnetization in emu per gram, I need the density of the material to convert to emu per cubic centimeter. I estimate the density to be 5 grams per cubic centimeter. I also have to convert to emu per cubic centimeter per Oersted by dividing by the 10 Oersted applied field. The estimated volume fraction is

(-4π)(-7.5 x 10-4 emu/g)(5 g/cm3) / (10 Oe) = 4.7 x 10-3 ≈ 0.5 %.

That's a very small number. I would not have gone public with half a percent superconducting volume. It's too small. You can easily be fooled because it's possible the system is just a diamagnetic. Maybe this 0.5% is actually not just a little 0.5% superconductor in there, but maybe it's just a weak diamagnet spread out over the 100%. Then this number would be entirely sensible. So, my first reaction was, "This could just be a diamagnet or a USO, an Unidentified Superconducting Object."

If I put a current through this material, as shown in figures 1A, B, and C, when 99.5% of the material is normal and resistive and only 0.5% is superconducting, I'm not going to get a short through the thing. I'm never going to get a zero resistance. So then I'm confused by Figures A and C and B, where they seem to have these ranges where they've got a current going through with essentially zero voltage.

The second paper that came out, the six-man paper appears to have the same data in figure 4A. You can tell because they both have what looks like an instrument glitch at the ninth data point of their field-cooled curve (FC). The problem is, in version one of the six-man paper, the units on the y-axis have changed; it's now susceptibility. Susceptibility is magnetization divided by the applied magnetic field. Hence, I should take the three-man paper value and divide it by 10 Oersteds to match the figure 4A in the six-man paper. The results differ by over a factor of 10,000. The susceptibility value makes no sense because it implies over 100% of the sample is superconducting. My guess is there is a missing exponent in their units in figure 4A. It does not engender confidence.

In version two of the six-man paper, the authors add a 10-4 exponent to the figure. It means there was a typo in the original version that has been fixed. The problem is if I take the magnetization data from the three-man paper and divide it by 10 to get susceptibility, it doesn't match the susceptibility in version two of the six-man paper. It looks like the same experiment. Did they do the experiment a second time? But that doesn't make sense because it's unlikely the instrumental glitch occurred at exactly the same point in two separate experiments. The number from the six-man paper in version two leads to 3.3% superconducting volume fraction. That's better. Maybe, maybe I'd want to publish with 3.3%. I probably wouldn't. And, I still don't see how 3.3% volume fraction would percolate through the system to create a short that leads to the zero resistance in the figures in the original paper. At this point, I don't know what to believe.

Another issue with the six-man paper is the magnetization of their Sample #2 shown in Figure 4A of both versions. The units change between the versions while the shapes of the curves are the same. The superconducting volume fraction estimate comes out greater than 100% for both versions. Again, it makes no sense. Also, you would expect the zero field cooled and field-cooled curves to drop down and turn flat, like the curves in Figure 1D of the three-man paper, Instead, the field cooled curve turns back up and you've got a funny bump in the zero field cooled curve. Either the data is very good and the sample is not what you think it is, or else the data is not very good.

There was another thing that bothered me. If I go to Figure 1A of the original three-man paper, there's a sudden drop from a finite voltage to zero. At these temperatures, I would expect a lot of rounding of this curve. There would be a sharp drop, but it wouldn't be infinitely sharp. You'd expect a rounding that would then curve and flatten out. Instead, when I zoom into it, I don't see any intermediate data points.

My conclusion is I remain optimistic that room temperature superconductivity is possible, but I am unconvinced that LK-99 is a superconductor.

ZIERLER: Let's zoom out for a second now, some sociological questions. As you mentioned, when this paper came out, your inbox was flooded, right?

TAHIR-KHELI: Flooded, yeah.

ZIERLER: Twitter, the science world, this has like been the buzz, like has not happened probably since the so-called Woodstock conference in 1987. Beyond your concerns with this paper, what is all of this latent energy that we have seen burst into kinetic energy because we want this to be true? What's the source of the excitement?

TAHIR-KHELI: It must be the room temperature claim. There is a reason superconductivity has been studied by so many important scientists over 100-plus years. There's something just amazing about nature being able to make a state of matter where there is zero resistance. It's not a tiny resistance; it's zero DC resistance.

ZIERLER: That's the true bar, right there, zero resistance?

