Superconducting FeSe studied by Mössbauer spectroscopy and magnetic measurements

1. Superconducting FeSe studied by Mössbauer spectroscopy and magnetic measurements A. Błachowski1, K. Ruebenbauer1, J. Żukrowski2, J. Przewoźnik2, K. Wojciechowski3, Z.M. Stadnik4 1Mössbauer Spectroscopy Division, Institute of Physics, Pedagogical University, Cracow, Poland 2Solid State Physics Department, Faculty of Physics and Applied Computer Science, AGH University of Science and Technology, Cracow, Poland 3Department of Inorganic Chemistry, Faculty of Material Science and Ceramics, AGH University of Science and Technology, Cracow,Poland 4Department of Physics, University of Ottawa, Ottawa, Canada

2. Fe-Se phase diagram The following phases form close to the FeSe stoichiometry: 1) tetragonal P4/nmm structure similar to PbO, called α-FeSe (or β-FeSe) 2) hexagonal P63/mmc structure similar to NiAs, called β-FeSe (or δ-FeSe) 3) hexagonal phase Fe7Se8 with two different kinds of order, i.e., 3c (α-Fe7Se8) or 4c (β-Fe7Se8) A tetragonal P4/nmm phase transforms into Cmma orthorhombic phase at about 90 K, and this phase is superconducting with Tc≈ 8 K.

3. Crystal structure of α-FeSe Superconductivity has been discovered for compound with excess iron FeSe0.88 [F.C. Hsu et al., Proc. Natl. Acad. Sci. U.S.A. 105, 14262 (2008)]. Aim of this contribution is to answer two questions concerned with tetragonal/orthorhombic FeSe: 1) what kind of defects makes excess iron: interstitial iron Fe1+xSe or selenium vacancies FeSe1-x? 2) is there electron spin density on iron ?

4. Fe1.05Se (FeSe0.95)

5. S = 0.46 mm/s Δ = 0.26 mm/s S = 0.55 mm/s Δ = 0.28 mm/s

6. Magnetic susceptibility measured upon cooling and subsequent warming in field of 5 Oe - point A corresponds to the spin rotation in hexagonal phase - region B shows transformation between tetragonal and orthorhombic structures - point C shows transition to the superconducting state

7. tetragonal A- relative contributions obtained from RT spectrum and kept constant in remaining fits S- total spectral shift versus RTα-Fe Δ - quadrupole splitting - absorber linewidth common for all lines orthorhombic orthorhombic and superconducting Change in isomer shift (electron density on Fe nucleus) for minor doublet (corrected for SOD) S80-SRT = +0.25 mm/s ↓ ρ80 - ρRT = –0.86 electron a.u.-3

8. Mössbauer spectra obtained in external magnetic field aligned withγ-ray beam Hyperfine magnetic fields on both Fe sites are equal to applied external magnetic field. Principal component of the electric field gradient (EFG) on Fe nucleus was found as negative on regular Fe sites, and seems to be positive for intercalated Fe atoms.

9. Conclusions 1. Dominant defects for excess iron are formed as intercalated iron on the octahedral sites between iron selenium layers. The electron density between iron selenium layers lowers upon transition to the orthorhombic phase making system more 2-dimensional. 2. There is no net electron spin density in the unit cell. Hence, there is no magnetic moment on Fe nuclei in the superconducting FeSe.