Mössbauer spectroscopy of iron-based superconductors A. Błachowski 1, K. Ruebenbauer 1, J. Żukrowski 2, J. Przewoźnik 2 11-family cooperation K. Wojciechowski 3, Z.M. Stadnik family cooperation J. Marzec family cooperation K. Rogacki 6, J. Karpinski 7, Z. Bukowski 7 1 Mössbauer Spectroscopy Division, Institute of Physics, Pedagogical University, Cracow, Poland 2 Solid State Physics Department, Faculty of Physics and Applied Computer Science, AGH University of Science and Technology, Cracow, Poland 3 Department of Inorganic Chemistry, Faculty of Material Science and Ceramics, AGH University of Science and Technology, Cracow, Poland 4 Department of Physics, University of Ottawa, Ottawa, Canada 5 Department of Hydrogen Energy, Faculty of Energy and Fuels, AGH University of Science and Technology, Cracow, Poland 6 Institute of Low Temperature and Structure Research, Polish Academy of Sciences, Wrocław, Poland 7 Laboratory for Solid State Physics, ETH Zürich, CH-8093 Zürich, Switzerland

Superconducting Materials

T c max = 56K 38K 25K 15K Fe-based Superconducting Families LaFeAsO BaFe 2 As 2 LiFeAs FeSe

There are two questions concerned with tetragonal/orthorhombic FeSe: 1) is there electron spin density (magnetic moment) on Fe? 2) is there change of electron density on Fe nucleus during transition from P4/nmm to Cmma structure?  -FeSe A tetragonal P4/nmm phase transforms into Cmma orthorhombic phase at about 100 K, and this phase is superconducting with T c ≈ 8 K.

Fe 1.05 Se P4/nmm a = (1) Å c = (1) Å

- point A - spin rotation in hexagonal phase - region B - magnetic anomaly correlated with transition between orthorhombic and tetragonal phases - point C - transition to the superconducting state Magnetic susceptibility measured upon cooling and subsequent warming in field of 5 Oe

Change in isomer shift S ↓ Change in electron density  on Fe nucleus  S = mm/s ↓  ρ = –0.02 electron/a.u. 3 tetragonal orthorhombic and superconducting orthorhombic phase transition

tetragonal orthorhombic and superconducting orthorhombic phase transition Quadrupole splitting Δ does not change - it means that local arrangement of Se atoms around Fe atom does not change during phase transition T (K)S (mm/s)Δ (mm/s)  (mm/s) (3)0.287(1)0.206(1) (3)0.287(1)0.203(1) (3)0.286(1)0.198(1) (3)0.287(1)0.211(1) (4)0.295(1)0.222(1)

Mössbauer spectra obtained in external magnetic field aligned with γ-ray beam Hyperfine magnetic field is equal to applied external magnetic field. Principal component of the electric field gradient (EFG) on Fe nucleus was found as negative.

LiFeP P4/nmm a = 3.698(1) Å c = 6.030(2) Å

Magnetization measured in ZFC mode

Mössbauer spectra of LiFeP T (K)S (mm/s)Δ (mm/s)  (mm/s) RT0.247(1)0.101(1)0.172(1) (1)0.112(2)0.224(1) (1)0.119(3)0.227(2) [FeP 4 ] tetrahedron coordination

122 family of Fe-based superconductors

BaFe 2 As 2 Velocity (mm/s)

Ba 0.7 Rb 0.3 Fe 2 As 2 T c = 37K Velocity (mm/s)

EuFe 2 As 2 Velocity (mm/s)

EuFe 2-x Co x As 2 T c = 10K Velocity (mm/s)

Conclusions FeSe 1. There is no magnetic moment on iron atoms in the P4/nmm and Cmma phases. 2. The electron density on iron nucleus is lowered by 0.02 electron/a.u. 3 at 105K during transition from P4/nmm to Cmma phase. LiFeP 3. There is no magnetic order in the superconducting LiFeP.