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Credit: Rick Soden
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Interview of Nai Phuan Ong by David Zierler on December 10, 2020,
Niels Bohr Library & Archives, American Institute of Physics,
College Park, MD USA,
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Interview with Nai Phuan Ong, professor of physics at Princeton University. Ong describes how he has managed to keep his lab running during the coronavirus pandemic thanks to remote data analysis. He recounts his childhood in Malaysia in a family of ethnic Chinese who had businesses in Penang, and he describes his Catholic schooling and how he became interested in science as a young boy. Ong describes the opportunities leading to his undergraduate education at Columbia, where he pursued a degree in physics. He explains his decision to enroll at Berkeley for graduate school, where he studied under the direction of Alan Portis and worked on developing a microwave technique to perform measurements of the Hall effect without making Hall contacts to the sample. Ong recounts his offer from the University of Southern California to join the physics department first as a postdoctoral researcher and then as a member of the faculty. He explains his decision to move to Princeton and describes some of the difficulties given what he saw as a low point for condensed matter physics in the physics department at Princeton at that time. Ong describes the significance of the prediction and discovery of superfluid helium-3, and he discusses how Phil Anderson introduced him to high-Tc superconductivity. He discusses his research on representing the weak field Hall effect in a geometric fashion, he explains why the cuprate Hall effect remains mysterious, and he describes his more recent work on quantum spin liquids and the Nernst effect. Ong describes the excitement surrounding research in novel ground states of Dirac electrons in graphene, and what the achievement of topological quantum computers would mean for his research. At the end of the interview, Ong explains why graduate students are among the rarest and most precious resources in science, and why he hopes to concentrate on the Karplus-Luttinger theory in the future.
This is David Zierler, oral historian for the American Institute of Physics. It is December 10, 2020. I’m so happy to be here with Professor Nai Phuan Ong. Phuan, it’s great to see you. Thank you so much for joining us today.
Thank you for the invitation. I’m honored.
Alright. So, to start, would you please tell me your title and institutional affiliation.
I’m a professor of physics in the Department of Physics at Princeton University.
When did you start at Princeton?
I started in 1985, so I’ve been here for quite a while.
Yeah, that’s a long time.
Yeah. Well, it’s (laughter)- yeah. Yeah, almost four- three and a half decades.
Phuan, just to talk about current events a little bit: in what ways has remote work and COVID affected your research, and in what ways have you been able to keep up with things, despite the difficulties?
Personally, I haven’t been affected that much, because the university has been very efficient in addressing the pandemic. Although it’s in lockdown, it’s only partial. Researchers can make a special request to come in if they wish. But we have to take the Covid saliva test once a week. And my graduate students take them twice a week. We’ve had minor outbreaks in the building, but nothing serious, and they’ve been very quick and efficient at tracing, and so on. But I feel-
So, your laboratory work has been able to go on pretty well?
Yes. Yeah, with appropriate social distancing. Masks are mandatory, whenever you step out of your office. And I would say a fifth of my colleagues are here, so yeah, it hasn’t been that bad an interruption. Coming in has been very therapeutic, actually, because I’ve been continuing my regular routine.
Phuan, are there aspects of your laboratory work that are now automated with computers, where you can do some data analysis from home if you wanted to?
Correct. Yes. In fact, my students know how to hook up their experiments so that they can even work from home and control the equipment remotely. But I try to discourage that. Sometimes, these experiments run for fifteen, twenty hours, so they lock up the lab and they go home to sleep. But the computer keeps churning away. So yeah, many experiments are automated. And I can-
In terms of collaborations with your colleagues, without being able to have conferences, have you found Zoom and videoconferencing to be a fairly effective substitute for in-person meetings and conferences?
Okay, in the sense that we can still talk to them. Right? And what’s good is that you can do this with colleagues from all over the world. But lacking this personal contact is a big impediment, because a lot of ideas come when you are in a one-to-one conversation in the corridors in some hotel. Then you get these ideas happening much more quickly, much more conveniently. I guess body language is very important in scientific discourse, and that’s what’s missing in these Zoom talks. When you say something wrong, or when you say something that’s interesting, you can see the reaction of the person you’re talking to. That, I feel, is missing in Zoom, because everyone comes prepared and camera-ready in Zoom.
(Laughter) Phuan, what about teaching? Have you had any experience teaching over Zoom to undergraduates this year?
Oh, yeah. Yeah. Before the summer, I taught an undergraduate course. And these are mostly freshmen, and the students have had a hard time adapting to it. They tend to be more quiet. They’re very afraid of asking questions for some reason that I can’t figure out. Because these courses are very interactive, and normally- so, we have twenty-five freshmen, and half of them would be eager to show off what they’ve read. But on the Zoom, they were very reluctant, and it’s hard for me to call on them, because you cannot reach out in Zoom to tap any one person.
It’s been more, I feel, like a YouTube talk, and I don’t think they did that well. You know, the benefit of Zoom is that they can go back and look at the video afterwards. But lacking this instant contact, where they can ask questions, was a big drawback. I hope the whole thing will be over in a year.
I hope so, too.
Because Zoom just doesn’t- it isn’t adequate.
It would be great to think of September, the start of the new school year, as things getting back to normal. Right?
Yes. Yes. Although, Princeton’s opening up in the beginning of February. Again, you know, under very strict social distancing conditions.
Right. Hopefully, that works out well. Phuan, let’s go all the way back to the beginning. Let’s start with your origins and your family, all the way back in Malaysia. Let’s start with your parents first. Tell me a little bit about them and where they’re from.
Well, they’ve had a much more interesting life than mine. They’re both from Chinese extract, in Malaysia. My mother’s ancestors came from China, many generations ago. And by the time she met my father, her family was fairly comfortable, middle or upper-middle class. They were basically traders or merchants. They ran several shops. My father entered Malaya illegally- well, he slipped into the country illegally, because at the time, it was under the British, and the British had very efficient sea patrol to guard against unwanted immigrants. My father told me that he was on a boat trying to evade the British patrol. So, they knew exactly when the patrol was coming. When they passed, my father jumped and swam to shore. Penang is an island. With the help of the local village clan, my father found work in my grandfather’s shop. To make a long story short, the Japanese invaded in 1940. When was Pearl Harbor? 1942?
’41. Correct. So, in late ’41, they had conquered Penang, Malaysia, and that led to the downfall of Singapore. It was brutal, because the army had come with a list of people whom they accused of supporting the Chiang Kai Shek regime. At the time, Japan and China had been at war for ten years or more. And of course, the Chinese immigrants were sending a lot of money to support the Chinese government. The Japanese army was brutal. They lined up people to be shot, soon after they invaded. But before the invasion, they dropped bombs. And the very first air attack killed my grandfather, my mother’s dad. They bombed a large tree that he was sheltering under because trees hid anti-aircraft guns in the China campaign. And since he was the head of the family, the family was placed in a very dire situation. The Japanese army’s brutality is well documented, they were rounding up unmarried women for nefarious purposes. So, my grandmother fled with her family to a distant village. My father offered to marry my mother. And even though he was from the wrong social class and much older, my grandmother agreed.
Your father was ethnically Chinese as well.
How did he first meet your mother? Where did they meet?
Remember, he was working with her father.
Her father’s shop. My dad was a worker, and they had known each other from that. Growing up, I always sensed a class barrier between my dad and the rest of the clan. He was just a poor, lowly worker, newly arrived from China. And my mother was this beautiful daughter of a fairly wealthy businessman. So, there’s no way that he could have asked for her hand and not be laughed at or kicked out of the shop. But during the war, that was a welcome arrangement. And so, they got married. The post-war years were very difficult. I was born three years after the war.
Phuan, how large was the Chinese community in Penang? How many people, do you think?
It was very large. Yeah. Well, Malaysia is now about fifteen to twenty percent Chinese. At that time, I’m not sure. Probably roughly the same fraction. But the Chinese were concentrated in the big urban centers, and Penang being one of the big seaports was almost ninety percent Chinese.
And how much mixing was there between the Chinese community and the Malaysian community?
There was a lot of mixing, because we did not regard ourselves as Chinese. You know? We regarded ourselves as- what’s the word for it? We were called “Straits Chinese,” the “straits” being the Strait of Malacca. Right? So, we were- you know, like half of us didn’t speak Chinese, or Mandarin. And we went to English medium schools. And there was no barrier between the Chinese and Malays. We often socialized together. It wasn’t until the nationalist movement came that things started to go south.
And what languages were a part of your childhood?
Okay. I spoke with my parents in a Chinese dialect, distinct from Mandarin, because they had very little schooling. Bless their memory. I spoke English with my siblings. There were seven of us. We learned Malay in school, but we rarely used Malay except when talking to folks in the village. But the educated Malays were also speaking English, mostly. So, it was a very mixed community.
And your school was in English?
Yeah. I went to a school, Saint Xaviers Institution, run by the Christian Brothers. The Christian Brothers ran several schools in Asia. The schools were inspired by the sixteenth century missionary Saint Francis Xavier. They were the most dedicated teachers. They lived in poverty, and they taught all the local kids (charging each student two dollars a month!). My school was excellent.
Now, was your family Christian? Is that why you went to this school?
No. Most of my classmates were not Christian. Although, of course, there were attempts to convert all these foreign heathens (laughter). But sometimes, we played along and attended Mass and so on, but I think no more than one or two percent were Catholic.
Phuan, when did you start to get interested in science?
I was raking my memory. You know, there was a library close to my school run by the British as part of their consulate. The country became independent from Britain in ’57, but the consulate remained as part of the British consul. It must have been when I was ten when I became a member of the library. My eldest sister Ay Whang, who had a subscription perked my interest. I started reading many books about science and airplanes, especially. I was fascinated with aircrafts. That was the start of my interest in science: I started to build toy airplanes from these kits, and I was fascinated by how they could fly. At an early age, I started to copy drawings of pistons and turbine blades in jet engines. Although I didn’t know very much, that really was what made me fairly technical compared to my classmates.
And in middle school and high school, did you have a strong education in math and science?
I had a good education in math. In science, it was not optimal- yeah, I wouldn’t do it the way I was taught (laughter). It was very- how should I put it? Sort of book learning, without emphasizing the way one actually does science. But the math training was very good. When I arrived at Columbia, I had had much more math than my American classmates. I’d had two years of calculus. I knew differential equations, and so on. So, I was the go-to guy for math among the physics majors. But I found that they were much better in physics, because they knew how to approach science from a curiosity-driven direction rather than simply book learning. Right? So, my training was basically, you know, you memorize these principles, Newton’s laws and Maxwell’s laws, and you apply them without wondering about the scientific process. And I found that my American classmates were much more adept at moving into uncharted territory. This came to the fore when we started to learn quantum mechanics, which was new to all of us. I had a very hard time, because I couldn’t grapple with all these concepts. It was all new, and I didn’t know how to learn new concepts. I had to unlearn my previous habits of learning.
Phuan, let’s back up a minute here, because it’s a very interesting story. What were the opportunities that allowed you to come to study at Columbia in the United States?
At the time- this is guesswork on my part, the U.S. was beginning its involvement in Vietnam. It had “lost China,” the civil war in China had gone badly, and the French had been kicked out of Indochina after the battle of Dien Bien Phu. I think all the private universities were very interested in recruiting foreign students from that part of the world. We were not aware of it, but my sister Ay Whang who had a strong influence on my early years got one of these scholarships. I think she arrived at Barnard College in 1962, I believe early sixties. So, I knew these scholarships were available.
