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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 2 Solid Mössbauer Spectroscopy Division, Institute of Physics, Pedagogical University, Cracow, Poland 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, AGH University of Science and Technology, Cracow, Poland 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 A synthesis was carried at 750°C for 6 days in evacuated silica tube. Subsequently the sample was slowly cooled with furnace to room temperature. Resulting ingot was powdered and annealed at 420°C for 2 days in evacuated silica tube and subsequently quenched in ice water. Experimental 1) Powder X-ray diffraction pattern was obtained at room temperature by using Siemens D5000 diffractometer. 2) Magnetic susceptibility was measured by means of the vibrating sample magnetometer (VSM) of the Quantum Design PPMS-9 system. 3) Mössbauer spectra were collected in temperature 4.2 K, in the range 75–120 K with step 5 K and in the external magnetic field up to 9 T. Efekt Mössbauera przejście jądrowe h Spektroskopia mössbauerowska Efekt Mössbauera - spektroskopia Ruch źródła względem absorbenta powoduje dzięki efektowi Dopplera zmianę energii kwantów V E V E c V 10 mm/s 1 mm/s 48 neV V hematyt Fe2O3 Oddziaływania nadsubtelne 1) Oddziaływanie elektryczne monopolowe elektrostatyczne monopolowe oddziaływanie ładunku jądra z ładunkiem powłok elektronowych Ze 2 c r 2 δR (ρ s ρ a ) δE ε 0 E0 Oddziaływania nadsubtelne 2) Oddziaływanie elektryczne kwadrupolowe oddziaływanie momentu kwadrupolowego jądra Q z gradientem pola elektrycznego q wytwarzanym przez powłoki elektronowe 2 3 1 e qQ EQ EQ EQ 2 2 2 Oddziaływania nadsubtelne 3) Oddziaływanie magnetyczne dipolowe oddziaływanie dipolowego momentu magnetycznego jądra z efektywnym polem magnetycznym H w obszarze jądra Em Hm I Em H I Zakład Spektroskopii Mössbauerowskiej Instytut Fizyki Uniwersytet Pedagogiczny ul. Podchorążych 2, 30-084 Kraków www.elektron.up.krakow.pl 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 tetragonal phase transition orthorhombic orthorhombic orthorhombic and superconducting Change in isomer shift S ↓ Change in electron density on Fe nucleus S = +0.006 mm/s ↓ ρ = –0.02 electron/a.u.3 tetragonal phase transition orthorhombic orthorhombic orthorhombic and superconducting T (K) S (mm/s) Δ (mm/s) (mm/s) 120 0.5476(3) 0.287(1) 0.206(1) 105 0.5529(3) 0.287(1) 0.203(1) 90 0.5594(3) 0.286(1) 0.198(1) 75 0.5622(3) 0.287(1) 0.211(1) 4.2 0.5640(4) 0.295(1) 0.222(1) Quadrupole splitting Δ does not change - it means that local arrangement of Se atoms around Fe atom does not change during phase transition 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.