Phase diagram of pnictide superconductors

Phase Diagram of Pnictide Superconductors

Interplay between magnetism, structure, and

superconductivity

B. Büchner

Institut für Festkörperforschung

IFW Dresden

High Temperature Superconductivity

in FeAs Compounds

OUTLINE

• Introduction

• Phase Diagram of LaFeAsO 1-x F x (and other pnictides)

Structural, magnetic, and superconducting transitions

• Superconducting State

Gap, penetration depth, relaxation rate

• Normal State Properties

Electronic Structure (XAS, PES, ARPES)

„Pseudogap“ (NMR, χ)

Electronic transport and spin fluctuations

Charge inhomogeneity (NQR)

Tuesday

Superconductivity in La(O 1-x F x )FeAs: The initiation

Y. Kamihara et al.,

J. Am. Chem. Soc. 130, 3296 (2008)

Februrary 2008

Picture: H. Takahashi et al.,

Nature 453, 376 (2008)

Dec. 2008: More than 600 papers on pnictide superconductors

Superconductivity above 50 K

Transition temperature (K)

Year of discovery

C. Hess et al., EPL (2009)

similar data: e.g. Ren et al. Chin Phys. Lett. 2008

Superconductivity above 50 K

La …. Sm … Gd

4πχ

4πχ

0 20 40

0.25

0.20

0.15

0.10

0.05

0.1

0.0

-0.1

-0.2

-0.3

0.1

1 (0h)

FC

ZFC

T (K)

FC

ZFC

2 (0.5h)

T (K)

0 20 40 60

0.1

FC

ZFC 5 (6h)

FC

ZFC

6 (12h)

0.0

-0.1

-0.2

-0.3

0.1

0.0

-0.1

-0.2

-0.3

0.1

4πχ

4πχ

0.0

FC

FC

0.0

4πχ

-0.1

-0.2

-0.3

0.1

ZFC

3 (1.5h)

ZFC

7 (24h)

-0.1

-0.2

-0.3

4πχ

4πχ

0.0

-0.1

-0.2

-0.3

FC

ZFC

4 (3h)

GdFeAsO 0.83

F 0.17

SE 1778/1-7

Milling Experiment

B = 20G

S. Wurmehl et al, work in progress

0 20 40 60

T (K)

Fe pnictides: A class of high T c superconductors

1111 systems

La, Ce … Gd-based

F, O doped

F doped REOFeAs

O defici. REOFeAs

Sm doped SrFFeAs

122 systems

Ba, Sr, Eu, Ca- based

K, Na, Co doped

T c up to 39 K

Intermetallic comp.

Single crystals

M. Rotter, M. Tegel,

and D. Johrendt

PRL 2008


0

FC

χ (% (4π) −1 )

-10

-20

-30

1111 systems

BaAs 2

Fe 1.9

Co 0.1

SE 1737 A

La, Ce … Gd-based

F, O doped

B = 20G

ZFC

F doped REOFeAs

O defici. REOFeAs

Sm doped SrFFeAs

0 5 10 15 20

T (K)

122 systems


K, Na, Co doped

T c up to 39 K

Intermetallic comp.

Single crystals


and D. Johrendt

PRL 2008


1111 systems

La, Ce … Gd-based

F, O doped

F doped REOFeAs

O defici. REOFeAs

Sm doped SrFFeAs

122 systems


K, Na, Co doped

T c up to 39 K

Intermetallic comp.

Single crystals


and D. Johrendt

PRL 2008


1111 systems

La, Ce … Gd-based

F, O doped

F doped REOFeAs

O defici. REOFeAs

Sm doped SrFFeAs

122 systems


K, Na, Co doped

T c up to 39 K

Intermetallic comp.

Single crystals


and D. Johrendt

PRL 2008

Medvedev, Felser et al.,

Nature Materials 2009

Interplanar distance may differ very much!

