Tag Archives: D. Jaksch

Soccer balls with no resistance (superconductivity)

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Known as a fullerene (also buckminsterfullerene, buckyballs, and/or C60), the soccer ball in question is helping scientists to better understand how to develop materials that are superconductive at room temperature. A Feb. 9, 2016 news item on Nanotechnology Now describes the latest in ‘soccer ball’ research,

Superconductors have long been confined to niche applications, due to the fact that the highest temperature at which even the best of these materials becomes resistance-free is minus 70 degrees Celsius. Nowadays they are mainly used in magnets for nuclear magnetic resonance tomographs, fusion devices and particle accelerators. Physicists from the Max Planck Institute for the Structure and Dynamics of Matter at the Center for Free-Electron Laser Science (CFEL) in Hamburg shone laser pulses at a material made up from potassium atoms and carbon atoms arranged in bucky ball structures. For a small fraction of a second, they found it to become superconducting at more than 100 degrees Kelvin – around minus 170 degrees Celsius. A similar effect was already discovered in 2013 by scientists of the same group in a different material, a ceramic oxide belonging to the family of so-called “cuprates”. As fullerenes have a relatively simple chemical structure, the researchers hope to be able to gain a better understanding of the phenomenon of light-induced superconductivity at high temperatures through their new experiments. Such insights could help in the development of a material which conducts electricity at room temperature without losses, and without optical excitation.

A Feb. 8, 2016 Max Planck Institute press release (also on EurekAlert but dated Feb. 9, 2016), which originated the news item, expands on the theme of superconductivity at room temperature,

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Andrea Cavalleri, Director at the Max Planck Institute for the Structure and Dynamics of Matter, and his colleagues aim at paving the way for the development of materials that lose their electrical resistance at room temperature. Their observation that fullerenes, when excited with laser pulses, can become superconductive at minus 170 degrees Celsius, takes them a step closer to achieving this goal. This discovery could contribute to establishing a more comprehensive understanding of light-induced superconductivity, because it is easier to formulate a theoretical explanation for fullerenes than for cuprates. A complete explanation of this effect could, in turn, help the scientists to gain a better understanding of the phenomenon of high-temperature superconductivity and provide a recipe for an artificial superconductor that conducts electricity without resistance losses at room temperature.

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In 2013, researchers from Cavalleri’s group demostrated that under certain conditions it may be possible for a material to conduct electricity at room temperature without resistance loss. A ceramic oxide belonging to the family of cuprates was shown to become superconductive without any cooling for a few trillionths of a second when the scientists excited it using 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 flash of light, the atoms in the crystal lattice change position. This shift in position persists as does the superconducting state of the material. Broadly speaking, the light-induced change in the structure clears the way for the electrons so that they can move through the ceramic without losses. However, the explanation is very dependent on the highly specific crystalline structure of cuprates. As the process was understood at the time, it could have involved a phenomenon that only arises in this kind of materials.

The researchers have included in the press release an image illustrating the latest work being described in the press release excerpt which follows this,

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 degrees Celsius. © J.M. Harms/MPI for the Structure and Dynamics of Matter

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 degrees Celsius.
© J.M. Harms/MPI for the Structure and Dynamics of Matter

The press release goes on to provide some technical details about the most recent research,

The team headed by Cavalleri therefore asked themselves whether light could also break the electrical resistance of more traditional superconductors, the physics of which is better understood. The researchers from the Max Planck Institute for the Structure and Dynamics of Matter, among which Daniele Nicoletti and Matteo Mitrano, have now hit the jackpot using a substance that is very different to cuprates: the fulleride K3C60, a metal composed of so-called Buckminster fullerenes. These hollow molecules consist of 60 carbon atoms which bond in the shape of a football: a sphere comprising pentagons and hexagons. With the help of intercalated positively charged potassium ions, which work like a kind of cement, the negatively charged fullerenes stick to each other to form a solid. This so-called alkali fulleride is a metal which becomes superconductive below a critical temperature of around minus 250 degrees Celsius.

The researchers then irradiated the alkali fulleride with infrared light pulses of just a few billionths of a microsecond and repeated their experiment for 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. It was hoped that this change in the structure could 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 to the researchers, meaning how easily electrons move through the alkali fulleride. The result here was an extremely high conductivity. “We are pretty confident that we have induced superconductivity at temperatures at least as high as minus 170 degrees Celsius,” says Daniele Nicoletti. This means that the experiment in Hamburg presents one of the highest ever-observed critical temperatures outside of the material class 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 they would like to do next is analyze the crystal structure during excitation with the infrared light. As was previously the case with the cuprate, this 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,” concludes Nicoletti.