Were you in contact with her? Would you learn about what she was doing in the states?
Yes. I knew there was this avenue. But most of my classmates, if they were going abroad, would go to England or Australia. England was obviously the prime attraction. Australia was the cheaper alternative. No one would ever think of going to the U.S., because of the following: the degree from Cambridge or Oxford was regarded as the gold standard, because Malaysia was a former colony.
And Australian universities were not too bad. Right? But the U.S., no one knew anything about the U.S. system, and it was basically a huge gamble to come to the U.S. Yeah.
What year did you arrive in New York?
And what were your initial impressions? Had you traveled abroad at all in your life?
Had you left Malaysia?
And what were your impressions when you landed in New York?
Trepidation, (laughter) and I guess bewilderment. Yeah. I’d never seen skyscrapers before.
Penang had not had tall buildings at this point.
No. No. Yeah, nothing above three or four floors. Yeah. So, seeing a skyscraper for the first time was a real eye-opener. Yeah, it was like a completely different planet. Right? This New York was very different from Malaysia.
But at least you could speak English. That was fine.
Yeah. So, that was okay. Right. I could get around, and I had read a lot about the U.S., obviously. I was ready to get acculturated, but there were still many things that were difficult to get around. The biggest problem was a sense of dislocation that all freshmen feel. Right? At Columbia, like all the Ivy Leagues, all the freshmen were captains of their class, stars, and so on. You are thrown in with everyone else, all equally accomplished, and you couldn’t make friends, and you miss all the support structure. I had had a lot of support structure back home, and that was all gone. It was like swimming alone, in this very competitive sea. But then the experience was also colored by- well, not colored but at that time, it was the height of the Vietnam War, and ’68 saw the start of serious campus unrest nationwide.
But even in ’67, when I arrived, the protest movement was approaching a crescendo at Columbia. On the first week during freshman orientation, the upperclassmen, were staging skits. They flashed a huge photo of Lyndon Johnson on the screen. Then a student came out and splashed red paint all over it. I was stunned. I was convinced that the police could come in and arrest the whole freshman class. And that took getting used to- such open protest could never have happened in Malaysia.
This is an exposure to democracy that you had never seen before.
Yeah, that’s right.
Phuan, what were your own politics? Did you have any strong feelings about the way that the students were protesting, or about the Vietnam War itself?
Well, you know, I underwent a transformation, as did 80 percent of the freshmen in Columbia. When I came, I was very pro-U.S. policy in Southeast Asia. But then, after- well, you know, after seeing weekly news clips of B-52s and Phantom F4s and the protest movement, I rapidly became radicalized. So, I started to empathize more and more with the protest movement, and my politics kept shifting left from that point on (laughter). When I was a sophomore, the first major campus upheaval in the U.S occurred at Columbia. I don’t know if you remember SDS (Students for a Democratic Society).
I didn’t know Mark Rudd who led the SDS branch at Columbia, but James Kunen (known for the “Strawberry Statement”) and others were my classmates. The New York Police didn’t know how to handle the protests, so they brought in mounted cops. There were, I remember, a dozen mounted cops forming a line bisecting the campus. Then they charged the students. The attack, which led to a lot of bloody heads, triggered the first campus shutdown.
It made it worse, you’re saying.
Oh, yeah. A lot worse, yeah. Thereafter, the campus shut down every spring semester. So, in my sophomore, junior, and senior years, we had no spring semesters. Everyone would emerge from the dorms and shut down the campus.
Phuan, in the classroom, how did you go about picking a major? Did you know you wanted to pursue a career in physics at this point?
Yes. I arrived determined to major in physics. I wavered a little bit because of my encounter with quantum mechanics. Our text, the book by Bob Dicke, was full of bras and kets. It was the first time I got a B in a course, and I was so upset that I went to see Professor William Happer. I don’t know if you know him.
He’s a very outspoken anti-climate-change physicist. He was my instructor in intro QM. I went in to see him, and I told him I just couldn’t make head or tails of quantum mechanics. He wasn’t very helpful, and I thought of changing to engineering, because I was much more comfortable with building things with my hands.
So, this means, Phuan, even at this early stage, that you would pursue experimentation more than theory in physics?
No. It wasn’t clear. I liked building things with my hands and enjoyed seeing them work. It just gave me a kind of sublime pleasure. But I found myself also rather strong in math and theory, although I didn’t know what theory meant at the time. I wasn’t decided. But my undergrad advisor, who was a particle physicist, Alan Sachs, dissuaded me. I will never forget the pain on his face when I told him I was switching(laughter). He just went, “ugh” (laughter). And I knew I had to switch back to physics.
He was upset that you were considering engineering.
Yes. And he couldn’t even mouth the disappointment.
So, he saw your talents. He was confident in your future success.
Well, I’m not sure he saw it, but anyway, he was quite effective in dissuading me. He was right. I attended my first few courses in engineering, and I was so bored that there was no question, I switched back right away. Columbia was a very powerful influence on my subsequent development as a physicist, because that was where I learned to unlearn all the bad habits of learning. Right? Instead of regarding physics as something that you inherit, and memorizing the rules, you learn how to ask questions. And that kind of approach, always pushing the boundary, asking questions, not believing something until you’ve convinced yourself by either experiment or deriving it yourself- yeah, that was kneaded into me for three years in Columbia, by several faculty members. That was a very valuable education. It is the hardest thing to duplicate, I think. As you know, the American university system is the envy of the world, but most people mistake the reason. They think that: oh, it’s because you have the best equipment and facilities. That’s not it. It’s because the university system teaches you how to think rather than acquire knowledge. Right? And this is a very difficult concept, which I try now to pass on to my graduate students. That’s a system that’s very hard to duplicate. The Japanese have tried it with mixed success, and now the Chinese. Right? The Chinese have sent their best students here. They go back. They get top positions, and they are trying hard to duplicate the American system. To do so, they will have to figure out how to transplant successfully the vibrant intellectual milieu of American universities.
Phuan, when you came to Columbia, was your intention that you were going to stay for the undergraduate degree and then return home? Or at what point did you realize-
I wanted to, yeah.
At what point did you realize that you would stay not only for your education, but that you would make a life for yourself in the United States?
It wasn’t clear until I got my first job. Because remember, this was the height of the Vietnam War.
I knew I would go to graduate school, because I wanted to get a PhD in physics. And I was really open to going back to teach in Malaysia, although the opportunities were scarce. Because being ethnic Chinese, I had a barrier. They usually awarded faculty positions to ethnic Malays. The job opportunities were also very few. So, it was by accident that I found my first job. I can tell you more about that if you wish.
And once I got my first job, then it just happened. There was no point at which I made a decision to stay. It was just from year to year.
And by the end of your undergraduate experience, how well defined were your identities as a physicist? At that point, did you know you were going to go into experimentation for graduate school?
No, it was still ambiguous and I remained ambivalent.
And so, what were your considerations when you were thinking about graduate school?
Let’s see. I got into Berkeley. Berkeley gave me a fellowship, which was very enticing (laughter). And let’s see. Where else did I get into? I was rejected by Princeton. I applied to Princeton (laughter). Let’s see. Where else? I think it was La Jolla. I got into La Jolla. But anyway, I landed in Berkeley, and wow, it was a completely different part of the U.S.
From highly urbanized New York to the California of the 1960s. Right? This was the flower children era, Grateful Dead, and all that. It was totally mind-blowing. But I still wasn’t decided on theory versus experiment. Towards the end of the second year, most of my classmates then had already decided, and I was quite late, because I couldn’t decide. I went in to talk to Charlie Kittel. I don’t know- do you know Charlie Kittel? He passed away in 2019 at 102.
I do. Yeah.
He was retired at that time, but he was a famous theorist. And I went in to talk to him, and he was surprised that I would talk to him about a thesis, because he must have been eighty at that time. Not quite- seventy. Yeah. Anyway, I noticed that he had a bed in his office, so he was, you know, quite close to, if not retired. So, he asked me, “What would you like to do?” And I told him, “Well, frankly-” no, at first, he asked for my name, which he carefully wrote down in this scrapbook, and then he asked, “Well, what would you like to do?” I said, “I’m not sure. I’m good in theory, and I’m good in experiment. Maybe I’d like to do both.” Then, without a word, he tore this page out of his book, crumpled it into a ball, and tossed it into the wastepaper basket (laughter). “Come back when you are sure.” So, there was no question I was headed for experiment. But none of the experimental groups would take me, because- I forget the reasons. I think they were full. Yeah. There weren’t that many that were taking students, and each one of them had the full quota of students, because I was late. It was at the end of my second year. Alan Portis, who became my PhD advisor, was unusual. He had left physics research for ten, fifteen years. I forget. He was active in education (as part of the team that wrote the Berkeley volumes on intro physics) and had served as the director of the Lawrence Hall of Science. But in 1973 his former student, Alan Heeger, the future Nobel Prize winner-
-had just discovered- claimed to have discovered high-temperature superconductivity (at sixty Kelvin!). Now, that was a false finding, but we didn’t know at that time. So, that motivated Portis to return to research. Portis had one opening, and I talked to him and [he] said, “Yeah, I’ll take you on.” And so, that’s how I started in experiment.
Was it called “solid state,” or was it “condensed matter” at the time?
At the time, it was solid state. Condensed matter was renamed, I think, by Phil Anderson or someone in Bell Labs, to include superfluid helium 3, which would become the central topic in the late seventies. At the time of my thesis, superfluidity in helium 3 had not yet been discovered. It was still called solid state physics.
And what was the laboratory like when you initially joined? What were the projects going on at the time?
Terrible (laughter). Portis had not engaged in research for ten or fifteen years. I was the only guy in the lab, and I had to build everything from scratch. One day, I said, “I need something called a lock-in synchronous amplifier,” because that’s the primary amplifier, invented by Bob Dicke, that everyone uses. And he said, “Oh. I have one upstairs in the storage room that you can look for.” So, I went up to the attic of Birge Hall, and I found this prototype lock-in from the early fifties powered by a row of large vacuum tubes (laughter). Although it was coated with dust, the tubes glowed menacingly when plugged in. Yeah. But that, more or less, was the kind of equipment I had. Actually, instead of doing experiments, I spent most of my time reading, because I was more interested in theory than doing experiments. So, I read. In hindsight, that was a good thing, because the department gave Xerox privileges to all the graduate students. There was no limit to how much you could Xerox. I remember that by the time I graduated, I had two or three cabinets of papers that I had Xeroxed and annotated. I read very deeply into each of these topics. And most of the reading was useless at that time, but they really set the stage for my research in subsequent years.
And what were you attracted to? What were the topics of most interest in your readings?
Almost everything actually, because at the time, there were very few review articles. So, if I read something by a famous guy, like Anderson, Landau, Kohn or Luttinger, I wouldn’t understand a word that they had written. And so, I would need to read back, and then back from that. And so, I would need to go back and back to the original papers, and then read forward again. And this was actually extremely valuable. It’s something that I tell all my students to do, because there’s nothing like learning from the old masters. You may not understand everything that they’re saying, but you get the gist of the story, and you get to know how these ideas- where they came from, how they evolved. And that has been a godsend, because the longer the roots are, the more profound the idea. And this would have echoes in my future, when I worked on things like the Berry phase, the Luttinger theorems, and so on, right up to the present. That was the most valuable life lesson. At Columbia, I learned how to think like a scientist. At Berkeley, I read very deeply; I had much more familiarity with the published literature in solid-state physics than most graduate students.