LaO 1-x

F x

FeAs „1111“

FeSe 1-x Te x

(Fe 2

P 2

)(Sr 4

Sc 2

O 6

)

T C, max = 17 K

H. Ogino et al., arXiv:0903.3314


1111 systems

La, Ce … Gd-based

F, O doped

F doped REOFeAs

O defici. REOFeAs

Sm doped SrFFeAs

122 systems


K, Na, Co doped

T c up to 39 K

Intermetallic comp.

Single crystals


and D. Johrendt

PRL 2008

Cuprates

Magnetism and Superconductivity

Cuprates



Cuprates

Heavy fermions

CePd 2 Si 2

Mathur, Julian, Lonzarich et al. 1998


Cuprates

Heavy fermions

CePd 2 Si 2

Mathur, Julian, Lonzarich et al. 1998


in FeAs Compounds

OUTLINE

• Introduction










Some of the people @ IFW involved in the research on FeAs …

Synthesis:

G. Behr, J. Werner, S. Singh, C. Nacke, S.Wurmehl et al.

M, Thermodynamics: N. Leps, L. Wang, R. Klingeler et al.

NMR, ESR:

H. Grafe, G. Lang. F. Hammerath, V. Kataev et al.

Transport :

X-ray diffraction:

Spectroscopy:

A. Kondrat, G. Friemel, C. Hess et al.

J. Hamann-Borrero

S. Borysenko, D.V. Evtushinsky, A. Koitzsch, J. Geck

A. Kordyuk, V.B. Zabolotnyy, T. Kroll, M. Knupfer et al.

Cooperations

μSR, Mößbauer H.H.Klauss, H. Maeter et al. TU Dresden

μSR H. Luetkens, R. Khasanov et al. PSI Villingen

Crystals

C.T. Lin, D. Inosov, V. Hinkov,

B. Keimer et al. MPI FKF Stuttgart

NMR N. Curro UC Davis

X-ray/neutron diffraction M. Braden et al.

U Köln

X-ray/neutron diffraction R. Feyerherm, D. Argyriou et al. HZ Berlin

DFG-Research Unit FOR538

Doping Dependence of Phase Transitions and Ordering Phenomena in Copper-Oxygen Superconductors

Resistivity and Susceptibility of La(O 1-x F x )FeAs

Preparation & Characterization

LaO 1-x F x FeAs, x=0 ... 0.2

Phase purity...

Lattice constants...

Preparation & Characterization

LaO 1-x F x FeAs, x=0 ... 0.2

Fluorine content...

Lattice constants...

Resistivity of La(O 1-x F x )FeAs

Two phase transitions in LaOFeAs

H.-H. Klauss et al., PRL 101, 077005 (2008)

LaOFeAs: Competing OP above T S ?

Lattice constant (Å)

8.74

8.72

5.70

5.68

XRD

c

b

a

Cmma

P4/nmm

dT

dp

N

=

TV

negative!

Δα

Δc

p

α (10 -6 K -1 )

12

9

6

3

Thermal

Expansion

0

0 50 100 150 200 250

T (K)

specific

heat anomaly

6

4

2

0

Δc p

(J/molK)

at T S :

upturn truncated

⎛ ∂V

⎞ ⎛ ∂S

⎞

⎜ ⎟ ∝ −⎜

⎟

⎝ ∂T

⎠ p ⎝ ∂p

⎠T

positive pressure

dependence!

