What Causes High-temperature Superconductivity?

A phase change at absolute zero temperature may provide key insights into the decades-old mystery of high-temperature superconductivity.

Diagram showing the properties of a material as temperature and chemical composition (phosphorus level in this study) are varied. — Image courtesy of Ames Laboratory

The Science

Researchers have found the first clear evidence of a superconductor’s quantum critical point—the point at absolute zero temperature at which tuning a material property, phosphorus content in this case, results in a phase transition that changes the electronic properties of the superconductor. The results demonstrate that such quantum critical behavior not only coexists with high-temperature superconductivity but is intimately connected with it.

The Impact

Superconductors that operate closer to room temperature than today’s cryo-refrigerated materials would have a variety of significant industrial applications. Finding quantum critical behavior in a superconductor may help solve the mystery of what causes high-temperature superconductivity and accelerate the search for still better materials.

Summary

Researchers may have taken a step toward linking high-temperature superconductivity and quantum critical behavior, perhaps the key to unlocking the mystery of what causes high-temperature superconductivity. In existing high-temperature superconductors, it hasn’t been clear until now if superconductivity prevents a quantum critical point or if quantum critical behavior is present but hidden by the superconductivity. A Japanese-American team, including Ames Laboratory and the Center for Emergent Superconductivity EFRC (research performed at the University of Illinois), studied a barium-iron arsenic superconductor with some of the arsenic replaced by phosphorous, B_aFe2(A_s1–xP_x)₂, to maximize the temperature at which superconductivity occurs (the transition temperature). The team measured the characteristic depth that a magnetic field penetrates below the superconductor’s surface. Known as the London penetration depth, it is a useful parameter when studying quantum critical behavior, because it provides direct information about the properties of the electrons in the superconducting material including at temperatures close to absolute zero. The team found that quantum critical behavior coexists with and may actually be protected by superconductivity. A better understanding of what drives high-temperature superconductivity should accelerate the discovery of new superconductors that operate even closer to room temperature.

Contact

R. Prozorov
Ames Laboratory, Ames, IA 50011 and Department of Physics
Astronomy, Iowa State University, Ames, IA 50011
prozorov@ameslab.gov 515-294-9901

Funding

Department of Energy, Office of Science, Basic Energy Sciences program at Ames Laboratory and at the Center for Emergent Superconductivity, an Energy Frontier Research Center (research performed at the University of Illinois). Work by co-authors from Japan and the United Kingdom supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan, the Japan Society for the Promotion of Science, and the Engineering and Physical Sciences Research Council (UK).

Publications

K. Hashimoto et al., “A Sharp Peak of the Zero-Temperature Penetration Depth at Optimal Composition in BaFe2(As1–xPx)2.” Science 336, 1554 (2012). [DOI: 10.1126/science.1219821]