TAHIR-KHELI: For DC. In AC, there's always a little bit that you can't escape. But at DC, there is zero resistance. There's something magical about that. Everything we hear about the weirdness of science, and quantum mechanics, and the electron can be in two places at the same time, who knows where it is or what it is—is it a wave, is it a particle—here's this thing in your face that you can almost touch. A room temperature one, you would be able to touch it, while it's superconducting.

ZIERLER: And, as you said earlier, you believe that a room temperature superconductor is out there.


ZIERLER: So you're primed—you want this to be—?

TAHIR-KHELI: I want to believe. As Fox Mulder said in The X-Files, "I want to believe."

ZIERLER: [laughs] That's right. You want to believe.


ZIERLER: Now, because we have the arXiv—with the X—anybody can just upload what they want to. It's not a peer-reviewed process.

TAHIR-KHELI: Right. Almost anybody.

ZIERLER: What's going on here? Let's now say you're a gatekeeper, you're a reviewer for Nature or Phys. Rev. B or wherever, and you look at this paper. Do you send it back to them and say, "You're onto something, but I want to see A, B, and C"? Is this kooky? Is this just lack of patience, "Come back to me when you have the right instrument"? What would your response be if you were peer reviewing this?

TAHIR-KHELI: I guess what I said was basically the equivalent of a referee report. What I said is not rocket science. Anybody who is knowledgeable about this stuff would have said the same things. They may even have been smart enough to pick out other things. I would have said, "I want to believe. At the very least, I need to see a resistance curve going above Tc and an estimate of the superconducting volume fraction. Also, you need to clear up your numbers on the figures so that everything is consistent."

ZIERLER: What are you seeing here? Is it sloppiness? Is it bullshit? Is it lack of patience? Should they not have submitted this until they were ready for something more? What exactly are you seeing?

TAHIR-KHELI: It appears to me that they don't know what they don't know. These people seem to be credible materials people; they know how to make stuff. I didn't see anything that jumped out at me that they don't know what they're doing. How they picked out this compound out of the zillions of others out there, well, that's the secret sauce that a materials scientist brings to the game. I don't claim to know that. But the problem—if you're a materials person, you see a room temperature superconductor as an instant trip to Stockholm. It's not obvious that anything else you're going to make is going to have that kind of appeal to the public. The problem with superconductivity, and that's why it's a graveyard of theorists and people who have made these claims, is that superconductivity is an electrical and a magnetic phenomenon, and generally the people who are making materials are not versed in the subtleties of both of those things. That's where I'm seeing the places where they're a little sloppy. I'm not sure they know exactly what they should be doing. I don't see any intent to defraud.

ZIERLER: There's no con going on here.

TAHIR-KHELI: I don't see it. I think these are perfectly reasonable human beings, and they found an interesting material.

ZIERLER: Do you think it's possible that they are going to get all of the criticism that you're representing, it's back to the drawing board, and they could return with something legitimate?


ZIERLER: So this is not fatally flawed.

TAHIR-KHELI: No, it's not looking good, but it's not fatally flawed. I don't actually know if the material is a superconductor or not. All that you've done is you've gotten me intrigued, but you haven't convinced me at all, and you've shown things that are sloppy, which makes me nervous.

ZIERLER: Jamil, does this call into question having a system like the arXiv where you can post sloppy and outrageous claims that work everybody into a tizzy? We're here talking about this. How much has this pulled you away from your own work. Is it problematic that, just sociologically, we have a scientific system that's capable of doing things like this? This could have been an obscure paper that was blocked at the peer process, and you wouldn't have even known about it, and we'd be none the wiser for it, right? Does that call into question the arXiv system as we currently have it?

TAHIR-KHELI: No. I prefer a freer market, because unfortunately, the journals are political, and they do block perfectly sound ideas.

ZIERLER: Your trajectory kind of speaks to that.

TAHIR-KHELI: Yes. I would take the free market over that, with the rough and tumble. Yes, caveat emptor, you have to be your own filter, but to tell you the truth, I do that even when I look at refereed journals, because most of the papers aren't very good, in my opinion. So. I'm doing it anyway. No, I think the system is kind of working.

ZIERLER: Are you motivated at all to see if you and Carver can try to reproduce this? Have you thought about that?

TAHIR-KHELI: Yes, on the first day, I looked at it, and thought, "Could we make this?" We briefly talked about it. We realized it wasn't that hard to make, but we're not really set up to make it, and we certainly do not have in this lab the equipment to do the magnetization measurement. That's a specialty piece of equipment. I think they said they used a Quantum Design machine. We don't have access to one, nor do we have the money to be buying one and setting one up and getting it to work on a short time-scale.

