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Mysterious increased transition temperature of iron selenide superconductor under high pressure

Hidden relationship between magnetism and superconductivity unveiled

Researchers at the University of Tokyo have shown that the mysterious fourfold increase observed in the superconducting transition temperature, or the temperature at which superconductivity emerges, of iron selenide (FeSe) when subjected to high pressure of 80,000 atmospheres is caused by the suppression of magnetism.

Among superconductive materials, which are capable of transmitting electricity without the loss of thermal energy, are iron-based superconductors expressing superconductivity at relatively high temperatures. In previous research, several unusual properties of FeSe, compared to other iron-based superconductors, have been reported. In particular, the origin of the fourfold increase in the superconducting transition temperature observed when subjected to high pressure remained one of the mysteries of condensed matter physics.

The research team of graduate student Kohei Matsuura and Professor Takasada Shibauchi, Department of Advanced Materials Science, and Professor Yoshiya Uwatoko, Institute for Solid State Physics at the University of Tokyo, and their collaborators, studied the effect of pressure up to 150,000 atmospheres, and temperature, on the electric and magnetic properties of FeSe crystals.

The team found that FeSe exhibits no magnetism at ambient pressure (1 atmosphere), but the application of 20,000-atmosphere pressure induces sudden magnetic order, which continues to rise until around 40,000 atmospheres; then at 60,000 atmospheres, the magnetism is suppressed while the superconducting transition temperature of FeSe rapidly increases.

This behavior was revealed for the first time by this study. It shows that magnetism inhibits superconductivity and that superconductivity with high transition temperature is realized by suppressing magnetism.

“Iron selenide becomes superconductive below minus 264 degrees Celsius at ambient pressure, but its superconducting transition temperature is enhanced to as high as minus 235 degrees Celsius under high pressure, which is an anomalous property among superconductors. To understand this mysterious behavior has been an important issue,” says Shibauchi. He continues, “With the help of Professor Uwatoko’s expertise on high-pressure measurement technique at the Institute for Solid State Physics, we found that the suppression of the magnetic order competing with superconductivity is an important factor to achieve a high transition temperature. I would expect that this result will help in the understanding of how to develop new high-temperature superconductors.”

The research outcome is the result of collaboration with Professor Yuji Matsuda, Department of Physics at Kyoto University, and researchers at the Chinese Academy of Sciences and Oak Ridge National Laboratory in the U.S.

Press release (Japanese)

Paper

J.P. Sun, K. Matsuura, G.Z. Ye, Y. Mizukami, M. Shimozawa, K. Matsubayashi, M. Yamashita, T. Watashige, S. Kasahara, Y. Matsuda, J.-Q. Yan, B.C. Sales, Y. Uwatoko, J.-G. Cheng, T. Shibauchi, "Dome-shaped magnetic order competing with high-temperature superconductivity at high pressures in FeSe", Nature Communications Online Edition: 2016/07/19 (Japan time), doi: 10.1038/ncomms12146.
Article link (Publication)

Electronic phase diagram of iron-based FeSe superconductor following change in pressure
High-temperature superconductivity (red) is realized in the pressure region where the dome-shaped magnetism (green) is suppressed.
© 2016 Kohei Matsuura.