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)

Peaks positions vs T for parent and doped conpounds

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

Thank you for your

attention