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, 4 Department of Physics, University of Ottawa, Ottawa, Canada

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 P63/mmc structure similar to NiAs, called 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.

Crystal structure of -FeSe 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 ?

Fe1.05Se

Spektroskopia mössbauerowska Efekt Mössbauera przejście jądrowe h Spektroskopia mössbauerowska

Efekt Mössbauera - spektroskopia Ruch źródła względem absorbenta powoduje dzięki efektowi Dopplera zmianę energii kwantów  V V  10 mm/s 1 mm/s  48 neV hematyt Fe2O3 V

Oddziaływania nadsubtelne 1) Oddziaływanie elektryczne monopolowe elektrostatyczne monopolowe oddziaływanie ładunku jądra z ładunkiem powłok elektronowych

Oddziaływania nadsubtelne 2) Oddziaływanie elektryczne kwadrupolowe oddziaływanie momentu kwadrupolowego jądra Q z gradientem pola elektrycznego q wytwarzanym przez powłoki elektronowe

Oddziaływania nadsubtelne 3) Oddziaływanie magnetyczne dipolowe oddziaływanie dipolowego momentu magnetycznego jądra  z efektywnym polem magnetycznym H w obszarze jądra

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Fe1.05Se

Magnetic susceptibility measured upon cooling and subsequent warming in field of 5 Oe - 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

Change in electron density  on Fe nucleus S = +0.006 mm/s tetragonal phase transition orthorhombic Change in isomer shift S ↓ Change in electron density  on Fe nucleus S = +0.006 mm/s ρ = –0.02 electron/a.u.3 orthorhombic orthorhombic and superconducting

Quadrupole splitting Δ does not change tetragonal T (K) S (mm/s) Δ (mm/s)  (mm/s) 120 0.5476(3) 0.287(1) 0.206(1) 105 0.5529(3) 0.203(1) 90 0.5594(3) 0.286(1) 0.198(1) 75 0.5622(3) 0.211(1) 4.2 0.5640(4) 0.295(1) 0.222(1) 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 orthorhombic orthorhombic orthorhombic and superconducting

Hyperfine magnetic field is equal to applied external magnetic field. 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.