1. Fe As A = Ca, Sr, Ba Superconductivity in system AFe 2 (As 1-x P x ) 2 Dulguun Tsendsuren Kitaoka Lab. Division of Frontier Materials Sc. Department of Materials Engineering Sc. Graduate School of Engineering Sc., Osaka Univ. Evolution from non-Fermi- to Fermi-liquid transport via isovalent doping in BaFe 2 (As 1−x P x ) 2 superconductors Kasahara et. al., Phys. Rev. 81, 184519(2010)

2. under high pressure 0 50 100 150 200 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) 1900192019401960198020002020 Year Hg La-Ba-Cu-O Discovery of superconductivity 1911 1986 High-T c cuprate superconductor 2006 Iron-based high-T c superconductor 77 163 1979 Heavy fermion superconductor CeCu 2 Si 2 heavy fermion system PuCoGa 5 Introduction History of Superconductivity

3. Introduction Iron-based Superconductors 11111111142226 Today’s talk Each system has FeAs layer 122122 Fe As

4. Introduction AFe 2 As 2 System CaFe 2 As 2 SrFe 2 As 2 BaFe 2 As 2 iso-valent doping Role of FeAs layer in 122 system CaFe 2 (As 1-x P x ) 2 SrFe 2 (As 1-y P y ) 2 BaFe 2 (As 1-z P z ) 2

5. Introduction Superconducting gap StructureSubstance T c [K] 42622CaAlOFeAs27 1111NdFeAsO55 122Ba 1-x K x Fe 2 As 2 38 StructureSubstance T c [K] 42622SrScOFeP17 1111LaFePO5 122BaFe 2 (As 1-x P x ) 2 31 Energy E Fermi Full gap Density of State gap Density of State Energy E Fermi Nodal gap gap 1.Spin-Lattice Relaxation Rate (by NMR) 2.Magnetic Penetration Depth 3.Thermal Conductivity 4.Specific Heat StructureSubstance T c [K] 42622CaAlOFeAs27 1111NdFeAsO55 122Ba 1-x K x Fe 2 As 2 38 StructureSubstance T c [K] 42622SrScOFeP17 1111LaFePO5 122BaFe 2 (As 1-x P x ) 2 31 1.Spin-Lattice Relaxation Rate (by NMR) 2.Magnetic Penetration Depth 3.Thermal Conductivity 4.Specific Heat

6. electronic spin Releases the energy T 1 : spin-lattice relaxation time nuclear spin Spin-Lattice interaction Energy Transfers in almost T 1 time I e Introduction Relaxation rate 1/T 1 by NMR

7. Introduction How to verify SC gap? Spin-Lattice Relaxation Rate (by NMR) Spin-Lattice relaxation time Full gap: Temperature Non-Linear relation Nodal gap: Temperature Linear relation

8. Exp. Result Resistivity of BaFe 2 (As 1-x P x ) 2 Resistivity: 1.T 0 Structure transition 2.T SDW AFM Order 3.T c on Superconductivity appears Resistivity reflects phase transition clearly as other transport properties

9. Transitions: Structure SDW onset T c Bulk T c Exp. Result Phase Diagram of BaFe 2 (As 1-x P x ) 2 Doping level (x) of P in BaFe 2 (As 1-x P x ) 2 At x = 0.26 T c max = 31 [K]

10. Highest T c is clearly related to AFM fluctuation Exp. Result Resistivity of BaFe 2 (As 1-x P x ) 2 Resistivity: Fermi-liquid:T c = 0[K] AFM fluctuation: (Non-Fermi-liquid) T c = 31[K]

11. Calculation Fermi Surfaces vs. Doping BaFe 2 As 2 BaFe 2 P 2 iso-valent doping (P at As) Ba 0.8 K 0.2 Fe 2 A 2 hole doping (K at Ba) Nodal gapFull gap 1.Full gap shows higher T c compared with Nodal gap 2.With 3D like FSs, SC gap becomes Nodal gap T c max = 38[K] 2D like FS T c max = 31[K] 3D like FS

12. T c max = 15 [K], at x = 0.05 1.SC occurs in tetragonal structure 2.In c-Tetra., FS changed into 3D 3.SC disappears in c-Tetra Exp. Result CaFe 2 (As 1-x P x ) 2 Fermi surfaces: Tetragonal (SC) c-Tetra. (NC)

13. Summary 1.Superconductivity occurs: 1.AFM fluctuation appears nearby high T c SC region 2.With structural change (Orthorhombic to Tetragonal) 2.Fermi Surface is structure dependent. In most cases, SC occurs when FSs are like 2D 3.Essence of Full gap is one of promising key to increase T c in Superconductivity

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