L. Wang et al., arXiv:0903.1235

α (10 -6 /K)

14

12

10

8

6

T s

LaOFeAs

Thermal Expansion

• large fluctuation region at T>T S

M/B (10 -4 emu / mol)

ρ (mΩ cm)

4

3

2

7

6

5

T N

100 150 200 250 300

T (K)

Magnetisation:

• linear T-dependence at high T,

only weak changes due to fluctuations

Resistivity

• Peak at T S

enhanced scattering in the fluctuation

regime

Strong link between

electronic and structural

degrees of freedom

Wang et al., subm. to PRB

Two phase transitions in (Pr,Sm)OFeAs

T N

T S

(220)

SmOFeAs

Kimber et al., PRB (2008)

• Structural Transition similar for all RE

• No transition in doped systems (x ~0.1)

REOFeAs: Structural changes

Thermal expansion α(10 -5 /K)

1.2

LaFeAsO

0.8

0.4

0.0

1.2

0.8

0.4

0.0

1.2

PrFeAsO

T Pr

N

GdFeAsO

T N

T μ N

T S

T S

CeFeAsO

SmFeAsO

T N

T N

T S

T S

1.2

0.8

0.4

0.0

0.0

0 50 100 150 200 250

T(K)

1.2

0.8

0.4


0.8

0.4

T N

T S

B = 0

0.0

0 50 100 150 200 250

T(K)

R. Klingeler et al., 2009, in print

REOFeAs: Structural changes


1.2

LaFeAsO

0.8

0.4

0.0

1.2

0.8

0.4

0.0

1.2

PrFeAsO

T Pr

N

GdFeAsO

T N

T μ N

T S

T S

CeFeAsO

SmFeAsO

T N

T N

T S

T S

1.2

0.8

0.4

0.0

0.0

0 50 100 150 200 250

T(K)

1.2

0.8

0.4


0.8

0.4

T N

T S

0.0

0 50 100 150 200 250

T(K)

B = 0

B = 9T

Phase transitions in undoped 122 compounds

BaFe 2 As 2

SrFe 2 As 2

D. Johrendt and R. Pöttgen, Physica C 2009

A. Jesche et al., PRB 2008

SDW magnetism in SrFe 2 As 2

Krellner et al., PRL `08, arXiv:0806.1043

Resistivity of Ba 1-x K x Fe 2 As 2

2,0x10 -2

3,0x10 -1

2,5x10 -1

Ba(FeAs)2, thin piece

up- curve

136.44 K

1,8x10 -2

1,6x10 -2

1,4x10 -2

ρ [mΩcm]

0 50 100 150 200 250 300 3,5x10 -1

2,0x10 -1

1,5x10 -1

1,0x10 -1

5,0x10 -2

0,0

1,2x10 -2

1,0x10 -2

8,0x10 -3

6,0x10 -3

4,0x10 -3

2,0x10 -3

dρ/dT [mΩcm/K]

0,0

0 50 100 150 200 250 300

T[K]

M. Rotter et al., Ang. Chem. 2008

Magnetic order of LaOFeAs

T= 13 K

57

Fe Mössbauer spectroscopy

counts (arb. units)

T= 78 K

_

T= 126 K

well defined magnetic

hyperfine field

commensurate magnetic

order below 138 K

B hyp (T 0) ~ 4.86 T

ordered moment ~ 0.3 µ B

itinerant magnet

T= 137 K

LaOFeAs

-4 -2 0 2 4

velocity (mm/s)


Magnetic order of LaOFeAs

C. de la Cruz, et al., Nature 453, 899 (2008)

M. Korshunov, I. Eremin, PRB 78, 140509(R) (2008)

I. Mazin et al., PRL 101, 057003 (2008)

J. Dong et al., EPL 83, 27006 (2008)

K. Kuroki et al., PRL 101, 087004 (2008)

S. Raghu et al., PRB 77 220503 (2008)

Magnetic order parameter in undoped LaOFeAs

Four band SDW theory

U=0.26eV

J=U/5

Q AFM

= (π,π)

Δ SDW

(0) = 31 meV

µ= 0.33 μ B

See:

