Princeton’s Robert Cava: From Superconductivity to Topological Insulators

Scientist Interview: March 2011

Robert J. Cava Twenty-five years ago this April, two researchers working for IBM in Zurich—Karl Müller and Johannes Bednorz—discovered the first material that didn’t have to be cooled to within a few degrees of absolute zero to be superconducting. The discovery, which would later win Müller and Bednorz the Nobel Prize, sparked one of the great research stampedes of modern science. In the quarter-century since the discovery of what researchers took to calling high-temperature superconductors, some 100,000 papers have been published on the subject.

One might assume that the researchers who made the key advances on superconductivity in the 1980s would be satisfied with having provided pioneering work in such an active field, but now another breakthrough has thrust some of them back into Science Watch’s spotlight on hot research.

The new discovery, known as topological insulators, doesn’t have the technological promise of high-Tc superconductors, but it does offer an opportunity for researchers to study an entirely new state of electronic matter, something they’d never imagined could exist. It may not be page-one science, but it’s producing some of the hottest papers in physics, as correspondent Simon Mitton discussed in his January/February 2011 Physics Top Ten column.

One of the principal players in this new research, as he was in the heyday of superconductivity, is Princeton University’s Robert J. Cava. Eight of Cava’s papers from the late 1980s on high-Tc superconductors have each been cited more than 500 times, while his April 1987 Physical  Review Letters article, “Bulk superconductivity at 91 K in single-phase oxygen-deficient perovskite Ba2YCu309-d” (R.J. Cava, et al., 58: 1676-9, 1987), has been cited more than 1,400 times. Nowadays, his latest work on topological insulators is once again attracting citations.

Cava is a co-author on three papers published since 2008 with over 100 cites each, beginning with an April 24, 2008 Nature article co-authored by a host of collaborators from Princeton—“A topological Dirac insulator in a quantum spin Hall phase”—that has garnered more than 200 citations (see adjoining table).

More recently, four reports by Cava and colleagues published in the last two years have been sufficiently cited to register as Hot Papers in the current bimonthly extraction. In all, according to Essential Science IndicatorsSM from Clarivate Analytics, his work over the last decade places him squarely among the world’s 1% most-cited physicists.

Cava, 59, received both his B.S. and M.S. degrees in materials science and engineering from MIT in 1973. He earned his Ph.D. in ceramics at MIT five years later before spending a post-doctoral year at the National Institute for Standards and Technology. In 1979, he joined Bell Laboratories in Murray Hill, New Jersey, where he stayed for the next 17 years. In 1996, Cava moved across the state to Princeton University, where he became a professor of chemistry and materials and, later, director of the Princeton Materials Institute and Chair of its Department of Chemistry. Cava is now the Russell Wellman Moore Professor of Chemistry at Princeton.

 
CAVA SPOKE TO SCIENCEWATCH.COM FROM HIS PRINCETON OFFICE

SW: Let’s begin at the beginning. How far have high-temperature superconductors come in the quarter-century since they were discovered, and what are the key research areas still being studied?

I’d say there are a couple of things that are still going on. First, there’s no universally accepted theory yet about why they work. We know a lot about them, and we have much of the phenomenology worked out, but we still have no theory about what makes them superconducting that the community as a whole accepts. That’s a remarkable situation, if you ask me. It goes to show how complicated physics can be sometimes.

There are so many interesting phenomena that occur in conjunction with the superconductivity that the whole package has not really been put together yet in a way that satisfies everybody, at least not like the BCS theory explains basic superconductivity. There’s nothing yet established that will go into the textbooks as explaining it. There’s a lot of action on the theoretical side, and it’s very sophisticated, but nobody has explained it all.

SW: How about the materials themselves? Have they evolved significantly from the materials used in the 1980s?

