1 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

2 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.

3 Crystal structure of -FeSeAim 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 ?

4 Fe1.05Se

5 Spektroskopia mössbauerowskaEfekt Mössbauera przejście jądrowe h Spektroskopia mössbauerowska

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

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

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

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

10 Zakład Spektroskopii Mössbauerowskiej Instytut Fizyki     Uniwersytet Pedagogiczny     ul. Podchorążych 2, Kraków

11 Fe1.05Se

12 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

13 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 = mm/s ρ = –0.02 electron/a.u.3 orthorhombic orthorhombic and superconducting

14 Quadrupole splitting Δ does not changetetragonal 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

15 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.

16 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.