Princeton’s Robert Cava: From Superconductivity to Topological Insulators
Scientist Interview: March 2011
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Now we have a preprint server run by Los Alamos, so people around the world can upload their papers, and the next day everybody on Earth can read it. So there was a real explosion of research on this new superconductor; it exploded all around the world simultaneously (see, for example).
Were you one of the people who jumped on it immediately? Did you work on it?
Well, this material had arsenic in it, and I’m at a university these days and didn’t want to work on it because I’d be exposing my students to arsenic. But then a researcher in China, M.K. Wu, who had worked on the original high-temperature superconductors with Paul Chu in 1986, discovered that you can get something very similar with lower-temperature superconductivity with iron and selenium (F.C. Hsu, et al., PNAS, 105: 14262-4, 2008). It’s not so toxic, so I started working on that with my students, and I have a couple of nice papers describing some chemical subtleties about this material and what happens to it under pressure where it becomes a very high-temperature superconductor. That was a lot of fun.
Your latest hot papers all have to do with something called topological insulators? What are those, and how did that research come about?
Topological insulators make for an interesting story about how science can work sometimes. I moved to Princeton in 1996 from Bell Labs, and Princeton has a long tradition of excellence in physics of all kinds. But they never had a new materials person making new compounds for them. The condensed-matter physicists were getting samples from all around the world, happily doing their thing, and they realized that one way to do good science in condensed matter is to get a new material that nobody’s worked on before and try to find something interesting in it.
That motivated Princeton to hire me, and we’ve had a long investment
of many years of working together since then.
We did some very nice work and established a good collaboration between
different people, particularly three or four physicists and myself. One of
the fellows in the group, Zahid Hasan, saw a 2007 paper from theorists at
the University of Pennsylvania, Charlie Kane and Liang Fu, and this paper
said that there are some insulators in the world in which the insulating
state comes about through a very unusual combination of properties of the
orbitals of the atoms involved (L. Fu, C.L. Kane, Phys. Rev. B,
76: 045302, 2007).
These orbitals, said the paper, make the insulating state different from what happens in other insulators. There’s a discontinuity at the interface between the outside world and this particular kind of insulator, and you end up with a metallic conducting layer on the surface of the insulator. That’s why they’re called topological insulators. That’s what the paper said, and it was a remarkable kind of theory paper in that it also said, you know, if you look right here, at this compound—and it named one—you should find this special metallic surface state.
Most theories don’t get that specific—they make a general statement, and then materials scientists are left to deduce what must be done to find a material to do it. This paper told you exactly where to look.
So what happened? Did you immediately go and follow Kane and Fu’s instructions?
"What’s striking about these topological insulators is that the electrons on the surface behave completely differently from other electrons," says Robert J. Cava of Princeton University.
The colleague I just mentioned, Zahid Hasan, a professor in our physics department with whom I’d worked before, came to me and said, “Bob, can you make this material right here? This is a really interesting paper and it’s got new kinds of physics in it and it makes this specific prediction about this specific material. Can you grow a crystal of it?” I said, “I already have it, because my postdoc Yew San Hor and I have been working on that compound for another reason.”
We reached into a drawer and pulled out a crystal that Yew San had already grown. Zahid went to work on it, and it took about year or year and a half of experiments to prove that what Kane and Fu predicted was true. It’s basically a new electronic state of matter on the surface of these crystals.
Were other people looking at this same crystal too, or were you the only ones?
No. We were it. And once that first material worked, Zahid said something to me like, “That’s great, Bob, we found one. But it would be really cool to find another one that hasn’t been predicted.” So I did some simple thinking, which had nothing to do with understanding the physics at all, because I don’t pretend to understand the physics. I was just thinking about what kinds of atoms were in this original material—bismuth and antimony—and what other elements are like those and which crystals can I actually grow, since I’m not a very good crystal grower. And I thought about what kinds of surfaces Zahid would need for his experiments, and one of these compounds I came up with actually worked—that’s bismuth selenide—and that’s another of our highly cited papers.
What is it about these topological insulators that makes them so interesting to study and has generated so much excitement?
What’s striking about these materials is that the electrons on the surface behave completely differently from other electrons. They have lots of interesting physics associated with them that’s just not present in normal electrons on either the surfaces or in the bulk of other materials. They act like they don’t have much of a mass, and they have some very peculiar properties related to their spin as well.
It’s the chance of a lifetime for this particular generation of condensed-matter physicists to explore a new electronic state of matter. It’s a new kind of electron to study, so there are a lot of theory papers about what kinds of exotic properties these electrons should have. And now, as people learn to grow these crystals and do interesting experiments on them, more experimental papers are being published. Because we have two or three of the earliest papers in the field, we’re getting a significant number of citations.
And it all happened because our chemists and physicists have this
collaboration and have all been working together every day. We have two
other physicists who do a lot of work in this area: Ali Yazdani and Phuan
Ong.
Yazdani is an expert in scanning tunneling microscopy and he’s been
looking at the electronic states of these electrons on the surface. He just
published a really interesting paper in Science (A. Richardella,
et al., 327[5966]: 665-9, 2010). Ong is an expert in the transport
properties of matter, studying how these electrons carry current. He and
his students found some very exotic physics associated with the electrons
on these surfaces (for example: J.G. Checkelsky, et al., Phys
Rev. Lett., 103: No. 246601, 2009).
It’s all a very cool feedback thing: the physicists ask, “Can you make a crystal that does this and this and this?” and we go back try to grow the crystal. And when they test it, they find it doesn’t do what they expect, so they tell us to change this or that, and that’s what we do. Sometimes, this type of feedback eventually leads to a successful discovery.
It’s been fun working on something that nobody really knows anything about, and working with people who do things completely differently from you. It’s just really great to have another new thing to work on at my age that’s as exciting as the high-temperature superconductors were. It won’t be as practical, but it’s a new thing—it has that excitement of being new. It’s really fun.
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KEYWORDS: Robert J. Cava, superconductivity, Princeton University, topological insulators, iron-based superconductors, Zahid Hasan, Charles Kane, Liang Fu.