Selected, Recent Highly Cited Papers by Robert J. Cava and Colleagues, Published Since 2005
(Listed by citations)
Rank Papers 2005-2009 % of field
1 G. Lawes, et al., "Magnetically driven ferroelectric order in Ni3V208," Phys. Rev. Lett., 95(8): No. 087205, 2005. 239
2 D. Hsieh, et al., "A topological Dirac insulator in a quantum Hall phase," Nature, 452(7190): 970-4, 2008.   220
3 Y. Xia, et al., "Observation of a large-gap topological-insulator class with a single Dirac cone on the surface," Nature Physics, 5(6): 398-402, 2009. 151
4 D. Hsieh, et al., "Observation of unconventional quantum spin textures in topological insulators," Science, 323(5916): 919-22, 2009. 109
5 T.M. McQueen, et al., "Extreme sensitivity of superconductivity to stoichiometry in Fe1+dsSe," Phys. Rev. B, 79(1): No. 014522, 2009. 90
SOURCE: Thomson Reuters Web of Science®.

Back through the mid-1990s, all the action was in interesting combinations of copper plus oxygen plus every element in the periodic table that could be tried with them to look for better high-Tc  superconductors. That part of the research is gone—it’s done. There have not been any new superconducting copper oxides of significance discovered in the last ten years or so. What’s always been going on in the background, though, is that people have figured out how to make really good wires and cables out of them.

A variety of high-temperature superconducting cables are now being tested in the power grid in various places. A high-temperature superconducting cable, for instance, is currently supplying power to 80,000 customers on Long Island. And the reason it’s being used in that particular spot is because they needed to get the cable through a congested area of infrastructure and still have it carry a large current, and the best solution was this superconducting cable. It’s working. It’s been in the grid for a couple of years now.

But we still have to solve a lot of problems—not just making superconducting wires and cables, but connecting them to ordinary wires and cables once we do. You can think of it as a 25-year development effort to get to this point. A lot of problems have been worked out, although not all of them, and it’s now often a matter of economics about whether it will be financially viable in different situations.

There are also some very advanced experiments being pursued to try to characterize superconducting materials. The physics tools to do this have gotten very sophisticated in the last 20 years. So research on high-temperature superconductors is still going strong, although it’s not as splashy for the general public as it was in the olden days.

SW: What do you consider the critical questions in high-Tc that still have to be answered?

Well, why do these things superconduct at all, and what’s going on in these materials at a very, very local scale? The most interesting physics is probably occurring at a scale of tens of angstroms. You can see wonderfully complicated things going on at the nanometer-length scale with different kinds of experimental probes. It’s not just a uniform sea of electrons like we learned a piece of copper is. It’s a very inhomogeneous distribution of electrons doing all kinds of crazy stuff. The more people look, the more complicated it gets.

SW: Are any researchers seriously looking for new high-Tc superconductors anymore, or is that part also done?

A small number of people are, and every once in a while a big surprise appears—somebody finds a new superconductor that nobody expected. In 2008, for example, a Japanese group found a new superconductor, a combination of iron, arsenic, lanthanum, oxygen, and fluorine that was superconducting at 26 Kelvin. What made it so interesting is that the superconductor seems to arise from the iron and arsenic, and the iron should typically give you a magnet.

Until the high-temperature superconductors came along, people thought magnetism and superconductivity were incompatible, whereas in many cases they’re probably just two sides of the same coin. You can change a magnet into a superconductor and a superconductor into a magnet by changing some chemical parameter.

So in 2008, a group of Japanese researchers discovered that iron and arsenic are the basis of a new class of superconductors whose superconductivity and magnetism seem to be related (Y. Kamihara, et al., J. Am. Chem. Soc., 130[11]: 3296-7, 2008; see also ).

After that big discovery, another group discovered that the temperature of the superconductor could go up to 50 or 60 Kelvin with the right combination of elements. That makes these the second-highest-temperature superconductors known, and with a whole new element involved—not copper anymore, but iron.

Of course, thousands of people also jumped onto this new one really fast. The interesting difference between now and 1986 is that back then you had to hear about the discovery through word of mouth. Somebody talked to somebody who talked to somebody on the other side of the world. Occasionally, a fax of a preprint appeared. Information didn’t travel very fast.

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