Prezentacja na temat: "Superconducting FeSe studied by Mössbauer spectroscopy"— Zapis prezentacji:
1Superconducting FeSe studied by Mössbauer spectroscopy and magnetic measurementsA. Błachowski 1, K. Ruebenbauer 1, J. Żukrowski 2, J. Przewoźnik 2,K. Wojciechowski 3, Z.M. Stadnik 41 Mössbauer Spectroscopy Division, Institute of Physics,Pedagogical University, Cracow, Poland2 Solid State Physics Department, Faculty of Physics and Applied Computer Science,AGH University of Science and Technology, Cracow, Poland3 Department of Inorganic Chemistry, Faculty of Material Science and Ceramics,4 Department of Physics, University of Ottawa, Ottawa, Canada
2Fe-Se phase diagramThe 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-FeSe3) 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.
3Crystal structure of -FeSe Aim of this contribution is to answer two questions concerned withtetragonal/orthorhombic FeSe:1) is there electron spin density (magnetic moment) on Fe ?2) is there change of electron density on Fe nucleusduring transition from P4/nmm to Cmma structure ?
12Magnetic susceptibility measured upon cooling and subsequent warming in field of 5 Oe - point A - spin rotation in hexagonal phase- region B - magnetic anomalycorrelated with transition between orthorhombic and tetragonal phases- point C - transition to the superconducting state
13Change in electron density on Fe nucleus S = +0.006 mm/s tetragonalphasetransitionorthorhombicChange in isomer shift S↓Change in electron density on Fe nucleusS = mm/sρ = –0.02 electron/a.u.3orthorhombicorthorhombicandsuperconducting
14Quadrupole splitting Δ does not change tetragonalT (K)S (mm/s)Δ (mm/s) (mm/s)1200.5476(3)0.287(1)0.206(1)1050.5529(3)0.203(1)900.5594(3)0.286(1)0.198(1)750.5622(3)0.211(1)4.20.5640(4)0.295(1)0.222(1)phasetransitionQuadrupole splitting Δ does not changeit means that local arrangement of Se atoms around Fe atom does not change during phase transitionorthorhombicorthorhombicorthorhombicandsuperconducting
15Hyperfine magnetic field is equal to applied external magnetic field. Mössbauer spectra obtained in external magnetic field aligned with γ-ray beamHyperfine magnetic field is equal to applied external magnetic field.Principal component of the electric field gradient (EFG) on Fe nucleuswas found as negative.
16Conclusions1. 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.3during transition from tetragonal to orthorhombic phase.