M. Korshunov and I. Eremin

arXiv:0804.1793


T N

LaOFeAs

T S

S

x=0: Stuctural and SDW Transistions

=

∂ ln

∂ε

Resistivity

• Peak at T S

ρ ∝

1

,

λ

enhanced scattering at higher T

• Inflection point at T N

reduced scattering + localization at T

T N

LaOFeAs

T S

S


=

∂ ln

∂ε

Resistivity

• Peak at T S

ρ ∝

1

,

λ




T N

LaOFeAs

T S

S


=

∂ ln

∂ε

Resistivity

• Peak at T S

ρ ∝

1

,

λ




T N

LaOFeAs

T S

S


=

∂ ln

∂ε

Resistivity

• Peak at T S

ρ ∝

1

,

λ




T N

LaOFeAs

T S

S


=

∂ ln

∂ε

Resistivity

• Peak at T S

ρ ∝

1

,

λ




Resistivity of ReOFeAs

Resistivity

• Peak at T S

enhanced critical scattering


reduced scattering + gap opening at T

ROFeAs (R = La, Pr, Ce, Sm)

ZF-μSR:

• Zero Field Muon Spin Rotation

Static magnetic order below T N

100% magnetic volume fraction

Commensurate magnetic structure

H. Maeter, et al., arXiv:0904.1563 (2009).

• T-dependence of μSR frequency

Second order transition at T N (Fe)

T N (R)

Why is CeOFeAs different ?

Moessbauer:

• Moessbauer spectroscopy

All compounds possess the same

ordered Fe moment (~0.35 μ B ) !

• Neutron scattering: 0.25 – 0.8 μ B

?

M. A .McGuire et al.,

New J. Phys. 11, 025011 (2009).

Ce magnetization above T N

Ce

CeOFeAs

• Magnetization in molecular field of

the Fe sublattice

Contributes to the same magnetic

Bragg peak as the Fe order

Creates a field at the muon site which

is proportional to the Ce magnetization

• This can be modeled by:

a Curie-like term to the μSR frequency

a calculation of thermal population of

crystal electric field (CEF) levels

f

T ) =

f

(

0

⎡ ⎛

⎢1

−

⎜

⎢⎣

⎝

T

T

N

⎞

⎟

⎠

α

⎤

⎥

⎥⎦

Fe sublattice

magnetization

γ

~

⎡ C ⎤

⋅ ⎢1

+ ⎥

⎣ T − Θ⎦

Curie-like Ce

magnetization

H. Maeter, et al., arXiv:0904.1563 (2009).

S. Chi et al., arXiv:0807.4986 (2008).

Interplay of Rare Earth and FeAs electronic systems

• LDA band structure CeOFeAs PrOCeAs

• Strong hybridization between Fe and Ce states

• CeOFeP is a moderate heavy fermion system

L. Pourovskii et al., EPL 84 37006 (2008)

• Pr states shifted 2 eV downwards

E. M. Brüning et al., Phys. Rev. Lett. 101, 117206 (2008)


Magnetism of lightly doped La(O 1-x F x )FeAs

H. Luetkens et al., Nature Materials 2009

Magnetism of lightly doped La(O 1-x F x )FeAs


Doping effect: LaFeAsO 1-x F x


1.4

1.2

1.0

0.8

0.6

0.4

0.2

LaO 1-x

F x

FeAs

x=0.00

0.0

0 50 100 150 200 250

T(K)



1.4

1.2

1.0

0.8

0.6

0.4

0.2

LaO 1-x

F x

FeAs

x=0.00

x=0.02

0.0

0 50 100 150 200 250

T(K)



1.4

1.2

1.0

0.8

0.6

0.4

0.2

LaO 1-x

F x

FeAs

x=0.00

x=0.02

x=0.04

0.0

0 50 100 150 200 250

T(K)



1.4

1.2

1.0

0.8

0.6

0.4

0.2

LaO 1-x

F x

FeAs

x=0.00

x=0.02

x=0.04

x=0.05

0.0

0 50 100 150 200 250

T(K)



1.4

1.2

1.0

0.8

0.6

0.4

0.2

LaO 1-x

F x

FeAs

x=0.00

x=0.02

x=0.04

x=0.05

x=0.10

0.0

0 50 100 150 200 250

T(K)