Here’s a link to and a citation for the paper,

Possible light-induced superconductivity in K3C60 at high temperature by M. Mitrano, A. Cantaluppi, D. Nicoletti, S. Kaiser, A. Perucchi, S. Lupi, P. Di Pietro, D. Pontiroli, M. Riccò, S. R. Clark, D. Jaksch, & A. Cavalleri. Nature (2016) doi:10.1038/nature16522 Published online 08 February 2016

This paper is behind a paywall.

Entangling diamonds

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Usually when you hear about entanglement, they’re talking about quantum particles or kittens. On Dec. 2, 2011, Science magazine published a paper by scientists who had entangled diamonds (that can be touched and held in human hands). From the Dec. 1, 2011 CBC (Canadian Broadcasting Corporation) news article by Emily Chung,

Quantum physics is known for bizarre phenomena that are very different from the behaviour we are familiar with through our interaction with objects on the human scale, which follow the laws of classical physics. For example, quantum “entanglement” connects two objects so that no matter how far away they are from one another, each object is affected by what happens to the other.

Now, scientists from the U.K., Canada and Singapore have managed to demonstrate entanglement in ordinary diamonds under conditions found in any ordinary room or laboratory.

Philip Ball in his Dec. 1, 2011 article for Nature magazine describes precisely what entanglement means when applied to the diamond crystals that were entangled,

A pair of diamond crystals has been linked by quantum entanglement. This means that a vibration in the crystals could not be meaningfully assigned to one or other of them: both crystals were simultaneously vibrating and not vibrating.

Quantum entanglement — interdependence of quantum states between particles not in physical contact — has been well established between quantum particles such as atoms at ultra-cold temperatures. But like most quantum effects, it doesn’t tend to survive either at room temperature or in objects large enough to see with the naked eye.

Entanglement, until now, has been demonstrated at very small scales due to an issue with coherence and under extreme conditions. Entangled objects are coherent with each other but other objects such as atoms can cause the entangled objects to lose their coherence and their entangled state. In order to entangle the diamonds, the scientists had to find a way of dealing with the loss of coherence as the objects are scaled up and they were able to achieve this at room temperature. From the Emily Chung article,

Walmsley [Ian Walmsley, professor of experimental physics at the University of Oxford] said it’s easier to maintain coherence in smaller objects because they can be isolated practically from disturbances. Things are trickier in larger systems that contain lots of interacting, moving parts.

Two things helped the researchers get around this in their experiment, Sussman [Ben Sussman, a quantum physicist at the National Research Council of Canada and adjunct professor at the University of Ottawa] said:

  • The hardness of the diamonds meant it was more resistant to disturbances that could destroy the coherence.
  • The extreme speed of the experiment — the researchers used laser pulses just 60 femtoseconds long, about 6/100,000ths of a nanosecond (a nanosecond is a billionth of a second) — meant there was no time for disturbances to destroy the quantum effects.

Laser pulses were used to put the two diamonds into a state where they were entangled with one another through a shared vibration known as a phonon. By measuring particles of light called photons subsequently scattered from the diamonds, the researchers confirmed that the states of the two diamonds were linked with each other — evidence that they were entangled.

If you are interested in the team’s research and can get past Science magazine’s paywall, here’s the citation,

“Entangling Macroscopic Diamonds at Room Temperature,” by K.C. Lee; M.R. Sprague; J. Nunn; N.K. Langford; X.-M. Jin; T. Champion; P. Michelberger; K.F. Reim; D. England; D. Jaksch; I.A. Walmsley at University of Oxford in Oxford, UK; B.J. Sussman at National Research Council of Canada in Ottawa, ON, Canada; X.-M. Jin; D. Jaksch at National University of Singapore in Singapore. Science 2 December 2011: Vol. 334 no. 6060 pp. 1253-1256 DOI: 10.1126/science.1211914

All of the media reports I’ve seen to date focus on the UK and Canadian researchers and I cannot find anything about the contribution of the researcher based in Singapore.

I do wish I could read more languages as I’d be more likely to find information about work which is not necessarily going to be covered in English language media.