Light pulses illuminate superconductivity in fullerenes

Superconductors have long been confined to niche applications, due to the fact that the highest temperature at which even the best of these materials lose their electrical resistance is -70°C. At the moment, superconductors are mainly used in magnets for nuclear magnetic resonance tomographs, fusion devices and particle accelerators.

All hopes for superconductivity at higher temperatures have been riding on ceramic materials known as cuprates. These materials lose their electrical resistance at relatively high temperatures, up to -120°C. For this reason, physicists refer to these materials as high-temperature superconductors.

Physicists from the Max Planck Institute for the Structure and Dynamics of Matter at the Center for Free-Electron Laser Science (CFEL) in Hamburg, Germany, led by Andrea Cavalleri, have now tried a different approach to making high-temperature superconductors. They shone laser pulses at a material made up of potassium atoms and carbon atoms arranged as fullerenes, also known as buckyballs, and found that this material becomes superconducting at around -170°C, although only for a fraction of a second. They report their findings in a paper in Nature.

In 2013, physicists from Cavalleri's group discovered a similar effect in a cuprate, showing that it could become superconducting without any cooling for a few trillionths of a second when excited with an infrared laser pulse. One year later, the Hamburg-based scientists presented a possible explanation for this effect.

They observed that, following excitation with the pulse of light, the atoms in the crystal lattice change position, clearing the way for electrons to move through the ceramic without losses. According to this explanation, the effect is very dependent on the highly specific crystalline structure of cuprates.

To gain a better understanding, the physicists then turned to fullerenes, which have a simpler chemical structure than cuprates. Fullerenes are hollow molecules consisting of 60 carbon atoms that form the shape of a football: a sphere comprising pentagons and hexagons. In particular, they studied the fulleride K₃C₆₀, in which positively-charged potassium ions bind with negatively-charged fullerenes to form a solid. This so-called alkali fulleride is a metal that becomes superconducting below a critical temperature of around -250°C.

The physicists irradiated this alkali fulleride with infrared light pulses lasting just a few billionths of a microsecond at a range of temperatures between the critical temperature and room temperature. They set the frequency of the light source so that it excited the fullerenes to produce vibrations. This causes the carbon atoms to oscillate in such a way that the pentagons in the football expand and contract. The physicists hoped that this change in atomic positions would generate transient superconductivity at high temperatures, in a similar way to the process in cuprates.

To test this, the scientists irradiated the sample with a second light pulse at the same time as the infrared pulse, albeit at a frequency in the terahertz range. The strength at which this pulse is reflected indicates the conductivity of the material, meaning how easily electrons move through it. Using this technique, the scientists measured extremely high conductivity. "We are pretty confident that we have induced superconductivity at temperatures at least as high as -170°C," says team member Daniele Nicoletti, also at the Max Planck Institute for the Structure and Dynamics of Matter. This is one of the highest observed critical temperatures outside of cuprates.

"We are now planning to carry out other experiments which should enable us to reach a more detailed understanding of the processes at work here," says Nicoletti. What the physicists would like to do next is analyze the crystal structure during excitation with infrared light, which should help to explain the phenomenon. The researchers would then like to irradiate the material with light pulses that last much longer. "Although this is technically very complicated, it could extend the lifetime of superconductivity, making it potentially relevant for applications," he explains.

This story is adapted from material from the Max Planck Institute for the Structure and Dynamics of Matter, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.