Phase diagram of LaFeAsO 1-x

F x

200

LaFeAsO 1-x

F x

dV/dT>0

T (K)

150

orthorh.

tetragonal

T C

from χ(T), μSR

100

50

SDW

magnetic

order

T N

from χ(T), μSR

T S

from χ(T), α(T)

T* from α(T)

dT N

/dp

Superconductivity of La(O 1-x F x )FeAs

Phase Diagram of La(O 1-x F x )FeAs


Phase Diagram of La(O 1-x F x )FeAs


Phase Diagrams of CeFeAsO 1-x

F x

and SmFeAsO 1-x

F x

CeO 1-x F x FeAs

Zhao et al., Nature Materials (2008), neutrons

• 100 % rare earth doping (5% lattice constant change!)

does not change the structural distortion, but a few percent Flourine

(electron doping) completely suppresses T s !

• gradual decrease of FeAs Neel temperature to zero ?

• coexistence of magnetism and superconductivity? Yes

Drew et al., Nature Materials 2009

Resistivity

CeO 1-x Fe x As

Tetragonal Orthorombic structural transition

via synchrotron XRD

0 % 5 %

(220)

(006)

10 %

15 %

superconducting

25 %

J. E. Hamann-Borrero, A. Kondrat

et al., in preparation

Zero field μSR: : CeO 1-x Fe x As

magnetic volume fraction

CeO 1-x

F x

FeAs

100%

80%

60%

40%

20%

0%

zero field μSR

0 20 40 60 80 100 120 140 160 180

temperature (K)

undoped

5% F-content

10% F-content

15% F-content

50%

Determine T N

(Fe) from magnetic volume:

• e.g.50% of signal shows static relaxation

• Strong reduction of T N

for x>0!

Appearance of µSR frequency

Long range coherent

sublattice magnetization

• Strong reduction of sublattice

magnetization for x>0

µSR frequency (MHz)

35

30

25

20

15

10

5

0

0 20 40 60 80 100 120 140 160 180

temperature (K)

H. Maeter, et al., in preparation.

• No long range order for

x=0.15

• but short range magnetic order

present with f µ

(T)=0!

Mean sublattice magnetization = 0

not visible in neutron scattering

Transverse field μSR: : CeO 1-x Fe x As

Static magnetic order

local field (G)

740

CeO 1-x

F x

FeAs

Frequency shift

• Additional diamagnetic frequency shift

• Bulk superconductivity T C

• Bulk static magnetism for 15%!

2,0

1,8

CeO 1-x

F x

FeAs

T C

730

25% F-content

1,6

25% F-Content

20% F-content

20% F-content

720

1,4

15% F-content

15% F-content

1,2

710

700 G TF-μSR

700 G TF-μSR

700

1,0

0,8

690

0,6

680

0,4

670

0,2

0 10 20 30 40 50

0,0

temperature (K)

10 20 30 40 50

relaxation rate σ S

(1/µs)

temperature (K)

Static line width

• Bulk superconductivity T C

• Bulk static magnetism for 15%!


CeO 1-x F x FeAs: coexistence of magnetism and SC?

Zhao et al., Nature Materials (2008), neutrons

Evidence for coexistence!

XRD, ρ, µSR,...


