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How Lasers Make Buckyballs Look Like Superconductors

Materials: Postassium-doped fullerenes display signatures of superconductivity under infrared light

by Matt Davenport
February 11, 2016 | A version of this story appeared in Volume 94, Issue 7

Credit: Nature
A K3C60 unit cell.
An image of a K3C¬¬60 unit cell.
Credit: Nature
A K3C60 unit cell.

An international research team has shown that materials can look and act like superconductors, even when they aren’t technically superconductors.

Researchers led by Andrea Cavalleri of the University of Oxford and the Max Planck Institute for the Structure & Dynamics of Matter goaded potassium-doped buckyballs, K3C60, into an exotic superconductor-like state using pulsed infrared laser light. This work could lead to switchable superconducting devices and better inform researchers working to design room-temperature superconductors, Cavalleri tells C&EN.

Potassium-doped carbon fullerenes are known superconductors, but only at temperatures colder than their critical temperature, about 20 K. Once they start superconducting, charge carriers flow without resistance and there is a characteristic change in how the material reflects light.

In the superconducting state, electrons and phonons—vibrations in the crystal lattice—become coupled, which is a spontaneous but persistent phenomenon. Put another way, a superconductor cooled below its critical temperature wants to stay a superconductor.

By firing laser pulses at powders of metallic K3C60, the team observed electronic and optical signatures of superconductivity at temperatures five to seven times the material’s conventional critical temperature, Cavalleri says (Nature 2016, DOI: 10.1038/nature16522).

These superconductive signatures, however, are short-lived—on the order of picoseconds. The researchers likely drive the material into an excited state that promotes electron-phonon coupling when they hit it with femtosecond IR laser pulses, Cavalleri explains.

This is interesting work, says physicist David Tománek of Michigan State University, who was among the first to study electron-phonon coupling in superconducting fullerenes. But because of the short lifetime of the excited state, “the physics community will be unlikely to accept the association of this effect with superconductivity,” he tells C&EN.

Cavalleri understands and points out that the team was careful to describe the behavior as superconductor-like. But he adds that his group is working to achieve long-term superconductivity using continuous-wave lasers, rather than pulsed light.

The team observed a similar effect in ceramic cuprate materials a few years ago, but Cavalleri says the new buckyball experiments should help researchers more fully explain the science behind the phenomenon. “In this case, the signatures are clearer,” he says. “There are textbook properties of superconductivity.”



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