IPF 2011: In search of a superconducting supertheory

Physicists looking at the history of superconductors have been keeping an eye on what the future might bring. Seamus Davis of Brookhaven National Laboratory, speaking at this year’s Industrial Physics Forum in Dallas, Texas, reviewed the recent history of high-temperature superconductivity development. He and his fellow researchers look for an all-encompassing theory that can explain superconductivity as a whole.

For years after the first superconductors were created, the highest temperature they were able to operate at—designated by T _c—hovered close to absolute zero and climbed at an excruciating rate of less than a degree a year new materials were discovered. Then in the late 1980s, with the discovery of copper oxide superconductors, T _c jumped suddenly, and the world hoped to see a revolution in technology as a result. Visions of levitating trains, superconducting server farms, lossless electricity transfer, and the like peppered the newspapers. Davis compared the anticipated technological revolution with the actual technological revolution that semiconductors brought about.

Alas it was not to be. After that initial cuprate jump, progress in raising T _c leveled off at about 135 K—high enough for laboratory and industrial endeavors, but too cold for consumer products. The discovery of pnictide superconductors in the late 2000s seemed to hold a lot of promise because their superconductivity resembles that of the cuprates but is different enough to suggest an alternative route to higher T _c. Despite an explosion in research, the highest pnictide T _c remains around 50 K.

What happened? Although the ideas for applications of superconductors were there, the materials themselves were not. Researchers in the field are still trying to devise a comprehensive theory of high-temperature superconductivity that could point the way to truly high-T _c materials. The one clear thing is that the answer, whatever it may be, is mighty complicated.

In his talk, Davis laid out several leading theories that physicists are currently investigating: strong correlations, antiferromagnetic spin fluctuations, local pairing, and quantum critical points. All the theories so far are incomplete. They explain aspects of why certain materials superconduct at higher temperatures, but none of them offers an all-encompassing explanation.

At the end of his talk, Davis called on the students in the audience to rise to the challenge before them. Because of their potential impact on the world, the discovery of materials that superconduct at room-temperature is one of the most tantalizing goals in all of physics.

“The challenge that I pose to you, is to just look at the facts and come up with an answer,” Davis said.

Davis showed a timeline of major milestones for superconductivity, from its original discovery in 1911 by Heike Kamerlingh Onnes, 1933 when Walter Meissner and Robert Ochsenfeld discovered their eponymous effect, on through to 1986 when high-temperature cuprates were discovered. On average, each major theoretical advance happened 26 years apart. According to Davis’s plot, 2012 is the year for the next big discovery.

Of course science doesn’t obey timetables and schedules. Who knows, maybe the pnictide superconductors discovered in the late 2000s will supply the next data point in Davis’s plot, or maybe the long-sought-after comprehensive theory is just around the corner. Nevertheless, Davis remains optimistic about the future.

“It’s a complicated problem no doubt,” Davis said. “I certainly believe we can understand what’s going on here, and we will achieve room-temperature superconductivity.”

Mike Lucibella