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Kitaoka lab. Takayoshi SHIOTA M1 colloquium N. Fujiwara et al., Phys. Rev. Lett. 111, 097002 (2013) K. Suzuki et al., Phys. Rev. Lett. 113, 027002 (2014) S. IIMURA et al., Nature Commun. 3, 943 (2012)

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Introduction Iron-based superconductors Sample LaFeAs(O,H) Results Experiment/Theory Summary

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introduction Superconductivity Superconductivity is caused by forming electron pairs

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introduction Superconductivity Electron-phonon interaction Spin or valence fluctuation Spin or valence fluctuation Spin fluctuation Pairing glue ? ?

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introduction Iron-based superconductors Ln Ae A Ln =La, Ce, Pr, Nd, Gd, Tb, Dy, Er, Y Ae =Ba, Sr, Ca, Eu A =Li, Na Ae 4 M 2 O 6 Fe 2 Pn 2 Ln Fe Pn O Ae Fe 2 Pn 2 A Fe Pn Fe Ch 42622111112211111 T c max =46KT c max =56KT c max =38KT c max =31K T c max =13K Distance between Fe-planes Long Short Long Distance between FePn-planes Fe Pn Ch M Ae O

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introduction FePn tetrahedral structure and T c h Pn Fe Pn Mizuguchi et al. Supercond. Sci. Technol. 23 (2010) 054013. = 109.5 C. H. LEE et al. J. Phys. Soc. Jpn., Vol. 77, No. 8 h Pn ~ 1.38 (Regular tetrahedron)

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introduction Electronic state Phase diagram The correlation between magnetic fluctuation and superconductivity? AF:antiferromagnetic SC:superconductivity

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introduction Electron doping effect k electron E Electron doping Electron doping Worse nesting Phase diagram Band structure Fermi- surface (,0) hole FF

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Sample LaFeAs(O 1-x H x ) x=0.36 T c =36K x=0.36 T c =36K x=0.08 T c =29K x=0.08 T c =29K x 0.04

Experiment m=+1/2 m=-1/2 H0H0 t < 0t = 0t > 0 Spin fluctuationT 1 measurement Electronic spin Dynamic susceptibility Nuclear magnetic relaxation rate (T 1 )

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75 As-NMR of LaFeAs(O,H) AFM fluctuations revive with the approach of the second AFM phase Worse nesting Fe As La (O,H) Experiment

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Superconductivity in LnFeAs(O,H) SC1 SC2 large small Bond angle 36K 47K 56K 55K SC1 : unique to La-1111 SC2 : universally to Ln-1111 Fe As La (O,H) Ln a c La : Double domes Ln(La) : Single dome Experiment

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Theory Origin of SC2 in LaFeAs(O,H) Fe As Fe t1t1 t2t2 dxy t 2 >t 1 in the largely doped regime

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Theory Fe As Fe large small Bond angle Electron doping t1t1 t2t2 dxy Origin of SC2 in LaFeAs(O,H)

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Theory Fe As The next nearest hopping within the dxy orbitals causes the enhancement of the spin fluctuation t 2 >t 1 in the largely doped regime As Fe dxy t1t1 t2t2 Origin of SC2 in LaFeAs(O,H)

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Theory SC1 SC2 large small Bond angle 36K 47K 56K 55K Fe As large small The next nearest hopping may be a key factor for appearance of higher T c dome Bond angle t2t2 Superconductivity in LnFeAs(O,H)

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Summary These NMR and theoretical studies were performed in LaFeAs(O,H). A new antiferromagnetic phase was discovered on further electron doping. The spin fluctuations revive with the approach of the second AFM phase. The next nearest hopping of electrons within dxy orbital may be a key factor for this revival of spin fluctuation and appearance of higher T c dome. As Fe dxy

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