And what was Portis’s style as a mentor? Was he central to the way you developed your dissertation?
No, he lost interest (laughter)- the discovery that Al Heeger had claimed, high-temperature superconductivity, turned out to be wrong. And my early lab work was showing that. But many other groups had also found the same conclusion. Then, Portis’ interest evaporated, he basically did not show up in the lab at all, which was fortunate, because I wasn’t there most of the time (laughter). I was in the library, xeroxing papers. I saw him roughly three times a year, because he had to sign my card to say that I’m still enrolled. Unfortunately-
Phuan, absent a more engaged mentor, how did you go about developing your thesis research?
I read. At the time, John Bardeen, so, this is how the discovery came about. Bardeen, whom everyone admired as one of the gods of solid state, had announced his new idea of superconductivity that he thought explained Heeger’s experiment, it’s called sliding conductivity. So, I read that, and I got completely mesmerized. I fell in love with that theory. When Portis said that Heeger would send him samples of these materials (the organic conductor TTF-TCNQ) that were supposed to exhibit Bardeen’s sliding conductivity, I jumped to join his group, even though it had no equipment. However, Portis himself didn’t really understand the problem. Bless his memory. Portis did not grasp the excitement about the phase transition to the charge-density wave state and sliding conductivity. So again, it was my good fortune, as I thought back in my life, it’s been one happy accident after another.
The next accident was that Pierre Monceau had heard that, in my thesis, I had developed a microwave technique to perform measurements of the Hall effect without making Hall contacts to the sample. This was an idea that Al Portis had, and I spent two years building a bimodal cavity (which supports two degenerate, orthogonal resonant modes). It didn’t work very well, but anyway, the idea was that, you place the sample in a bimodal cavity, and by looking at the Faraday rotation in a magnetic field, you can infer the Hall effect. This was a clumsy way to perform the Hall effect (roughly 1000 times poorer in resolution than DC). But amazingly, Pierre Monceau had heard about this idea, and so he arrived at Berkeley with these extremely tiny whiskers (of NbSe3) that he couldn’t put Hall contacts on. Today, of course, it’s a breeze. NbSe3 exhibits two impressive electronic phase transitions (at 142 Kelvin and at 49 Kelvin) to charge-density wave states. The Heeger material, TTF-TCNQ, also had a charge-density wave, but it did not exhibit sliding conductivity.
After about six months, I decided to put Monceau’s NbSe3 crystals in my microwave cavity. I had no hope of seeing its Hall effect, but that didn’t matter. In the cavity, we succeeded in measuring the resistivity profile versus temperature in an electric field oscillating at a frequency of ten GigaHertz. To our astonishment, it was completely different from the DC resistivity profile. The DC resistivity displays two enormous “humps” caused by the successive disappearance of the free electrons, as they condensed to form the two distinct charge-density wave condensates. However, neither hump was seen in the resistivity measured at 10 GHz. In hindsight, what we were seeing was that the microwave was rocking the charge-density wave condensate back and forth. The oscillations translate to enhanced AC conductivity that erased all traces of both transitions in the resistivity. This was at 6:00 pm, and Monceau was upset and hungry. Out came a stream of colorful, French swear words. He thought that we had burned the crystals. So, I said, “No, we don’t have enough microwave power to destroy the crystals.” Let’s check them by increasing the DC current through the crystals. As a result, we made our second discovery in the same night: as the DC current was increased, the two “humps” in the curve of resistivity vs temperature were sliced off as if with a sharp razor. The enhanced electric field was abruptly dislodging (depinning) the charge-density wave condensate from the host lattice, causing them to “slide.” By midnight, we knew we had seen what Fröhlich and Bardeen were talking about, although it wasn’t superconductivity. But it was electrical current conduction of a new type. So, we repeated the experiment. We did many measurements until- it must have been 4:00 or 5:00 in the morning, because I remember that, when we left the lab, dawn was breaking over the Berkeley hills. But we knew we had seen something new. You know? And that’s an unforgetable experience, because we were the first human eyes to witness this phenomenon.
And we knew it. We knew it. Because you know, when it happens, and it’s real, you instinctively know, because it was so clear.
Phuan, I wonder in what ways was your wide reading in theory was instrumental to the success of this experiment.
I think it was. Not to pat myself on the back. Because remember, I had done this deep-dive, and when Bardeen announced his ideas to explain the Heeger experiment, I started to read back, and I found the paper by Fröhlich. Fröhlich actually had the same idea ten years before. I think he was working at Bristol University (he later moved to Technion). I read that, and then I read about the various theorists who were attempting to make a serious calculation. I found a very important paper by Anderson, Patrick Lee, and Maurice Rice that said: well, the condensate will unstick, but it will not be superconducting. It will be more like friction, it will stick and slip. And that was amazing to me, that you could have a quantum entity that imitated the rigidity of a classical object- an extended object. I had read all these papers, and so when we saw it, it was unmistakable. I mean, it was- yeah. So, in a sense, both our eyes were primed, maybe mine more so than Monceau, because he hadn’t dug into these papers. But the hardest part of the writing up was to convince Al Portis that we had seen something new (laughter). And he-
What was his resistance? Why did he think this was something hard to believe?
I don’t know. You know? I never asked him. Because I didn’t get along with him. He thought I was a royal pain- well, anyway, we never got along. He tried to ascribe the finding to an extraneous effect, and he came up with various contrived models. All were unrealistic and easily disproved. So, he asked me- I did all the writing, to rewrite the manuscript. Monceau and I had prepared a manuscript announcing the discovery that we were about to send off to PRL. And Al said, “No, no, we’re not going to submit that, because this is all nonsense.” So, I rewrote the manuscript eight times. I wish I had kept every copy, but in those days, we did not use a computer. All you had were typed versions. And yeah, I must have rewritten it eight times, and he still wouldn’t let us submit it. In the end, he got Leo Falicov, a theorist at Berkeley, interested. They came up with this highly implausible idea based on Zener tunneling (laughter). The first announcement of our finding had the wrong theory behind it, which we never lived down (laughter). And that was published in PRL. So, that’s how it happened. You know? The two senior authors did not understand the phenomenon, and I was just a graduate student. This was so, even though I wrote the paper, I wrote everything. But after I graduated, and landed my job at USC, Monceau and I wrote a long paper with the correct interpretation, because in those days, after you publish in PRL, you are obliged to write a PRB. And then, we dropped Portis’ name and published on our own. But still, he wouldn’t give his blessing to submit it. In the end, I just had to override his objection. I regard that long PRB paper as the first announcement of this discovery.
Phuan, given how fundamental this discovery was, what questions in the field did it answer, and what questions were raised as a result of what you had just discovered?
In my mind, it showed that the prediction of Fröhlich and Bardeen was correct, that indeed, you had this amazing phenomenon in which electrons condense to form a condensate. Along one theoretical path, they become a superconductor, and along a different path, they become a charge-density wave condensate. The charge-density condensate is not superconducting, but if it could slide, it conducts an electrical current, although not as efficiently as a superconductor. So, in that sense, this was a big confirmation of their ideas. As I mentioned earlier, this is one of the four or five mechanisms by which electrical current can be conducted in a material. Right? The others are: ordinary conduction in metals and semiconductors, superconductivity, kinks in polyacetylene, and (far in the future) chiral edge modes.
Subsequently, the whole field became badly muddled by many researchers. Bardeen also muddied up the waters by claiming that the depinning process was a result of macroscopic quantum tunneling, a phenomenon that no one understood, and no one could parse and analyze what Bardeen meant. Because you know, I don’t know if you’ve ever met him. He can be difficult to talk to. Do you know the physicist Per Bak who passed away twenty years ago?
Per Bak told me about a party in Urbana that Bardeen had hosted. They were walking in his garden which Bardeen was very proud of. It had twenty or more trees. Bak asked Bardeen, “When did you plant this tree? When did you plant that? What is that tree?” And Bardeen just stood there silently nodding. Bak was concerned that he had said something offensive. Then after about five minutes, out came the information: “This was planted in 1922. This was 1935” (laughter). So, you know, Bardeen has this long-time lag when he talks to you. He’s processing the information before he lets it out. It was difficult to have a one-on-one conversation with him. He always came back with the same words, and with a time lag. Few knew what he meant. Eventually, the sliding conductivity field played out. All told, there were four materials known to exhibit sliding conductivity. As the field dried up, it was beset with these poorly posed ideas as well as conflicts between experimentalists. Few were paragons of virtue.
You mean, the science suffered as a result-
-because of the lack of clarity and communication.
I suspect some claims were fraudulent. Some of the data in PRLs were possibly fake (laughter). The sliding conductivity field acquired an odor. Either you couldn’t trust the data, or you couldn’t trust the theory. I was happy to leave it when I arrived at Princeton in 1985. That was like a fresh break for me.
Yeah. After your defense, what were the opportunities available to you? Were you looking at postdocs and assistant faculty positions at the time?
No, just postdocs. That was the depth- the height of the Vietnam War. I sent twenty letters to Bell Labs, and other places, and I got no offers.
And what year was this?
’71. No, I’m sorry. No, ’76. ’76.
So, you’re still- Cambodia invasion was ’75, I believe.
No, no, ’70. 1970.
’70. ’70. Right.
The evacuation from Saigon was ’75. This is right after.
Correct. This is the year after. All university groups in the U.S. had been badly decimated, and they were still recovering from the war. The only two offers I got was from Walter Hardy at British Columbia, and from Young Bae Kim at USC. Young Kim was an interesting character who had worked at Bell Labs. At Bell, Kim discovered that a large magnetic field or DC current could abruptly drive a clean type II superconductor (Niobium, I think) into its resistive state. He brought this problem to Anderson.
Phil Anderson, you mean?
Yeah. Phil Anderson. Right. It was surprising that Anderson took time off to listen to Kim. Out of that discussion came what’s called the Kim-Anderson flux-flow theory, the earliest evidence for flux flow in a type-II superconductor. The resulting Kim-Anderson theory was a pioneering step that eventually led to commercial superconducting magnets.
Subsequently, Kim arrived at USC to start a group in applied superconductivity. The job offer made to me was a part of a ploy, yet another happy accident that came my way. Kim had reserved five huge rooms in a brand-new science building, Seaver Hall, as part of an effort to recruit Doug Osheroff from Bell Labs. Osheroff was a likely bet for the Nobel Prize because of his discovery that helium-3 becomes a superfluid at 1 mK. The prospects of Osheroff coming were slim. Nonetheless, Kim tried hard to prevent these labs from going to the burgeoning quantum optics group by having a new recruit occupy them. I was too naïve to inquire about the boundaries of this offer. Although I had no equipment, again (laughter). I did have a nice problem to work on, the sliding charge density wave. The one nice advantage about the position was that Kim had a chemist in his group, James Savage, who succeeded in growing crystals of NbSe3. These crystals saved my career. Bardeen, unbeknownst to me, also helped. Although Monceau and I had sent him the original preprint in 1975, Bardeen never contacted either of us. Instead, he called the Office of Naval Research (he was on their advisory board), and said, “There’s a young man. His name is Ong. He just joined the faculty at USC. I want you to fund him” (laughter). It’s like a godsend. Right?
