Transcript Slajd 1 - M&oumlssbauer Spectroscopy Division

Nadprzewodniki na bazie żelaza FeSe i LiFeP
– badania metodą spektroskopii mössbauerowskiej
oraz pomiary magnetyczne
A. Błachowski1, K. Ruebenbauer1, J. Żukrowski2, J. Przewoźnik2,
J. Marzec3, K. Wojciechowski4, Z.M. Stadnik 5, U.D. Wdowik6
1
Zakład Spektroskopii Mössbauerowskiej, Instytut Fizyki,
Uniwersytet Pedagogiczny, Kraków
2 Katedra
Fizyki Ciała Stałego, Wydział Fizyki i Informatyki Stosowanej,
Akademia Górniczo-Hutnicza, Kraków
3 Katedra
4
Energetyki Wodorowej, Wydział Energetyki i Paliw,
Akademia Górniczo-Hutnicza, Kraków
Katedra Chemii Nieorganicznej, Wydział Inżynierii Materiałowej i Ceramiki,
Akademia Górniczo-Hutnicza, Kraków
5 Department
6
of Physics, University of Ottawa, Ottawa, Canada
Zakład Zastosowań Informatyki, Instytut Techniki,
Uniwersytet Pedagogiczny, Kraków
Superconducting Materials
Fe-based Superconducting Families
LaFeAsOF
1111
TC max = 56K
BaFe2As2
122
LiFeAs
111
FeSe
11
38K
25K
15K
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 100 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 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
P4/nmm
a = 3.7720(1) Å
c = 5.5248(1) Å
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.
Total electron spin density versus energy for the Cmma phase at null pressure
Spin-up and spin-down states are plotted separately in red and green colors, respectively.
Fermi level is marked by the vertical line.
This is obviously non-magnetic metallic system.
Phonon dispersion relations at null pressure and for the ground state
PHONON DYNAMICS IN TETRAGONAL/ORTHORHOMBIC PHASE
Total density of the phonon states versus pressure for the orthorhombic phase (DOS)
V. Ksenofontov, G. Wortmann, A.I. Chumakov et al.,
Density of Phonon States in Superconducting FeSe as a Function of Temperature and Pressure,
arXiv:1004.2007
Electron spin density versus energy for the hexagonal phase
A transition from the ferromagnetic insulating state
to the metallic state with very small magnetic moment
at high hydrostatic pressure
Energy gap and magnetic moment in the hexagonal phase
LiFeP
P4/nmm
a = 3.698(1) Å
c = 6.030(2) Å
Magnetization measured in ZFC mode
Magnetic hysteresis obtained at 2 K and 20 K
Magnetization measured in sweep mode
Mössbauer spectra of LiFeP
T (K)
S (mm/s)
Δ (mm/s)
 (mm/s)
RT
0.247(1)
0.101(1)
0.172(1)
77
0.356(1)
0.112(2)
0.224(1)
4.2
0.364(1)
0.119(3)
0.227(2)
[FeP4]
tetrahedron coordination
Conclusions
FeSe
1. There is no magnetic moment on iron atoms in the P4/nmm and Cmma phases.
2. The electron density on iron nucleus is lowered by 0.02 electron/a.u.3 at 105K
during transition from P4/nmm to Cmma phase.
LiFeP
3. There is no magnetic order in the superconducting LiFeP.