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Researchers Observe Light-Induced Lossless Electricity Transmission in Fullerenes

The general belief is that superconductors can only be used in a few applications as the finest of these materials tend to lose their resistance at -70°C. These days they are predominantly used in magnets for fusion devices, particle accelerators, and nuclear magnetic resonance tomographs.

Intense laser flashes remove the electrical resistance of a crystal layer of the alkali fulleride K3C60, a football-like molecule containing 60 carbon atoms. This is observed at temperatures at least as high as minus 170°C. (© J.M. Harms/MPI for the Structure and Dynamics of Matter)

A team of scientists at the Max Planck Institute for the Structure and Dynamics of Matter at the Center for Free-Electron Laser Science (CFEL) in Hamburg tested a material comprising carbon atoms and potassium atoms arranged in bucky ball structures with the aid of laser pulses. They observed that the material became superconducting for a less than a second at 100°K, which is about -170°C.

The same team discovered such an effect back in 2013 using a different material. The material was a ceramic oxide, which is part of the “cuprates” group.

Through their new experiments, the scientists hope to gain some insight into the occurrence of light-induced superconductivity at high temperatures as fullerenes possess a relatively uncomplicated chemical structure.

These insights may aid in the development of a material capable of conducting electricity at room temperature without any loss, and without optical excitation.

So far, ceramic materials referred to as cuprates were the potential candidate for achieving superconductivity at room temperature. At high temperatures of -100°C, these materials lose their electrical resistance. Therefore, physicists have labeled these materials as high-temperature superconductors.

The key focus of Andrea Cavalleri, Director at the Max Planck Institute for the Structure and Dynamics of Matter, and his colleagues is to help in the progress of materials, which lose their electrical resistance at room temperature.

The outcome from their fullerenes experiment brings them nearer their goal. It also provides a better understanding of light-induced superconductivity, as it is simpler to derive a theoretical explanation for fullerenes than for cuprates.

With a full explanation of this effect, scientists will be able to acquire a comprehensive understanding of the occurrence of high-temperature superconductivity and compile a formula for an artificial superconductor capable of conducting electricity without losing any resistance at room temperature.

Structural change clears the way for the electrons

In 2013, Cavalleri’s team illustrated that a material could conduct electricity at room temperature without losing resistance but under specific conditions. When the scientists used an infrared laser pulse to excite it, the ceramic oxide of the cuprates group became superconductive for a few trillionths of a second, without the need for cooling. A year later, the Max Planck Institute team offered a plausible explanation for this effect.

Subsequent to excitation with the flash of light, they noticed that the atoms in the crystal lattice change position. This change in position remains as does the material’s superconducting aspect.

The change brought about by the light in the structure offers a path for the electrons so that they can move via the ceramic material without any losses. This explanation however is based on the highly precise crystalline structure of cuprates.

Cavalleri’s team began looking at whether light could also break the electrical resistance of highly conventional superconductors.

The Max Planck Institute team comprising Matteo Mitrano and Daniele Nicoletti, were able to make a breakthrough discovery when they switched cuprates with the fulleride K3C60, which is a metal made up of Buckminster fullerenes.

These hollow molecules possess 60 carbon atoms which combine to form the shape of a football i.e., a sphere made up of hexagons and pentagons. The negatively charged fullerenes are able to adhere together to form a solid with the aid of intercalated positively charged potassium ions, which act as a type of cement. This metal is an alkali fulleride and it turns superconductive below a critical temperature of about -250°C.

One of the highest critical temperatures apart from cuprates

Infrared light pulses of only a few hundred billionths of a microsecond in duration were used to irradiate the alkali fulleride. The team repeated the experiment at a variety of temperatures between room temperature and critical temperature.

The frequency of the light source was fixed so that it excited the fullerenes to generate vibrations. This, in turn, causes the carbon atoms to swing in such a manner that the pentagons in the football begin to contract and expand.

The team anticipated that this change in the structure might produce transient superconductivity at high temperatures much like the process in cuprates. For a second time, the scientists irradiated the material with a light pulse at the same time as the infrared pulse but at a frequency in the terahertz range in order to test this.

The team was able to assess the material’s conductivity based on the strength at which this pulse is reflected. This goes on to prove that electrons could move easily through the alkali fulleride. The outcome was a very high conductivity.

We are pretty confident that we have induced superconductivity at temperatures at least as high as minus 170°C

Daniele Nicoletti, Max Planck Institute

Outside of the cuprates, the Max Planck Institute experiment is one of the highest observed critical temperatures.

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.

Daniele Nicoletti, Max Planck Institute

Going forward, the team plans to test the crystal structure during excitation using the infrared light. Just as in the case of the cuprates, this may help to explain the occurrence. Next, they would 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.

Daniele Nicoletti, Max Planck Institute

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