I knew nothing about applying for funding. I wasn’t even on the USC faculty. One day I got a phone call from Dick Brandt, a contract monitor at ONR, who said, “I want you to send me a white paper.” And I said, “What do you mean, ‘a white paper’? Papers are all white” (laughter). Brandt couldn’t believe how dumb I was, but he did give me funding, and that saved my hide, because a year later I was promoted to a regular faculty position.
Still without startup package. Right? The senior faculty said, “Now you have your own funding. We should make you a regular member.” And then I broke away from Kim’s group and set up my own group.
Phuan, did you see this as a natural progression from your thesis research?
Or was this a new area of physics for you?
It was a natural progression, because the discovery was made in the last six months of my thesis. And nothing had been done on this phenomenon, so I knew I wanted to do my best to explore it. A whole arena had opened up, and there was all this stuff to do. With the funding from the Navy, I could do it. Fortunately, no one else believed our experiment, except at Bell Labs. Patrick Lee persuaded his Bell Labs colleagues to jump in, and they did, but it was two years after the discovery. So, I had a breathing spell of two years to do some experiments. The first problem we attacked was a prediction by Patrick and Maurice Rice that if you put impurities into the system, they would act like pinning centers. Right? They will make it harder to de-pin the condensate. We verified exactly how it happened. The concentration dependence of the threshold field confirmed the weak pinning calculation of Lee and Rice. I think that was a big boost to the career of Patrick. He never told me so, and I know him very well. But I think that was the first major success he had at Bell, that he had made this prediction on how the de-pinning would depend on the impurity concentration, and the experiment beautifully verified that. So, that was a big accomplishment in the very early years.
And presumably, this research helped you secure a faculty position at USC.
No. Getting the funding from the Office of Naval Research-
Oh, that was actually more important.
Well, there was that. Yeah. They had no idea what I was working on, because the condensed matter group there consisted of myself, Kim, and a group investigating superfluid helium-3. And they had no idea what solid state physics is all about. But what they were concerned with was funding. Right? Because here’s a young guy who didn’t have a startup package, but he could attract funding. So, we should promote him.
In what ways was your work with Joe Brill so important?
That was the depinning versus impurity experiment, showing that there are two kinds of pinning: one is called “weak pinning,” which is much more interesting; and the other called “strong pinning.” Weak pinning is what Patrick and Maurice had calculated. In weak pinning, the threshold field, the minimum electric field you need to make it start sliding, varies as the square of the impurity concentration. We persuaded Savage to grow ten compositions with varying amounts of impurities, basically replacing niobium with tantalum, which he did (laughter). And then Joe Brill and I worked hard measuring these ten different compositions. And indeed, we verified that the threshold varied exactly as Patrick and Maurice had predicted.
Did you achieve tenure at USC?
Yeah. I got tenure after six years, yeah. And that’s under the regular schedule. Right. But then-
Were you happy at USC? Did you think you would spend your whole career there?
Yeah. I was convinced that, that’s it (laughter). You know, L.A. at the time was a very nice place to be, because you could go to the beach in half an hour from USC. Right? And you go to Chinatown or Little Korea in ten minutes. It was the whole universe, crammed into one city. And the air really wasn’t smoggy. It was a very pleasant climate. And I met my wife Delicia there as well. We had just bought a house. Let’s see. We got married in- oh, she’s going to kill me. I think we got married in ’82, maybe ’84, no ’82. And then the offer from Princeton came in ‘84, and she was heartbroken. She didn’t want to leave that house.
It was up in the hills of La Crescenta, a small-town northwest of Caltech, towards the mountains. And this cabin was beautiful. It was brand new, all wood, up in the mountains. I had to break the news to Delicia: we have to move to the East Coast, because Princeton just made me a professorship offer.
This was an offer that, academically, you could not refuse.
I would say so. Yeah. But that’s what I had to persuade her, because getting the best graduate students is extremely important for experimentalists. Not so much for theorists. Right? But for experimentalists, you want to at least have a shot at attracting a good fraction of the best students in the world; although they all came to the U.S, the top ones rarely chose USC. Now, Princeton had its drawbacks. At the time, it was very hostile to condensed matter physics.
Right. Condensed matter was in a low point at Princeton at this point.
Very low, yeah. A brief history. You know, condensed matter, solid state physics, had an early start at Princeton, one of the initial powerhouses. Bardeen and Conyers Herring were students of Eugene Wigner, and David Pines was also at Princeton. But then, after the fifties, they let it wither on the vine, I guess. They did have a few faculty, but they were not competitive in this mushrooming field. What made the faculty realize they had to change was the shifting graduate student body in the late seventies. The PhD applicants were changing. Right? More and more of the talented experimentalists were opting for condensed matter. The best prospectives would not even agree to visit Princeton, because there was no one here doing interesting experiments.
And so, where would they go? Like, Champaign-Urbana would be the place for them to go at this point?
Berkeley. Berkeley was huge.
Stanford was huge. Not so much Harvard at that time. But anyway, Cornell, Berkeley, MIT, Urbana, Stanford, and San Diego.
So Phuan, did you understand your recruitment as an institutional effort on the part of Princeton to revive the condensed-matter program?
No. You know- again, I was very naïve. I thought, wow, if they offer me the job, I’ll go. I knew nothing about the political landscape. There was another compelling reason, aside from the graduate students. Anderson was about to leave, and he was, of course, the leader of the entire condensed-matter effort. Without informing the chair, he had committed in ’82 to be the founding director of the new Institute of Theoretical Physics at Santa Barbara (now Kavli KITP). And luckily for us, me especially, Joyce Anderson refused to go, because she had had a traumatic experience with fire. She grew up in Chicago, and there had been a devastating fire, I think, in the forties. And she had to go out on the lake in a boat to escape. It was traumatic. Then she visited UCSB (laughter) and the Santa Barbara folks showed her and Phil the Reagan Ranch. The Reagan Ranch was perched on the peak of the Sierras. You could see on one side the Pacific Ocean, and on the other side, the deep hinterland. Right? But Joyce wasn’t interested in looking at the ranch. She was fixated by the hillsides and asked, “What are those brown patches?” They said, “Oh, that’s nothing. Every year a fire comes through. The Santa Ana winds whip them up.” And as soon as they came home, she said: “Phil, you can go.” See, this is what she told me, many years later. “But I’m staying in New Jersey” (laughter).
So, Phil knew that he couldn’t- you know, Phil is very dedicated to- he was very dedicated to Joyce. Val Fitch, who was the chairman at that time, assured him that the Department would do its best to build up the condensed matter experimental group. So, they tried. They thought that it would be just a cakewalk, but Princeton had such a bad reputation that no one that they identified would come. None of the stars in the field would agree to come. I think among the people they asked was Cherry Murray, who was- she’s gone off to accomplish great things. And maybe a few others. I never found out who the rest were. Then Phil, in desperation, after he told Santa Barbara he was staying at Princeton, asked Patrick, his old chum at Bell Labs: “Maybe we should look for young people, instead of the stars.” And Patrick said, “Well, there’s a young guy at USC who is doing charged density waves” (laughter). That’s how they found my name. It was just out of these vague connections that I had in the past.
And perhaps they knew you were naïve enough not to appreciate everything that was going on.
I was naïve. When I arrived, the pervasive scientific culture was challenging at times. The research organization was very hierarchical, like universities in pre-war Germany.
Did you take graduate students with you to Princeton?
Yeah, I took a postdoc and one graduate student. Yeah.
And what were your impressions of Princeton when you joined the faculty? Were you the first Asian on the faculty?
Yes. I was the first Asian, the first Chinese. I was traumatized by the reputation of the faculty. You would sit in these faculty meetings, and every other faculty member was a major star in his or her field. It was mostly men at the time. And I’d say to myself I don’t belong. Right? Because I was a total unknown. And few spoke except the eminent theorists. Arthur Wightman (the department historian) told me that Eugene Wigner often referred to the experimental faculty as “the hired help.” And they had all the power. The faculty meetings were scary. And you know, if you said the wrong thing, they would turn and look at you (laughter). And that’s enough to make you wish that a hole would open up and you could fall through it.
Phuan, did any of the senior faculty take you under their wing, serve as a mentor to you?
The more influential faculty were kind to me, because I suppose they realized that they had to do something about condensed matter. They were eager to help me succeed. I was given the best office available, which led to some grumbling among the B team. Sam Treiman was the chair when I arrived. With the help of Treiman, Fitch, Anderson, Wightman, George Reynolds, Joe Taylor and Ruby Sherr, I soon found my footing. But I was foolish. I didn’t ask for a sizeable startup package. The amount I asked for was so small that Treiman, who was negotiating with me, visibly choked. And I thought he choked because I was asking for too much money (laughter). He choked because of the opposite reason, but he coolly smiled and said, “We’ll manage.” Right? In hindsight, I had asked for nothing. Just a few pieces of electronics here and there. And that later turned out to be not quite enough.
Phuan, did you take this as an opportunity when you moved to Princeton with the new laboratory, with new instrumentation, to move into new areas of condensed matter?
I wish I could tell you, yes, I had the foresight to do that. But remember, I don’t think too far ahead. I was still mesmerized by my day-to-day problems: charge density waves. The decision to change fields was basically made for me, because I had lost my NSF funding before the move.
That’s another unfortunate story which I won’t go into unless you ask me.
I’m going to ask you right now. What happened? What happened with the funding?
Well, I had one NSF grant, which is extremely difficult to get for experimentalists, especially in this area. I had a misguided conversation with Kazumi Maki. Kazumi and Masako were good friends with me and Delicia at USC. One day he showed me an NSF proposal from Tony Leggett that he was refereeing. You know, the Tony Leggett. The proposal was only one page. Maki turned to me and said, “See? When you’re famous, you write one page.” And I said, “Excellent.” I, in my reptilian brain, thought that, you know, if you write a one-page proposal, that’s it. That shows how good you are. And in my renewal proposal, I wrote one page. That was extremely foolish of course. I lost my NSF funding before I got to Princeton. I learned later that Anderson had travelled to NSF to explain Princeton’s aspiration to rebuild the condensed matter group. And they told him: have him resubmit. Anderson comes back, but he never told me I should resubmit my renewal. He just said- his exact words were, “It will be alright.” And I had no idea what he meant by that – a comedy of reticence and ignorance. To this day, I have never submitted an individual investigator proposal to NSF. I later secured funding from a number of sources but not NSF. So, that’s ironic. Right? I arrived to start the condensed matter group. I had no federal funding except from ONR. But happily, high temperature superconductivity was discovered by Georg Bednorz and Alex Mueller the next year, ’86.
Yeah. What were some of the developments that allowed for this breakthrough in 1986?
