October 18, 2016

Insight on mechanism of unconventional superconductivity

Researchers at the U.S. Department of Energy’s Ames Laboratory and partner institutions conducted a systematic investigation into the properties of the newest family of unconventional superconducting materials, iron-based compounds. The study may help the scientific community discover new superconducting materials with unique properties.

Researchers combined innovative crystal growth, highly sensitive magnetic measurements, and the controlled introduction of disorder through electron bombardment to create and study an entire range of compositions within a class of iron-based superconductors. They found that the key fundamental properties—transition temperature and magnetic field penetration depth — of these complex superconductors were dependent on composition and the degree of disorder in the material structure.

“This was a systematic approach to more fully understand the behavior of unconventional superconductors,” said Ruslan Prozorov, Ames Laboratory faculty scientist and professor in the Department of Physics and Astronomy at Iowa State University. “We found that some proposed models of unconventional superconductivity in these iron-based compounds were compatible with our results, and this study further limited the possible theoretical mechanisms of superconductivity.”

That information will also serve as a resource for future research into unconventional superconductors.

A crystal sample of one of the iron-based unconventional superconductors studied by Ames Laboratory scientists. Their systematic investigation of this class of superconductors may lead to the creation of new materials with unique superconducting properties.

Science Advances - Energy gap evolution across the superconductivity dome in single crystals of (Ba1−xKx)Fe2As2



“This study fleshed out the knowledge of a class of materials more completely and in a way that will be helpful to the scientific community as they search for high temperature superconductors. Knowing how transition temperature is affected by composition, magnetic field and structural disorder gives science a better idea of where to dig—it contributes to the goal of discovery by design, creating materials that do exactly what we want them to do,” said Prozorov.

Abstract

The mechanism of unconventional superconductivity in iron-based superconductors (IBSs) is one of the most intriguing questions in current materials research. Among non-oxide IBSs, (Ba1−xKx)Fe2As2 has been intensively studied because of its high superconducting transition temperature and fascinating evolution of the superconducting gap structure from being fully isotropic at optimal doping (x ≈ 0.4) to becoming nodal at x greater than 0.8. Although this marked evolution was identified in several independent experiments, there are no details of the gap evolution to date because of the lack of high-quality single crystals covering the entire K-doping range of the superconducting dome. We conducted a systematic study of the London penetration depth, λ(T), across the full phase diagram for different concentrations of point-like defects introduced by 2.5-MeV electron irradiation. Fitting the low-temperature variation with the power law, Δλ ~ Tn, we find that the exponent n is the highest and the Tc suppression rate with disorder is the smallest at optimal doping, and they evolve with doping being away from optimal, which is consistent with increasing gap anisotropy, including an abrupt change around x ≃ 0.8, indicating the onset of nodal behavior. Our analysis using a self-consistent t-matrix approach suggests the ubiquitous and robust nature of s± pairing in IBSs and argues against a previously suggested transition to a d-wave state near x = 1 in this system.

SOURCES- Ames Lab, Science Advances

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