effect of the electron-phonon coupling on phonons in iron based superconductors · 2019. 4. 15. ·...
TRANSCRIPT
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Ilya Sergeev, PETRA III, Hamburg, Germany
Effect of the electron-phonon coupling on phonons
in iron based superconductors
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Outline
• Fe superconductors: overview
• Study of lattice dynamics in LnFeAsO
• Study of lattice dynamics in EuFe2As2
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Fe-superconductors. Overview
Crystallographic structures of Fe-superconductors
J. Paglione and R.L.Greene,
Nature Physics, 6(2010)645
• superconductivity originates within Fe layer
• suppression of magnetism by doping or by pressure leads to SC
• unconventional superconductors: magnetic(?) excitations are the “glue” of the Cooper pair
F. Wang and D.-H.Lee,
Science, 332(2011)200
/ pressure
Phase diagrams of Fe-superconductors
-
Measurements of phonons in FeSCs
nuclear inelastic scattering
Higashitaniguchi et al., PRB 78(2008)174507
inelastic neutron scattering
Christianson et al., PRL 101 (2008) 157004
inelastic X-ray scattering
Le Tacon et al., PRB 78 (2008) 140505(R)
Eg, R
Eg, FeAs
Eg, Fe
A1g, As
B1g, As
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Theoretical phonon calculations
Electron-phonon properties of LaFeAsO
Boeri et al., PRL, 101(2008)026403
The theory predicts Tc=0.8K due to
the phonon mediated Cooper pairing.
Much smaller than exp. Tc = 25K
Dependence of phonons on magnetism
T. Yildirim, Physica
C 469(2009) 425
122-family
Zbiri et al.,J.Phys.
Cond.Matt
22(2010)315701
1111-family
Wang et al.,Physica
C 472(2012)29
11-
family Theory predicts significant effect of the local
magnetic moment on the phonon structure. It
suggest to use phonons to reveal presence of the
Fe magnetic moment.
FeSe
-
Fe PDOS for parent 1111 compounds
Fe PDOS for different compounds
at room temperature
0.00
0.05
0.10
La57
FeAsO
0.00
0.05CeFeAsO
0.00
0.05 PrFeAsO
Fe
PD
OS
0.00
0.05 Nd57
FeAsO
0 10 20 30 40
0.00
0.05149
Sm57
FeAsO
Energy / meV
0.0
0.1
Sm
PD
OS
0 10 20 30
Energy / meV
LaFeAsO
NdFeAsO
0.00
0.05
0.10
PD
OS
/ m
eV
-1
SmFeAsO
0 10 20 30
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
PD
OS
/E2 / 1
0-4 m
eV
-1
-40 -20 0 20 40 60
101
102
103
104
NIS
/ c
ounts
Energy / meV
SmFeAsO at
295 K
70 K
57Fe nuclear
resonance
102
103
104
105
NF
S /
counts instr. function
Sergueev et al., PRB 87 (2013) 064302 Measurements at ID18. E = 0.7 meV
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Fe PDOS for LaFeAsO1-xFx at 0 and 296 K
0 10 20 30 40
0.00
0.05
0.10
Energy / meV
PD
OS
/ m
eV-1
296 K - above TN
0 10 20 30 40
0.00
0.05
0.10
LaFeAsO
52 K - below TN
Above and below Neel temperature
With doping Q. Huang et al. PRB 78 (2008) 054529
Phase-diagram for LaFeAsO1-xFx
0 10 20 30 40
0.00
0.05
0.10
PD
OS
(m
eV
-1)
Energy (meV)
parent @ 52K
0 20 40
0.00
0.05
0.10 doped @ 42K
shift ~ 0.8 meV
0 10 20 30 40
0.00
0.05
0.10
PD
OS
(m
eV
-1)
Energy (meV)
parent @ 296 K
0 20 40
0.00
0.05
0.10 doped @ 296 K
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Fe PDOS for parent / doped 1111 compounds
Room temperature Low temperature
LaFeAsO1-xFx
NdFeAsO1-xFx
SmFeAsO1-xFx
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How to find “peak” energy
0 10 20 30 40
0.00
0.05
0.10
PD
OS
(m
eV
-1)
Energy (meV)
Options to find peaks:
• Fit by peak function ? peak shape unknown
• Use COM position ? depends on chosen E-range
• Our solution: search or relative shift compared to
reference spectrum by least square fit. Obtain value
with statistical error.