So, this is how it happened— again, it’s ironic, because Mueller had been invited to a workshop at Los Alamos, which I attended along with Peter Littlewood and Paul Chaikin (laughter). This was eight months before the famous Bednorz-Mueller paper appeared. Mueller is quite a character. He stands up and says, “You know, I have something much more interesting to talk about than the topic today,” (a problem in ferroelectricity). “If you ask me, I will talk about that topic instead of this topic.” And none of us took him up on this! He would have told us everything eight months before it was published. Chaikin and I now look back and say, “Oh, God. We were idiots.” But anyway, Mueller went back and wrote up the discovery. He had been studying ferroelectricity. And he had this idea that, in a ferroelectric, the central positive ion is displaced from its equilibrium position, and that’s what gives you the electric polarization. He had this interesting idea that this could be a mechanism for superconductivity. It would somehow bind Cooper pairs. Not entirely crazy. There were theories about this. And so, he was searching for the right material, and he chanced on lanthanum-barium-copper-oxide. This perovskite had been studied by many chemists, but no one had ever cooled it below the boiling point of liquid nitrogen (seventy-seven Kelvin). Mueller and Bednorz were the first to do so and, to their astonishment, it went superconducting at around thirty Kelvin. And that was how they made the discovery.
Phuan, what were some of the technological advances that allowed for this to happen?
None. This could have happened in 1950.
It could have.
It could have, yeah. It was basically the motivation from a new way of thinking about superconductivity. I mean, honestly, most discoveries in condensed matter physics are accidental.
With my own, like this one, most of them are, including von Klitzing’s discovery of the quantum Hall effect. When theorists tell us that they predicted this, and that it was discovered, it rarely happens that way. I would say eighty percent of the time, it’s some schmuck working at midnight, and he or she stumbles onto something. Right? (laughter) Then what distinguishes the talented from the less-so is recognizing that you’re seeing something new. In most cases the discovery is accidental, except for superfluid helium. Yeah, I must say, superfluid helium-3 was predicted and indeed was discovered.
And in what ways, Phuan, did this potentially affect your career in 1986?
Oh, it was huge.
I had landed at Princeton but had lost my NSF funding. The field of charge density wave was evaporating, and it was rife with controversy and political fights.
So, you were really searching for a way forward yourself.
Half-searching, yeah. Yeah, like I said, I must confess, I was I was still trying to squeeze a few more PRLs out of this dead horse (charge density waves). Right? And then, Anderson, of course, he knew about the Bednorz-Mueller discovery. He was in Bangalore attending a conference where the announcement was made to the Indian physicists. Phil made a long-distance call to me from India, and asked, “Why aren’t you working on cuprate superconductivity?” To which I replied, “Superconductivity?” (laughter) And that was how Phil introduced me to high-Tc superconductivity. After reading the Bednorz-Mueller paper, I realized its transformative importance, but I had to find samples. AT&T had just broken up into Bellcore and six other companies. I got hold of a very talented chemist at Bellcore, Jean-Marie Tarascon, who agreed to send me samples. And so, one of the earliest findings was the following: what is the origin of this superconducting state? What is the parent state? And Anderson had this idea that it came from what’s called a Mott insulator. To show that, one would have to show that the charge carriers (or holes), which are the holes that you introduce by chemical doping, was proportional to the doping- that each strontium ion added to the lattice of the parent compound, donated exactly one hole carrier. The best way to show that is the Hall effect, which is right up my alley. My postdoc Zhao-Zhong Wang (now at Lab. de Photonique et Nanostructures) and I performed the Hall experiment and confirmed that the parent state is indeed the Mott state.
Phuan, did you see these advances purely from a basic science perspective, or were there clear industrial applications for this?
I wasn’t thinking of the industrial applications. Luckily, right? Because it’s taken- let’s see. It’s taken thiryt-five years to have them come to fruition. The technology has matured to the extent that superconducting magnets based on YBCO are reliable. There are six or more companies trying to achieve compact fusion confinement machines using high-Tc superconducting magnets. However, we focused exclusively on the fundamental question: why does it happen? Anderson would constantly be asking me, “What’s new in the lab?” Right? So, it was a very fruitful exchange of ideas and results that went back and forth. I believe our findings also influenced his thinking.
I wonder in what ways was this research attracting the attention of physicists from other fields, given how big a deal it was.
A number of high-energy theorists were thinking about moving into the field. Yeah.
T.D. Lee, of Lee and Yang.
T.D. started to work on a new theory of high-temperature superconductivity, using Feynman diagrams. Phil told me that T.D. and his collaborators had invited him to Columbia to give a colloquium (laughter). So, Phil visited and had a chat with T.D. about his theory. Phil was dreading that. Halfway through the conversation, Phil stood up and walked towards the door. T.D. stopped him midway and said, “No, no. You sit down. This is the chair that (Hideki) Yukawa sat in. Please sit down and listen to my theory.”
And so, Phil sat down. Phil has told me many stories, all very colorful (laughter), about his encounters with famous men. In this venture, T.D. worked with Richard Friedberg. Do you know Friedberg?
He was famously absent-minded (he once stuffed the laundry into the dishwasher). Friedberg taught us freshman physics using the Feynman Lectures, but at the slightest excuse, he would veer off into advanced topics. One I remember is calculating the path of a spinning sphere placed on a rotating turntable!
Friedberg was a boyhood genius. At Harvard, he had solved a very old problem in math. I forget which problem. Anyway, as a result, T.D. invited him down to Columbia and awarded him tenure.
In my senior year at Columbia, Friedberg was asked by T.D. to give a colloquium on his latest research. He starts talking about the Ising model in a magnetic field, explaining how he was trying to find a solution. Halfway through, T.D. stood up and commented at length on why he was wasting his time. Friedberg looked down at his shoes, then when T.D. sat down, he went back to the blackboard and continued from exactly where he left off (laughter). That impressed me.
Phuan, I would love to hear about this APS meeting known as the “High-TC Woodstock” meeting.
Oh, yeah. So, that happened in ’87. I think it was March- obviously in March. And by January, the community was trying to persuade APS to have a session, because it wasn’t scheduled, obviously. And they didn’t have any space. So, they said: okay, we’ll create a time slot, but it has to start at 6:00 in the evening. And they found a small room. The room was way too small for the crowd (laughter). And by 5:00, the room was chock full. And I knew I had to get in, because I was scheduled to give a talk. I had submitted an abstract. And my turn came at 3:00 in the morning. So, the whole session, I was happy to find out, is on YouTube, so you can hear the first talk. I think the first talk was Anderson, all the way to the last one at 6:00 in the morning. And mine was somewhere in the middle.
And Phuan, what was the big excitement to convey to that larger APS audience, who might not have been aware of the particular details of the discovery in the research?
We weren’t interested in doing that. Everyone in that room was working on this problem.
Okay. Okay. So, it was a room of specialists. They wanted to hear the details.
Exactly. And they were exchanging ideas and experimental results and trying to make contacts. Right? Trying to collaborate. We were all believers. We knew that something dramatic had happened, and this was like night and day. We weren’t interested in talking to the reporters. But most of the people in condensed matter physics were aware. They were aware that this was a momentous discovery. But very few could get in (laughter), because the speakers were taking up all the space. It was very exciting. The fact that it went to 4:00 or 5:00 am.
And what was your role in this? What were you presenting on?
I reported on our Hall effect results, which showed that superconductivity arose from the Mott insulator. The parent compound was a Mott insulator. In that early period- this is like five months after the discovery, there were many competing scenarios.
And one of them went back to my thesis project, charge-density waves. I knew what charge-density wave materials looked like, and I knew this had nothing to do with it. Several in the audience were surprised that I would ditch my old area, in favor of the Mott insulator picture that Anderson had proposed. No big breakthroughs came from that meeting, except that many people started to learn how to make the material and make better measurements. These were very early days.
And so, what advances came out as a result of the meeting? In what directions did the field go afterwards?
Well, the field was mostly propelled by experimental discoveries of more cuprates with ever higher TCs. In the period from ’86 to ’89, maybe ’90- so, that four-year period- every year, you had one or two new cuprates discovered. And the TC went higher and higher. Right? And there were other variants that were equally interesting. So, it was all- you know, the moment that discovery was announced, the whole community was in a gold rush. Folks would jump into (making) the new compound, and then the next one would come along three months later. So, it was a mad rush. You know? It was like a gold rush. And the theory struggled to keep up, because as soon as you made a prediction, it was falsified in some way. Yeah. So, things were happening very rapidly, and didn’t really settle down till the mid-nineties.
Yeah. And Phuan, in what ways was this all a very positive development for the state of condensed matter at Princeton?
Well, from my selfish point of view, there was no question, because I instantly forgot about the political struggles and maneuverings, because there was exciting science to do. And we were right in the thick of it, and Anderson kept badgering us for new results. So, it walled me off from the rest of the department, because I was intently focused on this problem. But I did help to recruit Paul Chaikin, who is now at NYU.
Paul was an old buddy from UCLA, and he came, but he did not work on cuprates. He was transitioning to soft condensed matter.
And so, he mostly worked in polymers and soft matter. We were beginning to build up a group at Princeton. Together we identified Bernhard Keimer, who is now one of the eminent condensed matter experimentalists in Germany. He’s a director at the Max Planck Institute at Stuttgart. We succeeded in recruiting him, and he became a valuable addition to the faculty. Then we just grew. Yeah. So, the department was then convinced that things would start to grow, and we added a lot of faculty then. Unfortunately, many of them left- probably five or six came. They stayed for three or four years, then they left, for one reason or another. In the mid-nineties. I recruited a top-notch chemist, Bob Cava, from Bell Labs. This would prove to be a great move that has benefitted many groups at Princeton. Much later, I also recruited Zahid Hasan, Ali Yazdani and Jason Petta, all-star acquisitions.
Phuan, what was the work that you did with Jeff Harris and T.R. Chien?
In 1987, soon after the Woodstock, Maw-Kuen Wu and Paul Chu discovered the first cuprate to crash the liquid nitrogen barrier, Y-Ba-Cu-O with Tc = 90 Kelvin. But there were no single crystals. Right? All you had were these sintered ceramics. Quite early, we succeeded in growing high-quality crystals, thanks to brilliant work by Zhao Zhong Wang. The Japanese group in Tokyo led by S. Uchida and Hide Takagi was our main rival. Luckily for us, Bell Labs never succeeded (laughter). They had two or three chemists working on this problem, but they never figured out. Our group and that of Don Ginsberg at UIUC were among the U.S. groups that succeeded. We focused on investigating the transport properties, especially the Hall effect. My students Jeff Clayhold, Ting Ray Chien and Jeff Harris carried out a series of superb Hall experiments on the crystals. We noticed that the Hall effect was most peculiar. It was very temperature dependent (unlike in conventional metals). We studied this for a long time and got Anderson interested. So, this, along with what’s called the “temperature-linear resistivity profile,” are the two major hallmarks of the cuprates. To this day, I believe they’re still not understood. After Anderson came up with a theory, the two-lifetime model, we realized that a good way to test it was to introduce impurities. As you introduce the impurities (Zn), you change the Hall effect. This allows a test of Anderson’s prediction. Chien’s thesis work was to carry out the full investigation. He found that the Hall angle behaved exactly as the two-lifetime theory predicted. But whether that theory is true or not is still an open question, because it was a very bold theory that people don’t quite understand. It wasn’t a microscopic calculation. It was basically: if things were such-and-such, what would it say? Right? So, it was an assumption built on an assumption. But amazingly, it was verified by Chien’s experiment.
These Hall experiments led to an important shift in our thinking. The “normal state” of the cuprates- “normal” meaning that you are above TC, is much more interesting than the properties below the transition temperature. Yes, YBCO is a superconductor, but experiments in the superconducting state don’t reveal how superconductivity arises. It is much more fruitful to study its properties above TC before it becomes superconducting. So, that was what we focused on.
