Superconducting FeSe studied by Mössbauer spectroscopy and magnetic measurements A. Błachowski 1, K. Ruebenbauer 1, J. Żukrowski 2, J. Przewoźnik 2, K. Wojciechowski 3, Z.M. Stadnik 4 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

Superconducting Materials

~55K ~40K ~20K ~10K Fe-based Superconducting Families LaFeAsOF BaFe 2 As 2 LiFeAs FeSe

Fe-Se phase diagram The following phases form close to the FeSe stoichiometry: 1) tetragonal P4/nmm structure similar to PbO, called β-FeSe (or α-FeSe) 2) hexagonal P6 3 /mmc structure similar to NiAs, called δ-FeSe 3) hexagonal phase Fe 7 Se 8 with two different kinds of order, i.e., 3c (α-Fe 7 Se 8 ) or 4c (β-Fe 7 Se 8 ) A tetragonal P4/nmm phase transforms into Cmma orthorhombic phase at about 90 K, and this phase is superconducting with T c ≈ 8 K.

Aim of this contribution is to answer 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 ? Crystal structure of  -FeSe

Fe 1.05 Se

- 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.

Conclusions 1. There is no magnetic moment on iron atoms in the superconducting FeSe. 2. The electron density on iron nucleus is lowered by 0.02 electron / a.u. 3 during transition from tetragonal to orthorhombic phase.