Iron-based superconductor: K0.8Fe2-ySe2

1. A new type Iron-based superconductor ~K0.8Fe2-ySe2~ Kitaoka lab Keisuke Yamamoto D.A.Torchetti et al , PHYSICAL REVIEW B 83 , 104508 (2011)W.Bao et al , arXiv:1102.0830v1 Feb (2011)

2. Contents • Introduction • Iron-based superconductor • History • Structure • Characteristics • Experiment ~K0.8Fe2-ySe2~ • Comparison to previous sample • NMR measurement • Summary

3. Introduction 1900 1920 1940 1960 1980 2000 2020 Year History of superconductivity Discovery of superconducting phenomenon 200 metal heavy fermion system 1911 high-Tc cuprate 163 Hg-Ba-Ca-Cu-O iron-based system under high pressure （ ） 150 Hg-Ba-Ca-Cu-O Tl-Ba-Ca-Cu-O Heavy fermion superconductor 1986 1979 Bi-Sr-Ca-Cu-O 100 Transition temperature (K) Y-Ba-Cu-O 77 High-Tc cuprate superconductor SmO F FeAs 50 0.9 0.11 MgB2 La-Ba-Cu-O LaO F FeAs PuCoGa5 Nb Ge 0.11 0.89 Nb 2006 Pb CeCu2Si2 NbN LaOFeP Hg NbC 0 Iron-based high-Tc superconductor

4. Introduction Iron-based superconductor 11 system 1111 system 122 system 111 system As Fe-Pnictide layer Fe Se LiFeAs FeSe LaFeAsO BaFe2As2 Tc max = 55K Tc max = 38K Tc max = 8K Tc max = 18K Pnictgen(15族元素)

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

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

7. Introduction K0.8Fe2-ySe2 Regular tetrahedron(109.5°) Nd Dy Pr Er Ba Ce K0.8 K0.8 Cs0.8 Cs0.8 La Li Na FeSe J.Guo et al , PHYSICAL REVIEW B 82, 180520R 2010 • Characteristics • isostructural to BaFe2As2 • relatively high Tc of ~33K • heavily electron-doped system

8. Experiment Fermi surface K0.8Fe2-ySe2 Previous superconductor Band structure Band structure electron hole Qian et al , arXiv:1012.6017v1 Dec (2010) Fermi suface Fermi suface Absence of the hole band electron electron hole hole nesting

9. Experiment Phase diagram K0.8Fe2-ySe2 Previous superconductor M.Fang et al , EPL, 94 (2011) 27009 TS ,TN≒ 140K~160K TcMAX≒ 50K TS,TN≒300K ~ 500K TcMAX≒ 33K Expect higher Tc AFM:antiferromagnetic(反強磁性) SDW:spin density wave (スピン密度波)

10. Experiment Ordered state K0.8Fe2-ySe2 Previous superconductor As Fe W.Bao et al , arXiv:1102.0830v1 Feb (2011) • unusual antiferromagnetic structure • Tetragonal（正方晶） • Fe vacancy order • Stripe magnetic order • Orthorhombic（斜方晶） M~0.5μB M~3.31μB

11. High temperature Low temperature DOS DOS ～kBT（small） ～kBT（large） Experiment E E EF0 EF0 Electron-dope K0.8Fe2-ySe2 D.A.Torchetti et al , PHYSICAL REVIEW B 83 , 104508 (2011) Similar to the electron-doped systems among Fe-based superconductors DOS : density of states

12. Experiment AFSF(antiferromagnetic spin fluctuation) (Tc~33K) (Tc~9K) • 1/T1T is enhanced toward Tc in both FeSe and the optimally doped Ba(Fe0.92Co0.08)2As2 • enhancement of the AFSF Tc Tc • In K0.8Fe2Se2 , there is no enhancement similarly to overdoped , nonsuperconducting Ba(Fe0.86Co0.14)2As2 (Tc~25K) Tc KxFe2-ySe2 Ba122(OVD) FeSe Ba122(OPT) (Tc~0K) electron Electron dope hole nesting Lack the hole bands near the zone center D.A.Torchetti et al , PHYSICAL REVIEW B 83 , 104508 (2011)

13. Summary • Introduction about K0.8Fe2-ySe2 • What is the reason of relatively high Tc ? • The structure of Fe-pnictide layer ? • Nesting between electron-band and hole-band ? • High TN and Ts? This sample gives us a new point of view about Iron-based superconductors