THE SAIF's WORLD

Resistance makes waves

Dec 23, 2013

A look inside a high-temperature superconductor: The Max Planck scientists in Stuttgart used resonant X-ray scattering to show the existence of charge-density waves inside the cuprate superconductors. The blue wavy lines indicate the incident and emitted radiation. The system of coordinates illustrates the orientation of radiation relative to the crystal axes a, b and c. Credit: MPI for Solid State Research

Even physics can give pointers for energy saving. An international team working together with the Centre for Quantum Materials, run by the Max Planck Society in conjunction with the University of British Columbia (Canada), is now in a position to provide materials scientists with tips for the development of high-temperature superconductors, in a bid to make them earn their name. The term is currently used to describe materials including ceramic cuprates, which lose their electrical resistance at significantly higher temperatures than conventional superconductors, but still well below the freezing point of water. In two complementary studies, the physicists have now established that superconductivity in cuprates collapses at a maximum of minus 135 degrees Celsius due to the formation of charge-density waves. These periodic fluctuations in the distribution of the electrical charges destroy superconductivity. Consequently, in order to find superconductors that drop to zero resistance at realistic temperatures, materials scientists must search for substances that are not subject to charge-density waves.

Nearly two per cent of the electrical energy generated by power stations is lost in the grid. In Germany alone, this is equivalent to the power delivered by a medium-sized coal power plant. These losses may increase in the future, as power from large offshore wind farms is transported to the landlocked south. Superconductors could provide a remedy if they were able to deliver power to consumers without loss or leakage, even in summery temperatures. In order to systematically search for such materials, however, physicists must first obtain an accurate picture of why today's best superconductors lose resistance in the first place, and how the temperature at which this happens can be raised - a puzzle researchers have been working on for some 30 years. Little by little, a picture is starting to emerge. Two studies by an international team involving the Max Planck Institute for Solid State Research, as well as the universities of Princeton and British Columbia and the Helmholtz-Zentrum Berlin, have now contributed a few more pieces to the puzzle.

"We have found charge-density waves in cuprates above the temperatures at which they become superconductive", says Bernhard Keimer. "Like superconductivity, these are caused by strong interactions between the electrons." The Director at the Max Planck Institute for Solid State Research in Stuttgart was involved directly in one of the two studies, and in an advisory capacity in the other.

Contest for states decided by a hair's breadth

Physicists have known for years that superconductivity can only arise in the first place if there is strong interaction between electrons. The fact is, forces – which current research assumes to be magnetic forces – bind electrons together to form Cooper pairs and these whizz through the crystal lattice unchecked. Researchers have also known for years that the strong interaction can induce other electronic phenomena such as magnetism or even charge-density waves, which are completely incompatible with superconductivity.

"These different states compete with each other in the materials", explains Keimer. "And which one wins is frequently decided by a mere hair's breadth." This means that whether a material is superconductive or not depends to a very high degree on its elementary composition and its structure, while chance also gets in on the act. Still, the current studies are giving the scientists more of a feeling for when and in what circumstances superconductivity occurs. "We are getting closer to the goal of predicting this state and developing materials that will be superconductors even at high temperatures", says the physicist.

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