Graphene shows signs of superconductivity

With extra ingredients, flat form of carbon displays evidence of resistance-free current

different versions of graphene

GRAPHENE ADDITIVE  Pure graphene (a) is a great conductor. But graphene doped with calcium (d) is a superconductor at low temperatures, a new study demonstrates. Trials with potassium (b) and cesium (c) showed no superconductivity.

J. Chapman et al/ 2015

Chalk up another superpower for the thinnest material on the planet.

When sprinkled with certain atoms, graphene — a flat sheet of honeycombed carbon atoms — conducts electrical current with no resistance at low temperatures, four research teams report. While graphene has awed scientists with its conducting prowess for over a decade, this is the first evidence that the wonder material can be a superconductor.

The research groups, which all reported their findings in separate papers posted online at the week of August 24, used different approaches to test for superconductivity and came up with some conflicting results. Nonetheless, “the evidence for superconductivity is completely convincing,” says Allan MacDonald, a theoretical condensed matter physicist at the University of Texas at Austin who was not involved with any of the studies. Follow-up research could help scientists better understand superconductivity and how it works in ultrathin materials, whose properties are strikingly different from those of their bulky counterparts.

There’s no material quite like graphene. It’s strong yet flexible, it’s an impenetrable wall for molecules trying to pass through and it’s a fantastic conductor (SN: 8/13/11, p. 26). But electrons in even the best conductors bump into walls and scatter, creating electrical resistance. Because graphene’s thicker cousin graphite can be coaxed to superconduct, theorists proposed that graphene doped with elements such as lithium could also shuttle current resistance-free.

Andrea Damascelli, a condensed matter physicist at the University of British Columbia in Vancouver, and his team carefully prepared graphene sheets and sprinkled them with lithium atoms at very low temperatures. Then the researchers fired photons into each sheet and analyzed the electrons that came out. Electrons usually repel each other due to their negative charge, but the physicists saw evidence that the electrons were partnering to form Cooper pairs, which glide through a material without scattering. The team concluded in a paper that will appear September 8 in the Proceedings of the National Academy of Sciences that lithium-doped graphene is a superconductor at temperatures as high as about 6 kelvins (–267° Celsius).

Another group, led by condensed matter physicist Rahul Nair of the University of Manchester in England, analyzed a stack of thousands of graphene layers. The researchers showed that graphene that’s doped with calcium atoms and chilled to temperatures of about 6 kelvins expelled magnetic fields, a signature of superconductivity known as the Meissner effect (SN: 8/8/15, p. 12).

Both Nair’s team and a Japanese group succeeded in getting graphene to superconduct with calcium but failed with lithium, which contradicts the results of Damascelli’s group and a South Korean team. Differing experimental approaches may be to blame, MacDonald says. Still, he says, the studies are important for detecting such an exotic phenomenon in an exciting new class of materials. “There are not many examples of truly two-dimensional superconductors,” he says.

Editor’s Note: This story was updated on September 14, 2015, to correct the date that the paper appeared online in the Proceedings of the National Academy of Sciences.

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