And what was the new way that you came to represent the weak field Hall effect in a geometric fashion?
That’s a different topic. I was struggling with how to understand the Hall effect in general within the conventional picture. I was sitting outside my accountant’s office, he was doing my tax returns, and running late (laughter). So, I’d been thinking of this problem, how to calculate the weak field Hall effect. I like to calculate, once you tell me what to calculate. As I stared at the calculation, I saw that it could be simplified in a way that should be in textbooks, but for some reason is not. It gives you, I think, a very clean and concise way to capture what the Hall effect actually measures. Mathematically, the geometric interpretation is a kind of generalized curvature. I still don’t understand it, but it exactly recovers the formula. And so, when I showed this to Patrick, he said, “Well, it’s just a construction.” And I was kind of hurt by that (laughter). I thought it was a valuable insight.
Subsequently, over the next three decades, after the paper was discovered by folks thinking about the Hall effect, it’s become the best way to understand the weak-field Hall effect. Roughly, the idea is the following: given an arbitrary Fermi surface in 2D, the lifetime of the electrons can vary in an arbitrary way around the Fermi surface. Right? Because that depends on the details of the scattering by phonons, or whatever. So, my picture is that if you tell me how it varies, the angular dependence, then as you go around the Fermi surface, the vector that represents the mean free path will draw, in its own space, a closed curve that may self-intersect, e.g. a trefoil knot. This is an abstract curve that is unrelated to the physical shape of the Fermi surface. It shows how the length of the mean free path varies with angle in the plane. The result is that the magnetic flux linking this curve gives precisely the Hall effect. Why that’s so is not understood, but I believe it may be a quantum result that carries over to the semiclassical regime. But anyway, it’s a representation that experimentalists now use to help them disentangle the Hall effect.
And in what ways did the research take off from here?
The cuprate Hall effect is still not understood. But then ironically, we found a major twist in the plot. We found starting in 1998 that what we regard as the normal state in the cuprates is not so. The problem is very rich. Recall that electrons condense to form the Cooper pair condensate. Right? Now, in the BCS theory, once you raise the temperature above TC, the condensate abruptly vanishes, together with all traces of superconductivity. So, you’re definitely in the normal state. Hence it doesn’t make sense apparently to talk about the condensate above TC.
However, in 1998 we found using the Nernst effect that this is not so in the cuprates; they are amazing materials. At temperatures high above TC, the condensate remains, but now it is so soft that it cannot support a supercurrent. It is still present in the crystal, but what happens is that vortices and antivortices will pop up spontaneously and cause the superconductor to lose its phase rigidity and consequently its ability to carry a supercurrent. Now, these ideas were anticipated for 2D films in 1978 by David Thouless and Michael Kosterlitz. They originate from very early ideas of Anderson on the phase rigidity or stiffness of the superfluid pair condensate. In the eighties, I had read an inspiring (but little cited) review Anderson wrote on phase rigidity and phase slip in helium-3 that featured, incongruously, a beautifully detailed drawing of a locomotive. This review and Anderson’s textbook Basic Notions of Condensed Matter Physics had a profound influence on my understanding of superconductivity. I gradually understood the concept of phase rigidity and its role in establishing long-range phase coherence. More importantly, I realized that it can vanish even when the pair condensate exists. This would be important many years later when we performed Nernst measurements on the cuprates.
In 1995, Steve Kivelson and Vic Emery wrote an influential paper showing that the cuprate condensate is very soft. The high temperature exacerbates the softness and greatly favors the spontaneous appearance of vortices. And that’s exactly what we saw using the Nernst effect. Above TC, the condensate survives but it is too soft to support long-range superfluidity. When I told Kivelson that, he was just ecstatic, because you know, we had never discussed this problem. Kivelson told me the Kivelson-Emery paper has since become his best-cited.
What kinds of questions were graduate students asking at this point, as they were looking to the future for their careers?
You mean now, or-
No, no. At the time, as a result of all of these advances.
Well, unfortunately, my star graduate students from the cuprate period did not stay in physics. They were attracted elsewhere. At that time, quantitative finance exerted a strong gravitational pull, because “quants” could make lots of money. That was also true of Anderson’s students. They did not even bother looking for academic positions. Many became millionaires before they reached forty. It was just exploding across the industry. Jeff Harris is now a wealthy finance manager. According to him, most traders who trade in, say, climate or corn futures are numbingly ignorant of statistics and science, and it is trivial to make a killing betting against them. Quants were foxes in the henhouse.
What has been the impact of the thermal Hall measurements on quantum spin liquids more recently?
Oh, yeah. This was something that we are currently working on. So, let me jump to the present. Spin liquids is, of course, a field that Anderson discovered theoretically in the seventies. He called it a new state of matter. And today, they are of great interest, because it’s one of the major unsolved problem- unsolved because the wave functions are all entangled. Even though the system does not have long-range order, the wave functions of all the different local moments, the spins, are so entangled that you cannot move one without affecting a hundred thousand neighbors. And this, of course, underlies quantum computing. So, folks are convinced that when they crack this problem, they would be able to use it for quantum computing, or at least create memory chips for quantum computers that are resistant to having their memory corrupted. But it’s a very difficult problem. The experimental probes to date have been neutron scattering and heat capacity and magnetization. That’s three very blunt tools that don’t really tell you very much about the quantum mechanics. Moreover, neutron scattering is inherently a noisy probe. What was missing was transport, which is a sharper tool for drawing out the quantum character of a system.
However, most of the spin liquids, in fact, all of them, are insulators. So, you cannot even think of measuring the electrical current. Right? Then you think: well, let’s measure the thermal current. You can still attempt to probe the system by transport, but now, the transport of heat rather than charge. The spin liquid will indeed conduct heat. But the new problem is that you are mostly detecting the phonon conduction- phonons comprise roughly ninety-five to ninety-nine percent of the heat current. So, how would you filter out the feeble spin current from the phonons? Fortunately, the thermal Hall effect allows you to do that because, in the thermal Hall effect, you apply a heat current- say along the x axis in a magnetic field applied along the z axis and search for a weak heat current flowing along the transverse y axis. Moreover, you should check that the y-axis current reverses when you invert the field, in accord with a Hall effect. Clearly, phonons could not do that. This was exactly the problem my student Kapeel Krishana and I had addressed in ’95: how to distinguish the Bogolyubov quasiparticles from the phonons below TC in the cuprate YBCO using the thermal Hall effect. So, starting in 2014, we started to look at the thermal Hall effect in spin liquids motivated by a calculation by Naoto Nagaosa and Patrick Lee. After six months, my student Max Hirschberger (now at U. Tokyo) succeeded brilliantly in establishing the first evidence of a thermal Hall effect in a bona fide quantum spin liquid using crystals of Tb2Ti2O7 grown by Cava. Performing a string of rigorous tests, Max proved that the spin excitations in the quantum spin liquid state are deflected in a magnetic field even though they are rigorously charge-neutral (hence the familiar Lorentz force is absent). That paper didn’t cause much of a stir, because not many grasped its message. But now, it’s really climbing in citation, because the hunt for the so-called Majorana excitations is currently in full swing. The latest excitement is that in 2018, Yuji Matsuda, my former postdoc now leading an active group in Kyoto, claimed to see Majorana excitations in the Kitaev spin liquid RuCl3 using the thermal Hall effect. We’re not so sure, because we are chasing after the same problem, and our own measurements don’t seem to confirm what he’s claiming. But anyway, it has exemplified how the thermal Hall effect brings in new information that’s directly relevant to the quantum nature of spin liquids.
Phuan, what is the Nernst effect, and how did you get involved in this research?
The Nernst effect was discovered in the forties but remained a little-used curiosity. We stumbled onto it because (laughter)- to be truthful, we walked backwards into it. I had a visitor in 1998, Zhuan Xu from Zhejiang U, who was looking for a good project. As a warmup, I suggested measuring the Thermopower in cuprates. In a temperature gradient applied along say the x-axis, electrons or holes diffuse to the cool end of the crystal. At steady state, the net charge accumulation at the cool end creates a “back” electric field that you measure and report as the Thermopower. By 1998, many such experiments had been done on cuprates. Hence, to do something new, we discussed applying a strong magnetic field along the z-axis. The Lorentz force skews the charge accumulation to one side to produce an electric field along the y axis that reverses sign if the magnetic field is reversed. That is the Nernst effect. Well, Xu performed the Nernst experiment, and came back the next day with a staggering result.
Unbeknownst to Walther Nersnt, the Nernst effect is the technique par excellence for detecting a vortex current. If a vortex liquid is present, the vortex Nernst signal is orders of magnitude larger than in the normal state. The applied temperature gradient drives a continuous stream of vortices along the x axis towards the cool end of the sample. Now imagine a line drawn along the y-axis transverse to the flow. Recall that the supercurrent swirling around each vortex core represents a 2π singularity in the phase field of the condensate wave function. Each time a vortex crosses this line, the singularity causes the phase difference between its ends to jump by 2π. By the celebrated Josephson equation, each jump of 2π translates into a tiny transient blip in the electric voltage measured between the two ends. With the passage of thousands of vortices each second, the blips integrate to a steady-state voltage difference that is easily detected as a large Nernst signal.
Xu had performed the experiment below TC in the cuprate lanthanum strontium cuprate LSCO and verified the appearance of the large vortex Nernst signal. As he raised the temperature above TC, he fully expected the vortex signal to vanish, leaving perhaps the much weaker normal-state signal. Instead, nothing changed. The vortex Nernst signal persisted with little change across TC, surviving to well above TC. And that made absolutely no sense, because one needs the condensate to exist before mentioning vortices. But it should have disappeared at TC. So, that was the beginning of our discovery that the condensate in cuprates, in fact, doesn’t disappear. And I persuaded Xu to repeat the experiment (laughter). He must have done it three or four times. By the end of that month, we knew we had found something remarkable, that the condensate doesn’t die above TC. In fact, it survives as a vortex liquid. Eventually we found that it survives to one hundred Kelvin above TC. There was an international conference that year, and even though the result was new, I felt I had to announce it. In my invited talk, I announced the Nernst effect finding, and was greeted with stony silence. Many confided later, “You must be doing something wrong.” Few were convinced even five years later, after we had shown that every cuprate family exhibits the high temperature vortex Nernst effect.
