講演者：堀金　和正 氏（ヴァージニア大学）
日時：１１月　１７日（金）　午後４時３０分から
場所： 青山学院大学　理工学部（相模原キャンパス）Ｌ棟６階　Ｌ６０３室

題目： 「Effect of structural parameter on superconductivity of the FeSe_{1-x}Te_x」


要旨：
　The discovery of superconductivity in the iron oxypnictide family of compounds has generated intense interest. The understanding of electronic, magnetic and structural properties of these compounds is the key to determining the mechanism. Lee et al. has revealed that Tc becomes maximum when FeAs4-lattices form a regular tetrahedron (As-Fe-As angle α= 109.47°) [1]. This result suggests a relationship between crystal structure and superconductivity. However, it is not clarified that this relationship is universal feature in the iron superconductor because there are few reports on the crystal structure analysis of lower α angle materials (α<105°). Moreover, it is difficult to know the effect of structure on superconductivity because ReFeAsO(1111) and MFe2As2(122) system changes not only crystal structure but also carrier concentration by replacing alkali metal or alkaline earth metal. More recently, superconductivity was discovered in α-FeSex system. Se-Fe-Se angle of the FeSe4 lattice is 104. 5° below Tc and Tc increases up to 14K by replacing Se2- with Te2-. Therefore, FeSe1-xTex is a good candidate material to research the relationship between crystal structure and superconductivity. In order to clarify the relationship between crystal structure and superconductivity, we have performed on neutron powder diffraction and high-pressure resistivity measurements in FeSe1-xTex.
The Tc of the FeSe1-xTex system increases with increasing α, suggesting that this relationship is universal feature and crystal structure plays an important role in the iron superconductor [2]. From the structural analysis, Fe(Se,Te)4-tetrahedrons distort greatly from the regular shape implies the improvement of Tc upon applying pressure. Thus, we have performed on the high-pressure resistivity experiments in FeSe0.5Te0.5, which shows the maximum Tc(=14K) under ambient pressure. The onset temperature of Tc increases rapidly from 14K to 26K applying up to 2GPa and the metallic phase was observed at P=14GPa [3]. In this talk, we also show the recent work about structure analysis under high-pressure.

Reference
[1] C. H. Lee et al., J. Phys. Soc. Jpn. 77 (2008) 083704.
[2] K. Horigane et al., J. Phys. Soc. Jpn. 78 (2009) 074718.
[3] K. Horigane et al. J. Phys. Soc. Jpn. 78 (2009) 063705
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