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Scientists explore why some materials are superconductive at high temperatures

Generally speaking, some energy is lost to resistance the second we attempt to use it for anything.  Superconductivity – the complete disappearance of electrical resistance in certain materials, usually at drastically low temperatures – is an exception to this rule. First discovered in mercury more than 100 years ago, superconductivity has since been revealed in thousands of alloys and chemical compounds as well as at least 25 other chemical elements.

Animation shows how a pump-probe experiment excites and images electrons. Image courtesy Stanford Institute for Materials and Energy Sciences.

Scientists and engineers have put superconductors to use in everything from the magnetic levitation that makes some high-speed trains seem to float to the high-energy particle research conducted with the Large Hadron Collider at CERN near Geneva, Switzerland. Even the now-common medical diagnostic tool known as the MRI (magnetic resonance imaging) is possible thanks to superconductivity. Greater understanding of high-temperature superconductivity could mean a whole new world of superconductor applications.

Typical superconductors only reveal their superconductive powers at extremely low temperatures, but some materials are only superconductive at very high temperatures. While scientists made this surprising discovery in the late 1980s, its full potential is still largely unknown and untapped. Materials with high-temperature superconductivity are unlikely characters, having proven themselves as excellent insulators long before they became an electron's easy ride.

So why do materials that are not ordinarily considered electron superhighways take on superconductive properties at high temperatures, but not at the low temperatures most superconductors require? A team of scientists led by researchers at SLAC National Accelerator Laboratory in Menlo Park, California, US, and Stanford researchers set out to answer this question and many others like it. The secret is in how electrons and phonons – atomic vibrations that dissipate excess energy – interact.

There’s no way to capture this extremely tiny, superfast interaction, but it's possible to create a close simulation using the supercomputer at the Department of Energy National Energy Research Scientific Computing Center at Lawrence Berkeley National Laboratory in California, US.  Researchers simulated what it would be like to shoot rapid-fire lasers at complex materials known to be high-temperature superconductors. By creating stop-action movies of the experiment – known as a "pump-probe experiment" – they were able to watch as the excited electrons interacted with and transferred their energy to the phonons.  The simulations used a total of 1 million central-processing-unit hours.

Details of this interaction reveal important qualities that underlie the material's properties. Researchers found that the higher-energy electrons lose their energy faster than the lower-energy ones. This also provides a more precise value for the material's equilibrium self-energy (ESE), a factor that helps scientists better understand electron-phonon interactions like those thought to enable high-temperature superconductivity. Previous techniques for determining ESE were fraught with errors, and produced widely varying results for the same material.

"We are creating a fundamentally new way of understanding and engineering complex materials based on how their electrons react when excited far from equilibrium," says the team's leader, Tom Devereaux, director of the Stanford Institute for Materials and Energy Sciences (SIMES), which is run jointly with SLAC. "Understanding – and ultimately controlling – the details of such electron behavior will help us design new materials relevant to fundamental energy science and applications."

This research is part of a much larger pursuit to understand how the world works in a more realistic context. Most science is based on theories and experiments involving balanced or nearly balanced systems, but our world is far more complex than that. Almost everything is constantly in motion, responding to stimuli and energy. Weather systems, photosynthesis, and even stock fluctuations are reactive events in constant flux. Simulations like these pump-probe experiments allow scientists to work with scenarios that are more likely to occur in our imperfect world.

"Current equilibrium-based theories apply to only a very small subset of the phenomena we observe around us," Devereaux says. "Even our vocabulary to describe non-equilibrium phenomena is desperately lacking."

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