Now, Bob Laughlin, who had just won the Nobel Prize for predicting fractionally charge quasiparticles in the fractional quantum Hall effect problem, was also a major presence in the high-Tc field. Bob stood up after my talk at a subsequent conference in Rio and said, “Phuan, you are full of-“ he used the “s” word. Now, Bob and I are good friends, so I took this as praise. The audience breaks into laughter, and I felt so- well, I knew I had to find another way to convince Laughlin. And that’s how my group conceived the torque measurement. The Nernst effect is arcane and unfamiliar to most folks, even to Anderson. Diamagnetism- what we learn in undergraduate physics is more familiar. In a magnetic field, electrons align their spin moments parallel to it. That’s paramagnetism. Diamagnetism is just the opposite. The moment- now not from the spin but arising from orbital currents- aligns antiparallel to the field. That’s a sure giveaway of superconductivity, which is one of the very few phenomena that exhibit diamagnetism. Now, if the condensate survives above TC, the sample should remain weakly diamagnetic because the vortex liquid is inherently diamagnetic. Suppose we liken vortices to deep depressions arrayed on a mattress. Then the nubs between them represent tiny patches of superfluid, each carrying a small diamagnetic moment. We set out to detect them. Instead of SQUID magnetometry which is woefully inadequate, we developed ultra-sensitive torque magnetometry with help from Mike Naughton (now at Boston College). The trick is to glue a crystal to a cantilever (a gold foil). A magnetic field set an angle to the copper oxide layers generates a torque that flexes the cantilever which we can detect with exquisite resolution. Using this torque magnetometry, Yayu Wang and Lu Li succeeded in measuring in staggering detail the magnetization of many cuprate families. They found that, in cuprates, the curves of the magnetization versus field (previously rarely investigated) are qualitatively distinct from the “textbook” form in superconductors such as Nb. Wang and Li confirmed that the diamagnetism persists above TC with a magnitude nominally proportional to the strength of the vortex Nernst signal. Hence despite the collapse of the phase rigidity and the vanishing of the Meissner effect above TC, the condensate betrays its presence by its large Nernst signal and diamagnetic signal. I believe the torque results finally convinced most of the community that, indeed, the pair condensate does survive well above TC. Anderson also came around.
What was going on in Tokyo with Uchida’s group, and what was the relevance of this to your research?
Well, they were very strong competitors in the period from ’87 to ’95. They were the leading experimentalists in Japan in high Tc research. I knew the senior member, Uchida, from my charge density wave days, because his thesis was based on my discovery. So, he and I had met at international conferences. Both of us switched to high-TC because we were comfortable with growing transition-metal chalcogenides, and so on. Uchida and Takagi were very strong competitors. We would barely finish an experiment, and the next week they would send us a preprint saying that they had done the same thing. We then returned the favor. It was like that, back and forth. I think the community recognized both groups as having contributed equally to the realization that the normal state was very strange. The Kamerlingh Onnes award, which I was fortunate to join them in, recognized both groups in 2006. But after that, we became collaborators. They would send me crystals if they succeeded in growing them.
What happened as a result from this collaboration?
Let me see. There were- okay, so, for example, when we had to test the Nernst effects and the diamagnetic signal on a family of cuprates that we couldn’t grow- this is the lanthanum-based cuprates, they generously supplied all the crystals to us- ten crystals that covered the full doping range. The first paper announcing the Nernst effect was based on Uchida’s crystals (laughter). It was a very fortunate collaboration, because we could cover the whole doping range in this paper.
Phuan, 2006 must have been a very special year for you. You received a major award, and you were elected to the AAAS.
I was fortunate. I believe that Anderson was influential in the AAAS election. The Kamerlingh Onnes award possibly reflected the consensus of the superconductivity community, at least, I hope. The same year, Kazumi Maki, my colleague from USC-
-also won the Kamerlingh Onnes award. And that was very nice. We shared the stage in the award ceremony. Unfortunately, he passed away four or five years later.
And that same year, you became interested in the novel ground states of Dirac electrons in graphene.
Right. So, my life changed (laughter).
That was a big one.
It was a chance encounter with Patrick. We were in Taipei in 2006 for a workshop. So, as usual, we’d say, “Hey, What’s up?” (laughter) And he told me what he was working on. And I knew, of course, about graphene. Right? The discovery of Andre Geim and Kostantin Novoselov was in early nineties. So, by the late nineties, all the integer quantum Hall effect experiments had been done, and it was nearly exclusively single-particle, non-interacting physics. But Patrick said, “No, no. There’s more interesting stuff in this material.” And so, I talked to him for a long time, and he had some specific predictions, which turned out in the end to be inapplicable. But at the time, I didn’t know that. And so, when I came back to Princeton, I told my student Joe Checkelsky (now at MIT) that this problem was potentially important, and we had to work on it. Joe learned how to exfoliate graphite on his own. And in the first experiment he did, he discovered this new ground state. We were setting out to reproduce a recent result from Geim and Novoselov, in which the conductance in the quantum Hall regime remained metallic up to very high magnetic fields. This was evidence for metallic chiral edge states, according to Patrick. Chiral edge-states are well known today, but this was 2005 or 2006. Patrick explained how chiral edge states could account for the Geim-Novoselov observation. So, after his first experiment on graphene Joe said, “No! What happened was that, at 7 Tesla, the resistivity increased abruptly by a factor of 5000, to values I can’t measure.” And we knew we had discovered a new ground state – the sea of 2D Dirac electrons becomes an insulator in an intense magnetic field. So, even though Patrick’s ideas did not work out, we had found something much more interesting. I sent white papers, by then, I knew about white papers (laughter), to seven federal agencies to extend this research, but none invited a full proposal.
Today, graphene is big business. Right? So, it was short-sighted of them, and I gave up and said, “To heck with it.” That was fortuitous because the next year, 2007, Charlie Kane attended a summer school at Princeton. He was also thinking about graphene, as was Duncan Haldane. Kane had brilliant insights into topological physics, motivated by graphene. In 1986 Haldane had imagined an artificial kind of graphene, now called Haldane graphene, that featured chiral edge states. Kane, working with Eugene Mele and Liang Fu (now at MIT), had elaborated on these ideas and generalized them to a few select bulk materials. One of them, bismuth antimony, was just what Cava and I had been investigating in thermoelectric experiments. Also, my student Lu Li (now at U. of Mich, Ann Arbor) had just discovered a new ground state in pure bismuth in an intense field using torque magnetometry, so we were primed for Dirac physics. Kane stood up at the end of a talk and said, “Oh, by the way, we have this crazy idea, and Duncan, I want to get your opinion about this.” And he rambled on and on about 2D Dirac electrons in topological surface states. Right then, Cava, myself, Hasan, Yazdani and Andrei Bernevig realized, wow, we have these crystals (laughter). We should start working on them. That’s how we got this head start. So, I forgot all about graphene and jumped into topological insulators. That’s how the field of topological insulators started. Again, you know, one of these series of happy accidents that pop up (laughter).
And what were you thinking about with 3D Dirac states?
Well, it was very exciting. Right? Because Dirac physics was what I had remembered from my Berkeley days.
There’s this intimidating 2-volume book, Bjorken and Drell. I don’t know if you’ve ever seen it.
I know it.
Graduate students in the seventies suffered through them. I dusted off my copies to relearn Dirac physics. The thought of doing Dirac physics in crystals was just intoxicating. Right? So, we knew we had to do it. But also, I had remembered about the chiral anomaly from my charge density wave days. And that, again, involved Dirac physics. We had to find Dirac materials before we could engage the anomaly. In 2010 we reported the earliest direct observation of the surface Dirac electrons in a topological insulator by verifying how the period of their quantum oscillations changes with the tilt angle of the magnetic field. These experiments demanded crystals in which the contribution of the unwanted bulk electrons is minimal. However, bismuth is a very fickle element. Its chalcogenides are difficult to purify. For the next few years, Cava and I were preoccupied with the difficult problem of reducing the bulk carriers. Eventually we succeeded in reducing the bulk conductivity by several orders of magnitude (laughter). By that time, the field of topological insulators had matured. Because the Dirac electrons on the surfaces of the bismuth-based topological insulators are strictly 2D, they do not exhibit the chiral anomaly. We needed materials with protected 3D Dirac states in the bulk.
The topological field began to shift to 3D Dirac bulk states in 2011 following an influential paper by Ashvin Vishwanath and coworkers. In 2012, Zhijun Wang and Xi Dai at IOP, Beijing, predicted that the 2 semimetals Na3Bi and Cd3As2 should feature symmetry-protected Dirac states in the bulk. The Dirac nodes are protected from gap formation by point-group symmetry (here, symmetry under 3- and 4-fold rotations about the z-axis). We abruptly turned our attention to both semimetals. Happily, Cava’s postdoc Satya Kushwaha succeeded in growing crystals which my group helped to optimize (this took 18 months). Our interest was in chasing down the fabled chiral anomaly. In quantum field theory, an anomaly is the breaking of a classical symmetry (in this case “handedness”) by quantum effects. In 1968 Steve Adler, John Bell and Roman Jackiw had discovered the first example, the chiral anomaly, in their successful explanation of the mysterious decay rate of the neutral π meson. In 1983, Nielsen and Ninomiya predicted that the chiral anomaly could be observed in bulk Dirac states in a crystal. Although I had long been aware of this prediction, it seemed beyond experimental reach. However, our successful growth of Na3Bi and Cd3As2 crystals suddenly brightened the prospects. In 2015, we published the successful observation of the chiral anomaly in an experiment on Na3Bi led by my student Jun Xiong (now a quant in high-frequency trading). In 2016, we followed up by the discovery by Hirschberger that the half-Heusler semimetal GdPtBi also exhibits the chiral anomaly. Subsequently, a third student, Sihang Liang, and I developed a simple “squeeze” test that allows us to distinguish the chiral anomaly signal from artifacts arising from inhomogeneous current density. In confirming the Nielsen Ninomiya predictions, these observations illustrate the amazing unity of physics over a vast range of energy scales. The reach of quantum field theory continues to amaze me.
When did you start to ask if the Hall effect could be produced by spin excitations that don’t carry a charge?
That was the thesis work of Max Hirschberger. So again, Patrick was an enormous influence.
(Laughter) He and Naoto Nagaosa, the leading Japanese theorist in condensed matter. They wrote a paper predicting that spin liquids should display a thermal Hall effect. So today, we understand it as meaning that they exhibit chiral edge modes, which produce a thermal Hall effect. But in their calculation, they had made a technical error, so the calculation wasn’t quite reliable. But Patrick had shown me that paper, and I came back and told Max: we’ve got to look for this, because our group was one of the few worldwide that had learned to measure it reliably. We got from Bob Cava beautiful crystals of terbium titanate a candidate spin liquid. Max found that the thermal Hall signal was actually rather large, larger than we had expected. So, I persuaded him to carry out several tests before we wrote up the results. Despite passing these tests, the phenomenon is still counterintuitive. For a neutral particle or excitation, you can’t even talk about the Lorentz force. Absent a charge, you can’t predict if it will swing right or left in a magnetic field. It made no sense to talk about the Hall effect. Yet, Max had established that a large thermal Hall effect exists.
What were some of the advances in topological insulators at this time?
The major advance did not come from our lab. Unfortunately, we missed the boat. The major advance was introducing magnetic ions to make it a magnetic topological insulator. Theory predicted that when you have a magnetic topological insulator, the bulk of the film becomes insulating. The Dirac nodes will develop a gap, resulting in an insulating state. However, it will leave behind chiral edge modes that display the quantum Hall effect. This is a quantum Hall effect at zero magnetic field. Right? The magnetization supplies the time-breaking mechanism to give you an anomalous quantum Hall effect. To us, it seemed like an impossible problem to control the magnetic impurities. But not so the Chinese. Qi-Kun Xue, who was trained in molecular beam epitaxy in Japan, is a leading MBE practitioner in the growth of oxide and chalcogenide thin films. After moving from IOP to Tsinghua, Xue teamed up with my former student, Yayu Wang, who had performed the Nernst effect experiments on cuprates. They started to make devices with a team of graduate students. And they succeeded. Yayu told me that, initially, it was like hunting blindly in phase space, and you didn’t know where to start. But once you hit the right composition, the system pulls you in, then it became easier to zero in to find the sweet spot. They must have measured a hundred samples before they found it. When they reported it, it was so stunning that it basically blew us away. You know? They found this beautiful quantum Hall effect that extended to zero magnetic field. Yes, I was proud that my former student did it, but I was somewhat envious. Yayu had just right combination of theory and set of skills to pull it off. That, in my mind, was the most exciting discovery in topological insulators.
