Photo of Unharnessing a new energy force in superconductivity
Summer 2012

Unharnessing a new energy force in superconductivity

Feature Quantum Materials 20.06.2012

WHEN PHYSICIST LOUIS TAILLEFER NEEDS UNPERTURBED TIME to be productive, he’s been known to skip town, leave his wife and kids behind in Sherbrooke and hide out at the InterContinental hotel in Montreal.

It was in late February 2007 when Taillefer – a professor of physics at the University of Sherbrooke; Director of the Quantum Materials Program at CIFAR; and a recent recipient of the Killam Prize, one of Canada’s most illustrious research awards – resorted to this strategy, in a desperate attempt to finish writing his CIFAR five-year renewal proposal. Locked away in his hotel suite, he could concentrate fully on the task. He ordered room service, worked as he ate, and ignored email.

But then on February 27 – he remembers the date, and for good reason – the phone in his hotel room rang (Taillefer doesn’t own a cell). It was a call from Toulouse, France, from one of his PhD students at the National High Magnetic Field Laboratory (NHMFL), where Taillefer and his team there were running experiments studying cuprate (copper-oxide) superconductors – a family of materials that conduct electricity without resistance.

“One way to describe what we do is that we ask questions of various materials,” says Taillefer. “We question the electrons in those materials and we get an answer back by measuring various fundamental properties. Sometimes the answer is what we thought, and sometimes it’s completely different. Sometimes it’s a very confusing, muddled answer that we can’t make sense of. And sometimes, of course, we are told something that we never expected.” To wit, on this occasion, the student at NHMFL was calling to report that their experiment had just defied all expectations and discovered “quantum oscillations.”

“For physicists,” says Taillefer, “quantum oscillations are the most pristine voice that electrons have to talk to you.” For instance, the existence of the oscillations would indicate immediately and surprisingly that the cuprate superconductor was a metal and not an insulator (as had been widely assumed). Physicists, however, had tried and failed to see quantum oscillations in cuprate materials before, and the established wisdom was that they would never be seen – it was essentially an abandoned quest.

Understandably then, Taillefer’s response to the report of this elusive quantum effect was one of disbelief, along with his trademark precision and clarity.

“What I appreciate with Louis,” says David Le Boeuf, the PhD student who made the perturbing phone call, “is that when you ask him a question, he will answer very simply. He has a way to explain physics that is very clear in his mind, so he gives a very clear answer.”

In this instance, Taillefer’s answer was “No, no, no. That’s not possible.” The anomalous wiggle in the data curve – waiting in his inbox, along with the other emails he’d been ignoring – simply couldn’t be the electrons saying, “quantum oscillations.” He told Le Boeuf to go back to the experiment, to question the electrons some more. “Try this test, and this test, and this test,” he instructed. And then Taillefer went back to writing his proposal. Perturbation mitigated – for the moment.

But as test after test dismissed suspicions and “artifacts” – potential errors in data – the team’s cautious enthusiasm mounted. “We got more and more excited, the more and more evidence we got for those quantum oscillations,” says Le Boeuf. And finally, they convinced Taillefer – that odd wiggle was indeed part of some real quantum oscillations. “It turned out to be the biggest discovery of my career,” says Taillefer. “It sure perturbed my report writing, but in a good way.”

THE BIG QUESTION UNDERLYING THE DISCOVERY OF QUANTUM OSCILLATIONS, FOR TAILLEFER AND ALL PHYSICISTS IN THE FIELD, IS: CAN WE FIND A ROOMTEMPERATURE SUPERCONDUCTOR?

It is a big question, with big stakes. So big that it made the “12 Events That Will Change Everything” list compiled by Scientific American, the time-honoured arbiter of humankind’s rapport with scientific advancement. The events predicted to “forever reshape how we think about ourselves and how we live our lives” included the following: polar meltdown, asteroid collision, deadly pandemic, human cloning, machine self-awareness and extra-terrestrial intelligence. And now, lo and behold – room-temperature superconductors.

