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, (2010)

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 superconductivity 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 History of Superconductivity

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

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

Introduction Superconducting gap StructureSubstance T c [K] 42622CaAlOFeAs NdFeAsO55 122Ba 1-x K x Fe 2 As 2 38 StructureSubstance T c [K] 42622SrScOFeP LaFePO5 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] 42622CaAlOFeAs NdFeAsO55 122Ba 1-x K x Fe 2 As 2 38 StructureSubstance T c [K] 42622SrScOFeP LaFePO5 122BaFe 2 (As 1-x P x ) Spin-Lattice Relaxation Rate (by NMR) 2.Magnetic Penetration Depth 3.Thermal Conductivity 4.Specific Heat

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

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

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

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]

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]

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

T c max = 15 [K], at x = 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)

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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