superconducting fese studied by mössbauer spectroscopy and magnetic measurements
DESCRIPTION
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 - PowerPoint PPT PresentationTRANSCRIPT
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, AGH University of Science and Technology, Cracow, Poland
4 Department of Physics, University of Ottawa, Ottawa, Canada
Superconducting Materials
~55K ~40K ~20K ~10K
Fe-based Superconducting Families
LaFeAsOF BaFe2As2 LiFeAs FeSe 1111 122 111 11
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-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.
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 ?
Crystal structure of -FeSe
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 the 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 the temperature 4.2 K, in the range 75–120 K with step 5 K and in the external magnetic field up to 9 T.
Fe1.05Se
- 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
Magnetic susceptibility measured upon cooling and subsequent warming in field of 5 Oe
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
orthorhombic
orthorhombicandsuperconducting
orthorhombic
phase transition
tetragonal
orthorhombic
orthorhombicandsuperconducting
orthorhombic
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
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)
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.