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Understanding electron pairing in magnetic semiconductors

Posted: 16 Jul 2013 ?? ?Print Version ?Bookmark and Share

Keywords:magnetic semiconductor? electron pairing? superconductivity?

A team of scientists at the U.S. Department of Energy's Brookhaven National Laboratory, Cornell University and collaborators has released the details of their study focused on understanding how some magnetic materials can be transformed to carry electric current with no energy loss. Using an experimental technique to measure the energy required for electrons to pair up and how that energy varies with direction, they identified the factors needed for magnetically mediated superconductivity as well as those that aren't.

"Our measurements distinguish energy levels as small as one ten-thousandth the energy of a single photon of light-an unprecedented level of precision for electronic matter visualisation," said Samus Davis, senior physicist at Brookhaven the J.G. White Distinguished Professor of Physical Sciences at Cornell, who led the research. "This precision was essential to writing down the mathematical equations of a theory that should help us discover the mechanism of magnetic superconductivity, and make it possible to search for or design materials for zero-loss energy applications."

The material Davis and his collaborators studied was discovered in part by Brookhaven physicist Cedomir Petrovic ten years ago, when he was a graduate student working at the National High Magnetic Field Laboratory. It's a compound of cerium, cobalt and indium that many believe may be the simplest form of an unconventional superconductor, one that doesn't rely on vibrations of its crystal lattice to pair up current-carrying electrons. Unlike conventional superconductors employing that mechanism, which must be chilled to near absolute zero (-273C) to operate, many unconventional superconductors operate at higher temperatures-as high as -130C. Figuring out what makes electrons pair in these so-called high-temperature superconductors could one day lead to room-temperature varieties that would transform our energy landscape.

The main benefit of CeCoIn5, which has a chilly operating temperature (-271C), is that it can act as the "hydrogen atom" of magnetically mediated superconductors, Davis said a test bed for developing theoretical descriptions of magnetic superconductivity the way hydrogen, the simplest atom, helped scientists derive mathematical equations for the quantum mechanical rules by which all atoms operate.

"Scientists have thought this material might be 'the one,' a compound that would give us access to the fundamentals of magnetic superconductivity in a controllable way," Davis said. "But we didn't have the tools to directly study the process of electron pairing. This paper announces the successful invention of the techniques and the first examination of how that material works to form a magnetic superconductor."

The method, called quasiparticle scattering interference, uses a spectroscopic imaging scanning tunnelling microscope designed by Davis to measure the strength of the "glue" holding electron pairs together as a function of the direction in which they are moving. If magnetism is the true source of electron pairing, the scientists should find a specific directional dependence in the strength of the glue, because magnetism is highly directional (think of the north and south poles on a typical bar magnet). Electron pairs moving in one direction should be very strongly bound while in other directions the pairing should be non-existent, Davis explained.


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