High pressure studies in iron-based compounds and recent study of pressure-induced superconductivity in iron-based spin-ladder material
(Nihon University, Japan)
报告摘要：Iron-based superconductor is well known as a high-Tc superconductor, which has a two-dimensional iron lattice and exhibits characteristic magnetic phases next to the superconducting phase. Large enhancement of Tc under pressure was found in the iron-based superconductor in the early stage. Our group reported some exciting results using high pressure technique. The technique will be presented in this talk. A diamond anvil cell was used mainly to generate high pressure. However, a cubic anvil press has advantage to generate hydrostatic pressure.
Among the study of the iron-based superconductor, we have an interest in iron-based ladder-type compound, which exhibits various kinds of magnetic ordering phase. They are one-dimensional analog of both stripe and block magnetism related to the iron-based superconductors [2, 3]. In these compounds, we discovered the pressure-induced superconductivity in the spin-ladder compound BaFe2S3 which exhibits the stripe magnetism at atmospheric pressure . With applying pressure, the insulator-metal transition was observed at ~11GPa, and just after appearance of metallic phase, superconductivity was observed. The dome-shaped superconducting phase was obtained in the P-T phase diagram, in which the maximum Tc was 17 K . We also studied the spin-ladder compound BaFe2Se3 which exhibits the block magnetism at atmospheric pressure. Although pressure-induced superconductivity was already found for BaFe2Se3 by other group , the interplay between superconductivity and magnetism remains open question. We confirmed the sudden drop in electrical resistivity for BaFe2Se3 under high pressure which is thought to be caused by superconductivity and studied the relation between magnetic ordering temperature TN and superconductivity by changing the content of excess Fe.
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 H. Takahashi et al., Nat.Mater. 14, 1008 (2015).
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报告人简介：Hiroki TAKAHASHI was born in 1958 at Hokkaido, JAPAN and received his B. Sci., Master of Sci. and Ph.D of Physics in 1986 all from Hokkaido University. He became a research associate (1986) at Institute for Solid State Physics (ISSP), University of Tokyo, and then moved to Nihon University (1996). He was promoted to full Professor of Physics, Nihon University (2003).
His main research field is superconductivity and magnetism under high pressure. He started to work for ISSP in 1986, when high-Tc cuprate was discovered. He developed cubic anvil press for low temperature with Prof. N. Mori to measure electrical resistivity, magnetic susceptibility, and so on, which realized excellent hydrostatic condition up to 10 GPa . He performed extensive studies on high-Tc cuprates using cubic anvil press and revealed fundamental properties of cuprate superconductors . During the study of cuprate superconductors, his group discovered pressure-induced metallization and superconductivity in the spin-ladder type cuprate material Sr14-xCaxCu24O41 , which has different Cu-O network from other cuprate superconductors.
In 2006, Prof. Hosono’s group in Tokyo Institute of Technology reported a new type of superconductor LaFePO. (Tc ~ 4 K). Prof. Takahashi was very interested in that material, because it has 2D crystal structure such as cuprate superconductors. He proposed the collaboration to Prof. Hosono, and the Tc was doubled under high pressure using a diamond anvil cell. In 2008, high-pressure study for LaFeAsO1-xFx was carried out as a successive collaboration with Prof. Hosono’ group and Tc was raised to 43 K. It was the highest Tc except for cuprate superconductors, and was published in Nature . He also found the maximum Tc in LaFeAsO1-xHx system is 52 K under high pressure . He discovered pressure-induced spin-ladder type superconductor in iron-based material BaFe2S3 . He successfully discovered the superconductivity in both the cupper-based and iron-based spin-ladder compounds under high pressure.
 N. Mori et al., High Pressure Research 24 (2004) 225.  H.Takahashi and N.Môri, Studies of High Temperature Superconductors Vol. 16, (Nova Science Publishers Inc. 1996).  T. Uehara et al., JPSJ65 (1996) 2764.  H. Takahashi et al., Nature 53 (2008) 376-378.  H. Takahashi et al., Sci. Rep.5(2015) 7829.  H. Takahashi et al., Nat. Mater.14 (2015) 1008.