Oak Ridge Today

ORNL scientists uncover clues to role of magnetism in iron-based superconductors

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Magnetism of Iron-based Superconductors

Oak Ridge National Laboratory scientists used scanning transmission electron microscopy to measure atomic-scale magnetic behavior in several families of iron-based superconductors. (Photo courtesy ORNL)

 

New measurements of atomic-scale magnetic behavior in iron-based superconductors by researchers at Oak Ridge National Laboratory and Vanderbilt University are challenging conventional wisdom about superconductivity and magnetism.

The study published in Advanced Materials provides experimental evidence that local magnetic fluctuations can influence the performance of iron-based superconductors, which transmit electric current without resistance at relatively high temperatures.

“In the past, everyone thought that magnetism and superconductivity could not coexist,” said ORNL’s Claudia Cantoni, the study’s first author. “The whole idea of superconductors is that they expel magnetic fields. But in reality things are more complicated.”

Superconductivity is strongly suppressed by the presence of long-range magnetism—where atoms align their magnetic moments over large volumes—but the ORNL study suggests that rapid fluctuations of local magnetic moments have a different effect. Not only does localized magnetism exist, but it is also correlated with a high critical temperature, the point at which the material becomes superconducting. [Read more…]

Five ORNL scientists rated among world’s most influential

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Ramamoorthy Ramesh

Ramamoorthy Ramesh

Five Oak Ridge National Laboratory physicists, including Deputy for Science and Technology Ramamoorthy Ramesh, have been named by Thomson Reuters as some of the best and brightest of our time.

The list consists of scientists whose work has been most frequently cited by peers as identified by Thomson Reuters platforms. Citation data was divided into two categories—2002-2012 and 2012-2013—with the latter labeled “hot papers,” ranking in the top 0.1 percent by citations in their field. Seventeen researchers earned this distinction while some 3,200 were included in the second section of the ranking with citations ranking in the top 1 percent for their field and year of publication.

Ramesh, who was actually listed in two categories—physics and materials science—was named to his position at ORNL in June 2013 after serving as the Plato Malozemoff Chair Professor of Materials Science and Engineering and Physics at the University of California, Berkeley, with a joint appointment as a faculty senior scientist at Lawrence Berkeley National Laboratory. He holds a doctorate in materials science from the University of California, Berkeley, and was elected to the National Academy of Engineering in 2011 in recognition of his contributions to the science and technology of functional complex oxide materials. [Read more…]

ORNL paper examines clues for superconductivity in an iron-based material

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FERMI Figure

A change of Hall and Seebeck effects point to large Fermi surface modification at the structural transition, preventing superconductivity at low temperatures. The change in the Fermi surface topology has been confirmed by angle-resolved photoemission spectroscopy. (Image courtesy ORNL)

For the first time, scientists have a clearer understanding of how to control the appearance of a superconducting phase in a material, adding crucial fundamental knowledge and perhaps setting the stage for advances in the field of superconductivity.

The paper, published in Physical Review Letters, focuses on a calcium-iron-arsenide single crystal, which has structural, thermodynamic, and transport properties that can be varied through carefully controlled synthesis, similar to the application of pressure. To make this discovery, researchers focused on how these changes alter the material’s Fermi surface, which maps the specific population and arrangement of electrons in materials.

“The Fermi surface is basically the ‘genetic code’ for causing a certain property, including superconductivity, in a material,” said Athena Safa-Sefat of the U.S. Department of Energy’s Oak Ridge National Laboratory, which led the research team. “We can make different phases of this material in single crystal forms and measure their structure and properties, but now we have Fermi surface signatures that explain why we can’t induce superconductivity in a certain structural phase of this material.” [Read more…]

ORNL study advances quest for better superconducting materials

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Superconductivity Pan Defects

Minghu Pan’s image of “clover-like” atomic defects—an example is circled—that result in strong superconductivity. (Photo courtesy ORNL)

Nearly 30 years after the discovery of high-temperature superconductivity, many questions remain, but an Oak Ridge National Laboratory team is providing insight that could lead to better superconductors.

Their work, published in Physical Review Letters, examines the role of chemical dopants, which are essential to creating high-temperature superconductors—materials that conduct electricity without resistance. The role of dopants in superconductors is particularly mysterious as they introduce non-uniformity and disorder into the crystal structure, which increases resistivity in non-superconducting materials.

By gaining a better understanding of how and why chemical dopants alter the behavior of the original (parent) material, scientists believe they can design superconductors that work at higher temperatures. This would make them more practical for real-world wire applications because it would lessen the extreme cooling required for conventional superconducting material. Existing “high-temperature superconductors” operate at temperatures in the range of negative 135 degrees Celsius and below. [Read more…]