Phase Diagrams of doped BaFe 2 As 2

BaFe 2-x Co x As 2

Ba 1-x K x Fe 2 As 2

Chu et al., PRB 2009

Rotter et al., Ang. Chem. 2008

Pressure induced Superconductivity in (Ba,Sr)Fe 2

As 2

Colombier et al., arxiv.org/abs/0904.4488v1


Phase Diagram of Ba 1-x K x Fe 2 As 2

Ang. Chem. 2008

Resistivity of doped pnictides

SmO 1-x

F x

FeAs – highest T c

Rotter et al.,

Ang. Chem. 2008

C. Hess et al., EPL (2009)

Resistivity of Co doped pnictides

2,0x10 -6

5,0x10 -6 Ba(Fe 0.95

Co 0.05

) 2

As 2

4,0x10 -6

1,5x10 -6

Ca(Fe 0.94

Co 0.06

)2As 2

, SE1754

cooling

ρ (Ωm)

3,0x10 -6

2,0x10 -6

ρ [Ωm]

1,0x10 -6

5,0x10 -7

1,0x10 -6

0,0

0 50 100 150 200 250 300

T in K

1,0x10 -7

Single crystals

0,0

0 50 100 150 200 250 300

T[K]

0 50 100 150 200 250 300

CaFe 1.85

Co 0.15

As 2

, SE1795D

1,5x10 -6 heating

4,0x10 -8

2,0x10 -8

2,0x10 -6

1,0x10 -6

1,0x10 -6

3,0x10 -6 0 50 100 150 200 250 300 -1,0x10 -7

Ca(Fe 0.95

Co 0.05

) 2

As 2

, SE1781B

cooling, 2mA

5,0x10 -8

ρ [Ωm]

0,0

-5,0x10 -8

dρ/dT [Ωm/K]

ρ [Ωm]

5,0x10 -7

0,0

T c

=21.2 K

0,0

0 50 100 150 200 250 300

T[K]

0,0

0 50 100 150 200 250 300

T[K]

-2,0x10 -8

Resistivity at the stuctural and SDW transistions

LaOFeAs

T S

ρ ∝

1

n

1

,

λ

Resistivity

• Peak at T S



reduced scattering

5,0x10 -6 Ba(Fe 0.95

Co 0.05

) 2

As 2

4,0x10 -6

Resistivity in Co doped systems

ρ (Ωm)

3,0x10 -6

2,0x10 -6

1,0x10 -6

Single crystal!!

• Reduction of scattering at T N

smaller

• Mean free path limited by disorder

due to Co atoms!!

0,0

0 50 100 150 200 250 300

T in K

Intrinsic sources of scattering masked by disorder

in Co doped single crystals!!!


in FeAs Compounds

OUTLINE

• Introduction










Superconducting Order Parameter (Gap)

How can we measure the superfluid density?

via the magnetic penetration depth λ

n s

/ m* is proportional to 1/ λ 2

Superconductivity in La(O 1-x

F x

)FeAs

• measure λ in vortex state of type II superconductor via field profile p(B)

transverse field µSR

•

B ext

In anisotropic powder superconductors P(B) shows Gaussian

shape

2 −4

ΔB

∝ λ

ab

How can we measure the superfluid density?

via the magnetic penetration depth λ

n s

/ m* is proportional to 1/ λ 2

• measure λ in vortex state of type II superconductor via field profile p(B)

transverse field µSR

•

Superconductivity in La(O 1-x F x )FeAs

1,0

CeO

B

0.75

F 0.25

FeAs

ext

50 K

1.6 K

muon spin polarization

0,5

0,0

-0,5

-1,0

0,0 0,2 0,4 0,6 0,8 1,0 1,2

In anisotropic powder superconductors P(B) shows Gaussian

shape

2 −4

ΔB

∝ λ

ab

time (µs)

Penetration Depth of La(O 1-x F x )FeAs

Proximity of

Magnetism?

H. Luetkens et al., PRL 101, 097009 (2008)

Penetration Depth of La(O 1-x F x )FeAs

T dependence of λ: consistent with BCS s-wave

B dependence of λ: not simple s-wave: d, ext. s, multiband

H. Luetkens et al., PRL 101, 097009 (2008)

Superconducting Properties of Co doped Ba122

0.2

0.0

-0.2

-0.4

-0.6

FC

T c

= 23.0(2)K

4πχ

-0.8

-1.0

BaFe 1.8

Co 0.2

As 2

BKZ 8

B = 20G

μSR: no trace of magnetism

-1.2

Bridgeman

-1.4

-1.6

ZFC

-1.8

0 10 20 30

H. Luetkens, C. Nacke et al., in preparation

Superconductivity of Co doped Ba122– TF-μSR

• Superfluid density (full line

shape analysis) for field

applied in different direction.

