Kitaoka Lab. Mariko Nitta

1. (Review Article) Kitaoka Lab. Mariko Nitta Nature Physics Volume: 6, Pages: 645–658 Year published: (2010) 10.1038/NPHYS1759

2. Contents • Introduction - Discovery of Fe-pnictide superconductivity - Four structures of Fe-pnictide superconductivity - Common and different features between Fe-pnictide and cuprate - Phase diagram of Fe-pnictide superconductivity - Superconducting energy gap and its symmetry • Experiments & Results - Jump in electronic specific heat at SC transition - Thermal conductivity - Relaxation rate 1/T1 • Summary

3. Introduction Discovery of Fe-pnictide superconductivity in 2008 1111 system Fe:ferro magnetic element LaFeAsO1-y Fe As O La Superconductivity magnetism Tcmax～28 K

4. Introduction Four structures of Fe-pnictide superconductivity all systems have FeAs layer distance between FeAs layers Block layer 111 system 122 system 11 system 1111 system BaFe2As2 FeSe LiFeAs LaFeAsO1-y

5. Introduction Common and different features between Fe-pnictide and cuprate CuO2 layer (1)2D layered structure FeAslayer (2)electron/hole doping causes superconductivity εF εF parent compound (3)non-doped state shows magnetism Fe-pnictide ; metal Cuprate ; Mott insulator E E La2-xSrxCuO4 (LSCO) k k electron doping 122 system BaFe2As2 single band multi band

6. Introduction Fe-pnictide superconductivity Ba1-xKxFe2As2 BaFe2-xCoxAs2 BaFe2As2-xPx BaFe2As2 Tetragonal Tetragonal Tc(K) Tc(K) T0 AFM AFM Orthor Orthor SC SC apply pressure doping level Lattice shrinking & electron/hole doping Tetragonal Orthorhombic Superconductivity appears

7. Introduction superconducting energy gap and its symmetry εF ele hole E multi band s+-wave k nodal s+-wave d wave Sign changing between hole band and electron band バンド間で符号反転が起こる。

8. Experiment (1) Jump in electronic specific heat at SC transition Energy gap : large →Jump in specific heat at SC transition temperature Tc

9. Results (1) Jump in electronic specific heat at SC transition K-doped Ba122 gap size : large Tc increases with gap size ⊿C/Tc(mJ mol-1K-2) Co-doped Ba122 gap size : small Tc(K) BCS type SC

10. Experiment (2) Thermal conductivity conduction electron HEAT COOL HOT Carrier : electron and phonon Conduction electron is responsible for the thermal conductivity electron state at Fermi level Thermal conductivity measurement at 0K

11. Experiment (2) nodal gap NS(E) N0 EF EF +Δ0 Thermal conductivity ～Doppler shift～ Full gap NS(E) full gap N0 EF EF +Δ0 nodal gap

12. Results (2) Thermal conductivity KFe2As2 Ni-doped Co-doped P-doped 10% Co-doped Thermal conductivity (normalized) Thermal conductivity (normalized) residual DOS atεF particular for nodal gap K-doped 4.8% Co-doped Magnetic field (H/Hc2) Magnetic field (H/Hc2) Full gap nodal gap

13. Experiment (3) Release the energy • Relaxation rate 1/T1 by NMR What isT1?? T1 ~ spin-lattice relaxation time I e nuclear spin electronic spin spin-lattice interaction Energy- transfer Time constant T1 1/T1 measurement is a good probe for Fermi surface !

14. Results (3) • compare 1/T1in1111,122 K-doped,122 P-doped “Ba122 K-doped” “Ba122P-doped” “La1111” Ba Ba K Fe Fe Fe As As As P O La

15. Results (3) compare 1/T1in1111,122 K-doped,122 P-doped “La1111” “Ba122 K-doped” “Ba122P-doped” 57Fe- NMR 75As-NQR ~T5 ~T3 nodal two gap two-gapped type s wave (unconventional SC)

16. Possible Scenario Ba1-xKxFe2As2 LaFeAsO BaFe2As2-xPx (LaFePO) As P Full gap nodal two gap Fe Fe As and P-height relate to gap symmetry???

17. Summary ？ mechanism of Fe-pnictide superconductors εF gap symmetry … full gap s+- type or nodal gap s+-type E La1111 Tcmax~28K Ba122 P-doped Tcmax~30K k Full gap nodal two gap electronic structure is described by multi-band picture