16 March 2015

Research may answer important superconductivity questions

Are electrons stronger in pairs? MagLab physicists released new research published in Nature Communications that could help answer a looming question about the strength of Cooper pairs in high temperature superconducting materials.

MagLab physicist Oskar Vafek.

MagLab physicists Oskar Vafek

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TALLAHASSEE, Fla. — Scientists at the Florida State University-based National High Magnetic Field Laboratory have made a discovery that will help determine how strongly paired electrons are in a high temperature superconductor; information that could advance magnet technology in things like MRI machines, levitating trains and power grids.

In the journal Nature Communications, Associate Professor Oskar Vafek and postdoctoral research associate Vladimir Cvetkovic lay out a new method for scientists to measure the binding strength of what's called a Cooper pair in superconducting materials.

And that information could be key to scientists understanding these superconductors.

"High temperature superconductivity is still a complex and fascinating puzzle," Vafek said. "Each new finding may offer a piece that helps us complete the picture."

Superconductors are materials that conduct electricity without resistance at very cold temperatures (below about negative 200 degrees Celsius). A better understanding of how they work could lead to advances in any number of areas including medicine and magnet technology.

Scientists have long believed that a key to superconductivity is that electrons pair up to travel easier through the material. These electron couples are called Cooper pairs and the theory based on them earned a Nobel Prize in 1972. But despite more than 40 years of research, there are still many mysteries left to uncover about these electron partners.

One big question is just how strong these Cooper pairs are in copper-oxide based superconducting materials. For example, Yttrium barium copper oxide superconducts at a temperature of 90 Kelvin or about negative 183 degrees Celsius.

Vafek uses mathematical models to study condensed matter theory, specifically superconductivity and quantum phase transitions.

In this newest theoretical work, he and Cvetkovic discovered that when the pairs are broken and subjected to external magnetic field, they may create a phenomenon known as the thermal Hall effect.

This effect occurs when a magnetic field is applied perpendicular to an applied heat current. The resulting temperature gradient acquires a component perpendicular to the heat current and the magnetic field.

The duo calculated whether this effect was dependent on certain variables, including the value of the Cooper pair amplitude and the magnitude of the external magnetic field. They discovered that this thermal Hall effect is largely independent of the structure of the magnetic field flux lattice and is, instead, intrinsic to how strongly the electrons are bound in Cooper pairs. The temperature dependence of the thermal Hall effect may therefore help scientists understand the basic properties of a material, including the binding strength of the Cooper pairs in external magnetic field.

Vafek is excited about the future extensions of this theory, as well as the possible experiments that it may guide and inspire.

"Being able to calculate how strongly the electrons are bound may allow us to better understand how high temperature superconductivity works, and how it is suppressed by external magnetic field," he says.

Who knows? That could even provide a new path toward achieving room temperature superconductivity.

The National High Magnetic Field Laboratory is the world’s largest and highest-powered magnet facility. Located at Florida State University, the University of Florida and Los Alamos National Laboratory, the interdisciplinary National MagLab hosts scientists from around the world to perform basic research in high magnetic fields, advancing our understanding of materials, energy and life. The lab is funded by the National Science Foundation (DMR-1157490) and the state of Florida. For more information, visit us online at nationalmaglab.org or follow us on Facebook, Twitter, Instagram and Pinterest at NationalMagLab.