Yeah. And Phuan, just to bring the conversation up to the present day, can you talk a little bit about your work in quantum materials and the support by the Moore Foundation to help make this possible?
The dream we have is to move into the second stage of the quantum revolution and to explore fractional statistics. A hot topic is looking for evidence for excitations that have fractional statistics. What that means is the following: if you interchange electrons, their wave function picks up a sign of -1. The -1 may be regarded as a change in phase of π. By contrast, if you exchange bosons, you pick up a +1 or a phase change of 2π. One may ask, are there excitations that acquire under exchange a phase change of say 2π/3? This would show that the excitations are neither fermions or bosons. Such excitations are called anyons. Theorists were talking about this twenty years ago, starting with Frank Wilczek and others. Now, there is increasing evidence that these particles exist in condensed matter. In the fractional quantum Hall effect, the Laughlin quasiparticles are fractional with a phase angle of 2π/3. Under repeated exchange, the world-lines of anyons undergo braiding. The braiding cannot be undone, and so we have a way to preserve the memory of what we did to the anyons. We may use topology to store quantum information.
It may be even more promising. Once we learn how to manipulate anyons, we can imagine many quantum devices. They could measure things that we can’t even dream of because they exploit the entangled nature of the wave function. In the past year, experiments were performed on anyon states in the fractional Quantum Hall effect. One was done at Purdue, and the other in France. They showed convincingly the reality of fractional statistics.
What impact do you think this research will have on quantum computing?
Well, if topological quantum computers could be realized, they would be much more robust than the current generation that Google has. The technology of quantum computing that Google and IBM have been trumpeting is based on the physics of the 1970s: Josephson junctions, qubits and so on. That’s fine, but the quantum states are very fragile. Quantum qubits can store information, but if you breathe on them, the states evaporate. So, the devices have to be kept very cold. The Google experiment is run at ten milliKelvin. Topological quantum computing holds the promise of protecting this fragile quantum state using topological concepts. It’s obviously twenty years in the future. But I think cautiously, the experiments are showing that they exist, and I wish I were twenty years younger, because I know this is going to be very exciting. So, that’s one of topics that I convinced the Moore people that I would start working on. And then the other one is the thermal Hall effect, which is increasingly tool of choice to probe the quantum spin liquids. But that is along the same line of thinking. Because that will teach us to understand entanglement, not just between a few qubits. Current quantum computer qubits involve entanglement over a few dozen spins.
The quantum spin liquid, however, has a hundred thousand spins entangled, and yet, you can still talk about excitations. We recently found that in the quantum spin liquid state, there exist Shubnikov oscillations, as if we had electrons occupying a Fermi surface. We call them quantum oscillations. And we think that the spin liquid has excitations. Even though they are neutral, they are behaving like good old electrons, except they’re neutral. So, this shows that they are real. They are these very strange excitations. I think the quantum world is very strange- sometimes too weird for our classical minds to wrap around. But by doing the right experiments, we develop the language to describe them. We may not understand them, but we can at least make predictions that can be tested.
Phuan, for the last part of our talk, I’d like to ask you two broadly retrospective questions and then a forward-looking question. So, first, I’d like to point out the obvious. You, of course, had a very difficult relationship with your graduate advisor, and yet in your own career, you have had so many tremendously successful graduate students.
Yes, that’s my point. You’ll be humble, I’m sure, but I can say it for you: you’ve had tremendous success as a graduate advisor. So, I wonder then, what negative lessons- “negative,” meaning, what did you learn not to apply as you became a senior person in the field, and a graduate advisor yourself. What did you learn from your own experiences, and how did you turn them inside-out to create this tremendous legacy of graduate students that you’ve had over the years?
Actually, I don’t know (laughter). Because I don’t think about this. But you know, as a side remark, I had a colleague once who told me that although his dad had grown up as an orphan, he was a wonderful parent. Perhaps, kids who grew up starved of parental guidance themselves become very good parents, because they realize or overcompensate for what they had missed. Right? I don’t really know- but I don’t consciously wake up to say, “Oh, I’m going to treat Student A or B very well,” because honestly, I didn’t feel that Al Portis had neglected me. I was happy to be left alone to read papers in the library. I didn’t feel deprived in any way. But what was lacking in my early career was professional guidance. I would have been less naïve, I think, if a mentor had told me, “Oh, you should never write a 1-page proposal, or you should ask for a big startup package.” I was oblivious to all this. But students these days are so savvy. They come armed with all these expectations, so they already know all these things. When I mention them, they just laugh, “Oh, I know all about that.”
But I think a guiding idea, I don’t really have any profound ideas on how to be a good advisor, but an important thing is perhaps the following: that you view graduate students as a scarce resource, because these are kids who are probably the cream of the cream in terms of talent in physics. And there are not many of them. Right? In fact, we find that each year among the domestic students, we would be lucky to see six or more students in this field who want to do experiments, who understand how to use lock-in amplifiers, perform e-beam lithography and (today) have some familiarity with topological concepts and quantum field theory. Out of the, perhaps, 10,000 undergraduates who graduate in the U.S. each year, we have only five or six in this elite group. We can add another five from abroad. And that’s it: ten students that the top ten US Physics Departments compete for. It’s a scarce resource, and I treat them as such. I try to keep them happy. Sometimes, unfortunately, one would make a dumb mistake, like blowing up an amplifier. It’s hard not to lose your temper, because there goes $6,000 up in smoke. The wise move is to just turn on your heels and slowly walk out of the room. After I calm down, I return; this resets the atmosphere. I rarely show my impatience (laughter). And they know. They appreciate this, and they will try ten times harder. Right? Because they know that you have given them the respect not to yell at them in front of their cohorts. We ought to treat them like a scarce resource, which they are.
There’s nothing scarcer than that.
Maybe one of them will accept Princeton’s offer, and then I have to compete with my colleagues to recruit that one. It’s very competitive.
(Laughter) Meaning, you really can see a gem of a graduate student. You’ve gotten to that point where you’ve known enough of them, you recognize real talent when you see it?
Well, not always. Not always. It’s hard to spot talent. But maybe after a year in the lab, you can tell.
Phuan, looking over the course of your career, is there any research in particular that you’re most proud of, either because it was most meaningful to you personally, or because you see it as above and beyond in terms of scientific contribution, the rest of the things that you’ve done?
Let’s see (laughter). Well, I would say the finding on the Nernst effect and the diamagnetism, showing that, in the cuprates, the condensate survives well above TC. I think for me, that was an intellectually rewarding discovery. A measure of how impactful a finding is – the number of people who disbelieve it and remain dubious for years afterward. That is a measure of how counterintuitive and surprising the finding is. And the Nernst effect has got to take the cake. Five or six years later, folks in the audience would still yell out their objections. Now, I think the community has, by and large, accepted it. You can tell when experimentalists, especially, have accepted it, because folks who work in the lab are hardcore skeptics. You can tell that they have accepted it when they propose the same idea but claim to own it (laughter). Then you know they really believe it. Fortunately, the original publications are archived.
To me, the Nernst experiment was the hardest result to sell. The charge density wave discovery was more or less instantaneous. You know? When Bell Labs jumped in, they reproduced the experiment right away. The work on the Berry curvature and the topological insulator, we were in the company of many people. And so, it was very hard to say we did this first, or some other group did. The chiral anomaly, which we established experimentally in 2015: that was also a very satisfying discovery. I had read about the anomaly quite early. So, when Dirac semimetals were discovered, we knew to search for it, and succeeded. This area is still evolving, but that may turn out to be an important finding as well.
Phuan, for my last question, I’ll come back to a comment you made previously. You said you wish you were twenty years younger. So of course, as you know, physicists never retire, so you don’t have to be twenty years younger. You could just live a long and healthy life. And with that in mind, what are the things in your own career that you’re most excited about, things that you look to the future and you say, “Just as I was part of fundamental discovery, ten, twenty, thirty years ago, here are the things that I can be a part of into the future, that can be just as fundamental”? What are those things that you might imagine, looking to the future?
You know, I’ve never done that, so I can’t think about it (laughter). I’m so immersed in day-to-day struggles, I haven’t thought much into the future. I don’t know. I mean, I would be happy to be able to function at the same level as now. Of course, one has to recognize the gradual decline in mental acuity, and so on. But, touch wood, so far, so good. I continue to function at the same level, I believe. So yes, if I could squeeze out another ten, fifteen years- like I said, because this is a very exciting era. I know it’s coming. One can point to specific experiments that show that it’s there, that I hope to participate fully in. And I think it could happen. But of course, that’s a function of the funding, a function of graduate students. But luckily, the students coming in, the graduate students who sign up with me, continue to come, and so I think- yeah, I can be reasonably hopeful that the stream of students will continue. It’s a question of whether nature is cooperative and enable us to see, to do something, then by serendipity stumble onto- ah, that’s exactly what we’re looking for. So that, I think, would be the most satisfying. I do not wish for anything more than that (laughter). Because that’s what’s nice about being an old professor. You don’t have to worry about a lot of things that you used to struggle with. I have all the equipment. And yeah, so this hearkens back to my earlier comment, that I have a reasonably complete library of important papers in this field stored in my head, and so I can fish out some phenomenon by memory. And then catch up quickly. That, I feel, has been the most helpful part of my training at Berkeley, that I had this chance of accumulating all the original papers.
All that reading served you well, from you graduate school days.
I believe so, yeah. Initially, I didn’t think so, but it seems that all these things that we talked about are related.
It all comes together. It all connects.
Yeah. The Berry phase, which is the underlying phenomenon in many topological phenomena, was actually in the original Karplus Luttinger paper. Luttinger did not know that. In my junior year at Columbia, Luttinger taught us thermodynamics, and he was an excellent teacher, but somewhat aloof. But I got to know him. In the late sixties, Luttinger became interested in optics experiments. As a lab assistant, I had to bring him lenses and mirrors. And I got to know him a little bit. At Berkeley, I read up on his theory for the anomalous Hall effect and realized the importance of his early papers on transport. Karplus and Luttinger identified the role of the Berry curvature in the AHE in 1954, roughly two and a half decades before Michael Berry’s work.
Luttinger and Karplus discovered what we now call the Berry connection. They didn’t know where it came from, and most physicists felt that it was nonsense, an artifact of a particular Feynman diagram. Of course, when I read Luttinger’s paper in ‘73 or ‘74, I didn’t understand it either. However, when the Berry phase was discovered in the eighties and Qian Niu related it to the AHE, it instantly connected. It was just the Karplus-Luttinger theory. These deep ideas have a long reach. They keep on giving. Right? And it’s been a rich lode that I continue to mine; it hasn’t disappointed yet.
Phuan, it’s been a great pleasure to spend this time with you. Thank you so much for sharing all of your insights over the course of this career, and it’s been a great pleasure. Thank you so much.
Thank you for asking me. Yeah. It was very enjoyable for me as well.