In conveying the promise of such a superconductor, Taillefer offers this analogy: The supercon is to the average electrical conductor (like copper) as the laser is to the lightbulb. It is a more coherent and orderly form of electricity on the atomic level. And thus, it is electricity that would be much more powerful in its applications – applications which, for the most part, we cannot yet imagine. Currently, the best high-temperature superconductors – the cuprates – operate below 164 Kelvin, or roughly -100oC – halfway to room temperature. “We are missing a factor of two,” says Taillefer. “We are looking for this factor of two that will bring superconductivity to room temperature. Then you can have a wire on your desk, and it would be superconducting as you look at it.”

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Louis Taillefer – Director of CIFAR’s Quantum Materials program – works with a dilution refrigerator that cools down superconductors and other quantum materials to –273.1˚ Celsius, a few millidegrees above absolute zero. IMAGE, COURTESY OF LA TRIBUNE / IMACOM BY JOCELYN RIENDEAU

A room-temperature superconductor would be a major technological revolution, in part because it would completely change the way we transport electricity, enabling, for example, green energy – such as wind power and solar power – to become much more efficient. “You could use the solar power of the Sahara and safely transmit the electricity generated there to Europe by going underneath the Mediterranean,” he explains. Traditional power lines, by contrast, would have to be high above the ground, making them vulnerable to disaster or attack – and costly. Similarly, the cuprate superconductors would be too expensive since they would have to be continuously cooled. A room-temperature superconductor, then, would be a momentous change in how we transmit energy.

“That’s the application that you can imagine,” says Taillefer. “But I believe [that] if you had a superconductor at room temperature, then human ingenuity would kick in. A lot of people would play around with this stuff on their desk and they would try things.

“Imagine if a laser only worked at minus-100. How many applications of a laser would you have? Very few. And same for the transistor. If you had to cool down a transistor to use it, you wouldn’t have Google. If we bring superconductivity, this quantum coherent form of electricity, to the point that it is accessible to everybody, then I think things that we can’t imagine will happen. It would make electricity qualitatively different. There are applications we can predict, like transmitting electricity, but I think the most exciting applications are those that are not predictable.”

WHILE FINDING THE ROOMTEMPERATURE SUPERCONDUCTOR that will spark a technological revolution is the ultimate applied goal, Taillefer says that “in practice, what it means is just trying to understand what makes some materials really good superconductors, much better superconductors than others.”

The unexpected discovery of quantum oscillations changed the paradigm in the field, he says, because it revealed a new principle at the heart of electron behaviour in cuprate superconductors – still the best hightemperature superconductors, but still a long way off from room temperature.

Alas, progress isn’t predictable, it isn’t linear; the physicists and the cuprates aren’t gaining a few degrees every month. Rather, they need another unexpected discovery, another breakthrough.

And as the magnanimous Taillefer is quick to point out, discoveries the likes of these do not occur without collaboration. With the quantum oscillations discovery, for instance, it took the combination of three kinds of experts, brought together through the CIFAR Quantum Materials program. Says Taillefer, “That’s really exactly what CIFAR aims to do – bring together people with different skills and experience, from different places, to work closely on a long-term basis and advance big questions.”

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A polycrystalline pellet (YBa2Cu3O7), cooled down to –196° Celsius, floats around a magnetic track at the University of British Columbia. IMAGE, COURTESY OF NSERC AND SUPERCONDUCTIVITY (BONN & HARDY) AND QUANTUM MATERIALS (DAMASCELLI & SAWATZKY) LABS AT THE UNIVERSITY OF BRITISH COLUMBIA

First, there were the materials experts – the modern-day alchemists who grew crystals of the highest quality, the best cuprate crystals in the world. Three CIFAR Fellows at UBC – Doug Bonn, Walter Hardy and Ruixing Liang – had been perfecting crystal growth for 15 years. “Liang is the secret weapon in the entire Canadian activity in this field,” says Bonn. “He brought the growth of very high-purity crystals of complicated materials to a really high art. Liang’s gradual success at improving the quality of the materials, year after year, even decade after decade, got these to a level of purity where we didn’t know it at the time, but we had hit the level required to do this quantum oscillation experiment.”