Interpolation: D(E)
Theoretical function for LSF:
F(E) = β D( E(1+α) )
α = E / E – relative “peak” shift
β (1/ α) – scaling factor
0 10 20 30 40
0.00
0.05
0.10
PD
OS
(m
eV
-1)
Energy (meV)
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Peaks positions vs T for parent and doped conpounds
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Raman scattering data with 1111 compounds
Raman scattering of NdFeAsO1-xFx Zhang et al., PRB 79 (2009) 052507
Eg of As and Fe
at ~ 16 meV (130 cm-1)
weak or invisible for
1111 family
T-dependence of other modes
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Raman scattering on BaFe2As2 Chauviere et al. PRB 80 (2009) 094504
Baum et al., PRB 98(2018) 075113
Raman scattering data with 122 compounds
Gap between B2g(1) and B3g
(1):
theory : 2.8 meV
exp : 1.2 meV
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EuFe2As2. NIS measurements at room T
0 10 20 30 40
0.00
0.05
0.10
Fe P
DO
S
IP
OP
Energy (meV)
0.0
0.2
Eu P
DO
S
ab
c
EuFe2As2
NIS on single crystal
0 10 20 30 400.0
0.1
Energy (meV)
Fe P
DO
S
EuFe2As2
NIS on powder
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Phase diagram of EuFe2As2
Ren et al., PRB 79 (2009) 094426
Phase diagram of EuFe2-xNixAs2 NIS on EuFe2As2 at low and room T
102
103
104
105
-40 -20 0 20 40
101
102
103
104
NIS
/ c
ounts
Energy / meV
295 K
25 K Eu57Fe2As2
NF
S /
counts
instr. function
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T – dependence of phonons in EuFe2As2
0.48
0.52
E16/E
32
TN=190K
0 50 100 150 200 250 300
2
4
6
8
/ m
eV
Temperature / K
gap ~ 0.5 meV
25K
170K
205K
10 11 12 13 14 15 16 17 18 19 20
Energy / meV
295 K
235K
90K
Fe
DO
S /
me
V-1
EuFe1.8Ni0.2As2, 25K
T – dependence of the phonon line position
and width obtained by Lorentz fit
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Theory proposition for spin-dynamics
Nature Physics 5 (2009) 141
* domain walls fixed
* all domains with same direction
(x/y symmetry broken)
* dynamic domain walls
* all domains with same direction
(x/y symmetry broken)
* dynamic domain and twin walls
* twins at different directions
(x/y symmetry conserve)
/ pressure
• AF magnetic
• orthorhombic
• paramagnetic
• orthorhombic
• paramagnetic
• tetragonal
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Conclusion
• There are anomalies in the T – dependence of the phonon structure
for FeSc of 1111 and 122 families.
• They can be related to the structural transition, spin-lattice coupling,
e-ph coupling, …???
• Characteristic E scale of anomalies is 0.1 – 1 meV. E –resolution of
current HRM is 0.7 – 1 meV
Investigation of the T evolution of the phonon anomalies requires
monochromator with 0.1 meV E resolution (spectrograph).
Other useful capability of the spectrograph is simultaneous
measurements of different samples ( doped / parent FeSc )
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Acknowledgment
U. Pelzer
R. Rüffer,
A. Chumakov
J.-P. Celse
M. A. McGuire
A.S. Sefat
B.C. Sales
D.Mandrus
R. P. Hermann
D. Bessas
M. Angst
W. Schweika
A. Möchel
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Thank you for your
attention