Ilya Sergeev, PETRA III, Hamburg, Germany Effect of the electron-phonon coupling on phonons in iron based superconductors
Ilya Sergeev, PETRA III, Hamburg, Germany
Effect of the electron-phonon coupling on phonons
in iron based superconductors
Outline
• Fe superconductors: overview
• Study of lattice dynamics in LnFeAsO
• Study of lattice dynamics in EuFe2As2
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
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
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
Fe PDOS for parent / doped 1111 compounds
Room temperature Low temperature
LaFeAsO1-xFx
NdFeAsO1-xFx
SmFeAsO1-xFx
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)
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
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
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
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
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
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
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 )
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