Next were the measurement experts. “I was the one who pushed the button,” says CIFAR Associate Cyril Proust, formerly Taillefer’s post-doctoral associate, now a Research Director at the NHMFL in Toulouse. And Proust pushed – literally – a real, big red button that instantly produces a huge magnetic field. He had been working for years to refine the facility’s sensitivity – its ability to run very low noise electrical measurements in a magnetic field a million times the Earth’s field. When he started, the noise produced by the measurement would have been larger than the size of the oscillations. But he notes, “I was working to improve the set-up for general purposes, not even thinking of searching for quantum oscillations.”

Neither, for that matter, was Taillefer – the third expert in this collaborative triumvirate: the explorer experimentalist. He brought the vision, the focus on the important problems, and the intuition for where to search. And while never in his wildest dreams did he expect to see quantum oscillations, he is known to have a knack for interrogating electrons. And, most importantly, once he got the unexpected result, he knew how to proceed with the puzzle it presented.

“He is a remarkable mix,” says Andrew Millis, a theoretical physicist at Columbia University. “He’s a master of some pretty difficult experimental techniques, and he has good theoretical chops in terms of being able to talk to theorists about his data and tease out what it is telling him.”

Millis also played a key role in this scientific process, being the fourth collaborative leg that kicked in soon after the discovery. Once a discovery is made, experimentalists rely on theorists, such as Millis, to explain the observed behaviour, to figure out what is going on. As a CIFAR Associate, Millis got word of Taillefer’s results before they were made public. And among the many theoretical proposals that rapidly came in from theorists, including those at CIFAR, his holds the day so far. Millis proposed that the quantum oscillations were due to “stripe order” – a magnetic phase, which, it seems, may be at the heart of why cuprates are such good superconductors.

With this theoretical model, Millis notes, “We can account for a reasonable variety of the features that Taillefer and company observed. Does that prove that that’s the only picture? No. There are things that agree very nicely with Taillefer’s subsequent experiments and there are things that don’t. So, exactly what’s going on is still somewhat up in the air.”

Says Taillefer, “There are some details of the story that we don’t fully understand, but this is the right road. That much is clear. Basically, these quantum oscillations were the tip of the iceberg. The iceberg is the fact that underlying superconductivity in these cuprates is an unexpected magnetic phase.” And now Taillefer and his colleagues believe that hightemperature superconductors in general – including a new family of iron-based superconductors discovered by Japanese researchers in 2008 – all rely on a force of magnetic origin that is associated with a phase, such as stripe order.

“Bottom line,” Taillefer continues, “superconductivity is an unusual state where electrons pair. In a metal, electrons move independently; they do their own thing and they carry electricity. But superconductivity is a new state of matter whereby below a certain critical temperature, suddenly, spontaneously, all the electrons pair up. It’s an unnatural thing for them to do, since they [should] strongly repel. But there is a force that nevertheless binds them together. With conventional superconductors – like aluminum and lead, and niobium-based alloys like those used in MRI machines – we know it’s an ionic force involving the vibrating atoms. Whereas with these high-temperature superconductors, what’s emerging is that it’s a different kind of force.”

All in all, five years on from the discovery of his career, Taillefer has a good story to tell with his next renewal report. “I just submitted it last Monday,” he says. “It brought back fond memories of the last time I did it.

“But now we’re already onto the next trail.” Back then they knew nothing about quantum oscillations and stripe order and iron-based superconductors, which are currently all the rage.

“Now it’s a totally different scene,” says Taillefer. “It’s a time of convergence and there seems to be a common story emerging. We are really getting at the heart of the matter, the heart of the force behind high-temperature superconductivity. And then we’ll go after that missing factor of two!”

 

Siobhan Roberts is a Toronto-based science writer whose second book, Wind Wizard: Alan G. Davenport and the Art of Wind Engineering, will be published this fall by Princeton University Press.

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