A new type Iron-based superconductor ~K 0.8 Fe 2-y Se 2 ~ Kitaoka lab Keisuke Yamamoto D.A.Torchetti et al, PHYSICAL REVIEW B 83, (2011) W.Bao et al, arXiv: v1 Feb (2011)

Contents Introduction –Iron-based superconductor History Structure Characteristics Experiment ~K 0.8 Fe 2-y Se 2 ~ –Comparison to previous sample –NMR measurement Summary 2

History of superconductivity under high pressure SmO 0.9 F 0.11 FeAs LaO 0.89 F 0.11 FeAs LaOFeP Hg-Ba-Ca-Cu-O （） Tl-Ba-Ca-Cu-O Bi-Sr-Ca-Cu-O Y-Ba-Cu-O MgB 2 NbGe NbN NbC Nb Pb high-T c cuprate metal iron-based system Transition temperature (K) Year Hg La-Ba-Cu-O Discovery of superconducting phenomenon High-T c cuprate superconductor 2006 Iron-based high-T c superconductor Heavy fermion superconductor CeCu 2 Si 2 heavy fermion system PuCoGa 5 Introduction 3

Iron-based superconductor LaFeAsOBaFe 2 As 2 LiFeAs FeSe Fe As Se 1111 system122 system 111 system 11 system T c max = 55K T c max = 38K T c max = 18K T c max = 8K Fe-Pnictide layer Introduction Pnictgen(15 族元素 ) 4

Iron-based superconductor Introduction Band structure Fermi suface Phase diagram 5 hole electron nesting hole electron

Iron-based superconductor FeSe Dy Pr Ce La Er Ba Na Li Nd Regular tetrahedron(109.5°) Anion height α Fe Pnictgen 6 Introduction

K 0.8 Fe 2-y Se 2 Characteristics isostructural to BaFe 2 As 2 relatively high T c of ~33K heavily electron-doped system Introduction Cs 0.8 K 0.8 Cs 0.8 FeSe Dy Pr Ce La Er Ba Na Li Nd Regular tetrahedron(109.5°) TCTC TNTN TSTS 1111~55K~140K~155K 122~38K~135K~140K K 0.8 Fe 2-y Se 2 ~33K~560K~580K 7 J.Guo et al, PHYSICAL REVIEW B 82, R 2010

Fermi surface Experiment Previous superconductor K 0.8 Fe 2-y Se 2 8 Qian et al, arXiv: v1 Dec (2010) Band structure Fermi suface Band structure Fermi suface hole electron nesting hole electron hole electron Absence of the hole band

Phase diagram Experiment Previous superconductor AFM:antiferromagnetic ( 反強磁性 ) SDW:spin density wave ( スピン密度波 ) M.Fang et al, EPL, 94 (2011) T S,T N ≒ 140K~160K T cMAX ≒ 50K T S,T N ≒ 300K ~ 500K T cMAX ≒ 33K Expect higher T c K 0.8 Fe 2-y Se 2

M~3.31μ B Ordered state Experiment Previous superconductor Stripe magnetic order Orthorhombic （斜方晶） unusual antiferromagnetic structure Tetragonal （正方晶） Fe vacancy order M~0.5μ B 10 Fe As W.Bao et al, arXiv: v1 Feb (2011) K 0.8 Fe 2-y Se 2

Electron-dope Experiment D.A.Torchetti et al, PHYSICAL REVIEW B 83, (2011) E DOS E F0 ～ k B T （ large ） High temperature E DOS E F0 ～ k B T （ small ） Low temperature Similar to the electron-doped systems among Fe-based superconductors K 0.8 Fe 2-y Se 2 DOS : density of states 11

Lack the hole bands near the zone center AFSF (antiferromagnetic spin fluctuation) 1/T 1 T is enhanced toward Tc in both FeSe and the optimally doped Ba(Fe 0.92 Co 0.08 ) 2 As 2 enhancement of the AFSF K x Fe 2-y Se 2 Ba122(OVD) Experiment D.A.Torchetti et al, PHYSICAL REVIEW B 83, (2011) TcTc In K 0.8 Fe 2 Se 2, there is no enhancement similarly to overdoped, nonsuperconducting Ba(Fe 0.86 Co 0.14 ) 2 As 2 TcTc (T c ~25K) (T c ~0K) (T c ~33K) (T c ~9K) TcTc 12 FeSe Ba122(OPT) nesting hole electron Electron dope

Summary Introduction about K 0.8 Fe 2-y Se 2 What is the reason of relatively high T c ? –The structure of Fe-pnictide layer ? –Nesting between electron-band and hole- band ? –High T N and T s ? This sample gives us a new point of view about Iron-based superconductors 13

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NMR spectrum NMR Intensity H Introduction Zeeman interaction m=+1/2 m=-1/2  ℏ H 0 Zeeman splitting μ : moment of nuclear spin( 核磁気モーメント ) γ : nuclear gyromagnetic ratio( 核磁気回転比 )

Knight shift NMR Intensity H electron Introduction

Nuclear spin-lattice relaxation time T 1 thermal equilibrium state excited state Electron system Lattice system Nuclear spin-lattice relaxation time( 核スピン格子緩和時間 ) Exchange of energy thermal equilibrium state T1T1

H.Kotegawa et al, arXiv: v4 12 Apr 2011D.A.Torchetti et al, PHYSICAL REVIEW B 83, (2011) 1/T 1

1/T 1 T Experiment H.Kotegawa et al, arXiv: v4 12 Apr 2011 FeSe (Fe 0.9 Co 0.1 )Se electron dope doping suppresses the AFSF and SC K 0.8 Fe 2 Se 2 character of the band near the Fermi level is different form FeSe H.Ikeda et al, JPSJ (2008)

77 K vs 1/T 1 T Experiment H.Kotegawa et al, arXiv: v4 12 Apr 2011 Korringa relation κ << 1 κ >> 1 ferromagneticantiferromagnetic K orb =0.02% A hf χ spin (T =0)=0 The non linear relationship κ is not very far from 1 the increase toward low temperature Spin correlations are not so strong, but AF spin correlations are developed κ : korringa ratio( コリンハ比率 )