ZIERLER: And by "we" you don't mean just you and Carver? Caltech does not?

TAHIR-KHELI: I'm sure there is equipment on campus to do this measurement, but my sense was that there are labs that are already set up to do this; they're going to beat you to it anyway.

ZIERLER: Do you think the race is on now, to try and reproduce this? Do you assume that's what's happening?

TAHIR-KHELI: Yes. I bet you a lot of the smart people have found all the squirrely things I found, but they're keeping their mouths shut, because maybe some slight alteration to this material is the holy grail. So, keep your mouth shut while people are all busy worrying about this one, and you're trying, mixing, and doing different things. What if you put a niobium in there, or a silver rather than a copper? I don't know, whatever. There might be some combination. They've pointed out a class of material that I don't think the superconductivity materials field was looking at. So, that's what I would do.

ZIERLER: For the last part of our talk, I want to just ask some questions about working with Carver. He's a legend. He's an incredible person. What's it like? What have you learned from him, as a person and as a scientist?

TAHIR-KHELI: He is my hero. I've never met anyone like him. The five or six years of working directly with him in the lab have been one of the greatest gifts of my life. To have that kind of time, in the lab, working with Carver, has been an amazing experience. I think I got more time with him than his graduate students got. When we started this project, neither of us knew the answer, so I wasn't learning from Carver where he was just showing me something that he already knew. We were both making it up as we went along, so I got to watch—I got to watch a modern Michael Faraday, how they fish around, and they sniff their way around, to find something from almost nothing. It was one of the greatest experiences of my life.

ZIERLER: How do you understand Carver's intellectual trajectory from microchips and VLSI to what he is doing now with you? What's the connecting point?

TAHIR-KHELI: I think it's the same thing as me; he wants to know what the electron is.

ZIERLER: Is that a knowable answer? This is really philosophical, but just—Carver coming to campus, two weeks out of every month, right? Obviously it's for the love of science. He never wants to stop.


ZIERLER: But is there a knowing at the end of all of this? Or is it just really about the journey?

TAHIR-KHELI: Carver once said, "Well, Jamil I'm pretty certain that even in your lifetime, we won't know everything, so, you can have fun for the rest of your life trying to understand." So I think the answer is, it's the journey. And if you get one teeny bit of insight that's new or original or helps you understand a few other things, that's worth it.

ZIERLER: Last question. Finally, Jamil, what does this research mean for you, in terms of keeping the conversation going? Does this change any direction for you? Does it provide a path that might not have been apparent before? Is there now a resurgence in all of the excitement, the race to achieve room temperature superconductivity, for which you might contribute or might positively contribute to what you're trying to achieve?

TAHIR-KHELI: It has not changed what I'm doing day-to-day because we believe we have something that could become a useful superconducting technology in the near future. I think it's good that people are aware of superconductivity and its amazing potential. It hasn't been fully exploited by human beings yet. So it's exciting. But you can also see it might turn a lot of people off, if there's too many people making claims, and so now we're cynical. Isn't the trick that you don't want to be too credulous or too cynical, right?

ZIERLER: That's right.

TAHIR-KHELI: That's really the trick in life.

ZIERLER: There seems to be a poetic repetition here, because what's happening now seems to be a phenomenon that you've experienced as a graduate student.

TAHIR-KHELI: Yes, yes, yes. The odds are not in favor of these people, if I was to bet. But I wouldn't take that bet, honestly, because I could be wrong. So I think it's exciting.

ZIERLER: And they could spark the next person to challenge it, and who knows what happens as a result.

TAHIR-KHELI: Maybe in this class of materials, there's something cool, and somebody might find it. I think people need to hunt. We need more people trying wild ideas in superconductivity. However, the academicians have no new ideas and because of tenure they might be lost for at least another generation. The engineers appear to have run out of ideas too. The big hope is that somehow the cuprate tapes will be good enough to get fusion over the hump and that will justify 30+ years of effort. Perhaps the only people left on the planet who are willing to swing for the fences are the LK-99 type people.

ZIERLER: Jamil, it has been a great pleasure spending this time with you.

TAHIR-KHELI: Thank you.

ZIERLER: I want to thank you so much for doing this.

TAHIR-KHELI: Thank you, thank you.