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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
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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.
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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 ?
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Fe1.05Se
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Spektroskopia mössbauerowska
Efekt Mössbauera przejście jądrowe h Spektroskopia mössbauerowska
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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
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Oddziaływania nadsubtelne 1) Oddziaływanie elektryczne monopolowe
elektrostatyczne monopolowe oddziaływanie ładunku jądra z ładunkiem powłok elektronowych
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Oddziaływania nadsubtelne 2) Oddziaływanie elektryczne kwadrupolowe
oddziaływanie momentu kwadrupolowego jądra Q z gradientem pola elektrycznego q wytwarzanym przez powłoki elektronowe
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Oddziaływania nadsubtelne 3) Oddziaływanie magnetyczne dipolowe
oddziaływanie dipolowego momentu magnetycznego jądra z efektywnym polem magnetycznym H w obszarze jądra
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Zakład Spektroskopii Mössbauerowskiej Instytut Fizyki
Uniwersytet Pedagogiczny ul. Podchorążych 2, Kraków
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Fe1.05Se
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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
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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
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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
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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.
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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.
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