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

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 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

EcV

E

V 10 mm/s1 mm/s 48 neV

V

V

Efekt Mössbauera - spektroskopia

hematyt Fe2O3

Ruch źródła względem absorbenta powoduje dzięki efektowi Dopplera

zmianę energii kwantów

Oddziaływania nadsubtelne1) Oddziaływanie elektryczne monopolowe

elektrostatyczne monopolowe oddziaływanie ładunku jądra z ładunkiem powłok elektronowych

)ρ (ρ δ ε

δ00

22

asRE

rcZeE

Oddziaływania nadsubtelne2) Oddziaływanie elektryczne kwadrupolowe

oddziaływanie momentu kwadrupolowego jądra Qz gradientem pola elektrycznego q wytwarzanym przez powłoki elektronowe

22

1

2

3 2qQeEEE QQQ

Oddziaływania nadsubtelne3) Oddziaływanie magnetyczne dipolowe

oddziaływanie dipolowego momentu magnetycznego jądra z efektywnym polem magnetycznym H w obszarze jądra

I

HmEm

I

HEm

    Zakład Spektroskopii Mössbauerowskiej    Instytut Fizyki

    Uniwersytet Pedagogiczny    ul. Podchorążych 2, 30-084 Kraków

www.elektron.up.krakow.pl

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.