Wind parks in the North Sea, plans for enormous solar power plants in North Africa: whilst the technologies for alternative power production are making huge strides, the question of efficient energy transportation still remains a major challenge. Innovative technological approaches are needed especially to avoid large energy losses and high voltages during transportation.
Together with his team and specialist colleagues from Switzerland and South Korea, the experimental physicist Professor Alfred Leitenstorfer, director of the Center for Applied Photonics (CAP) at the University of Konstanz, has succeeded in making an important contribution to the understanding of high-temperature superconductors.
A material analysis using extremely short laser impulses indicated that an interaction between the lattice vibrations of atoms and the elementary spin direction of the electrons could be responsible for superconductivity at high temperatures. The results will be published in the June edition of “Nature Materials”. The article is already available in the journal’s online edition.
Superconductivity is one of the most useful and the most impressive quantum phenomena. One of the key features is that electrical currents can be transported with no loss at all. However, the standard superconductor is associated with one considerable drawback: the materials have to be cooled down immensely before their electrical resistance is reduced to nil – and sometimes this temperature is close to absolute zero. Previously, the so-called transition temperature, at which the superconductors reach this point of no resistance, could only be very slowly increased.
That was before the discovery of cuprate, a high-temperature superconductor based on copper. Cuprate superconductors enable transition temperatures to be reached using nitrogen and even conventional means as a cooling agent instead of the more expensive helium. However, the present temperature that can be achieved is still around minus 100 degrees Celsius – far below room temperature.
The superconductor effect derives from the interplay between electrons which join to form so-called Cooper pairs, although they should in fact repel each other. How exactly this effect occurs in high-temperature superconductors is still largely unexplained. However, it is known that the effect in conventional superconductors is due to an interaction between electrons via quantized lattice vibrations, the so-called phonons. But more is needed to explain high-temperature superconductivity. It is assumed that the strongly bound Cooper pairs in high-temperature superconductors cannot be exclusively explained through the electron-phonon pairing process.
At CAP, in Professor Alfred Leitenstorfer’s department and the Konstanz working group of Dr Jure, a compound belonging to the family of iron pnictides was analysed. This family of basic materials for a new type of superconductor was discovered only a few years ago. Professor Leitenstorfer has just turned down an appointment as director of the Max Planck Institute of Microstructure Physics in Halle in favour of his Chair for Experimental Physics at the University of Konstanz. At the CAP the world’s most precise measurements in the infrared spectral range are being carried out with time resolutions of smaller than the oscillation of light. An extremely brief laser impulse was used to induce vibration in the crystal lattice of the material. This procedure is comparable to setting a pendulum into motion with the blow of a hammer, only on a different scale. During these oscillations with a frequency of five terahertz, that’s five billion oscillation cycles per second, the physicists established a link between the distortion of the crystal lattice and a wavelike order in the electron spins. “It’s a very important and surprising piece of information, that at such a high frequency and within such a brief expanse of time, the spins gain direction when the crystal lattice is distorted in a way that is impossible under conditions of equilibrium,” Alfred Leitenstorfer remarks. Both the orientation of the electron spins - the momentum of the electrons - and the vibration impulses of the atomic lattice belong to the microscopic freedom range of a solid which, when excited, also produces the temperature.
In the USA three large independent power grids have already been connected at very short distances with cooled high-temperature superconductors, so that energy excesses and deficits can be balanced out between them. “The vision is to simply lay pathways of high-temperature superconductor cables in the ground – without complex cooling techniques,” says Alfred Leitenstorfer. But to engage in the targeted development of materials with the necessary high transition temperatures, a microscopic understanding of the effect is of paramount importance. The latest results may well contribute a significant step in this direction.Original publication: K. W. Kim, A. Pashkin, H. Schäfer, M. Beyer, M. Porer, T. Wolf, C. Bernhard, J. Demsar, R. Huber & A. Leitenstorfer: Ultrafast transient generation of spin-density-wave order in the normal state of BaFe2As2 driven by coherent lattice vibrations, Nature Materials (2012).
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