• Pronounced anisotropy of the

penetration depth λ.

• Assuming λ a = λ b one can

calculate the low temperature

values:

• λ a (T=0) = 220 nm

• λ c (T=0) = 1100 nm

H. Luetkens, C. Nacke et al., in preparation

Superconductivity of Co doped Ba122– TF-μSR

Superconductivity in Ba 1-x K x Fe 2 As 2

µSR: Phase separation into

commensurate SDW and

superconducting volumes

~ 2

• stronger temperature

dependence of λ -2

large and small gap ?

μSR: 50 % non-magnetic

R. Khasanov, PRL 09

Gap on the inner Г-barrel, ГM

cut1

4

Intensity (arb. u.)

2

0

-2

-4

10 K

15 K

20 K

25 K

30 K

35 K

40 K

45 K

fit

Δ (meV)

10

8

6

4

2

0

Resistance (arb. u.)

0

10

20

30

T (K)

T c

=32 K

40

50

-30 -20 -10 0 10

ω (meV)

D.V. Evtushinsky et al., PRB 2009

Anisotropy of Gap according to ARPES

D.V. Evtushinsky et al., PRB 2009

Similar results see eg. Ding et al EPL 2008

Penetration Depth of Ba 1-x K x Fe 2 As 2 , T c = 32 K

Δ large: 9 meV

(ARPES)

Δ small 1.1 mev

(ARPES: < 4 meV)

D.V. Evtushinsky et al., New J. Phys. 2009

Penetration Depth and Uemura Plot

Uemura plot:

T C

scales linearly with the superfluid

density, i.e. T C

∝ 1/ λ 2 ∝ n s

/m*

120

100

Bi2223

h-doped

cuprates

T C

(K)

80

60

40

LN 2

La214

Y123

e-doped

cuprates

SLCO

La-Fx This work

Nd-O0.85 This work

Sm-O0.85 This work

La-F0.08 Carlo et al.

Ce-F0.16 Carlo et al.

Nd-F0.12 Carlo et al.

Sm-F0.18 Drew et al.

20

PCCO

NCCO

0

0 20 40 60 80

λ -2

ab (μm-2 )

The Fe-based superconductors are close to the Uemura line

for hole doped cuprates hope for higher T c in the future

Gap and Spin Lattice Relaxation Rate (NMR)

75

As NMR on La(O 1-x F x )FeAs: T 1

•No Hebel Slichter coherence peak, no expon. decay

•T 3 dependence below T c is indicative for line nodes

10

H 0

c

⇒ suggestive of d-wave

superconductivity

coherence peak

and exp decay

T 1

-1

(s

-1

)

1

0.1

T 3

T c

0.01

LaO 0.9 F 0.1 FeAs

10 100

Temperature (K)

H.-J. Grafe et al., PRL 101, 047003 (2008)

75


1

published data H||ab peak

new data H||ab peak

theory Parker & Mazin

At low temperatures

deviation from T 3

extended s +/-

wave - scenario

(s -1 )

0.1

LaO 0.9 F 0.1 FeAs

Mazin et al.,

PRL 101, 057003 (2008).

T 1

-1

*

Parker et al. PRB 2008

0.01

Chubukov et al. PRB 2008

T 3

1E-3

1 2 3 4 5 10 20

Temperature (K)

or dirty d-wave…

75


100

10

T c

~T 3

~T

1

(s -1 )

T 1

-1

0,1

0,01

1E-3

~T 4.8 ~T

10 100

T (K)

LaO 0.9

F 0.1

FeAs

LaO 0.9

F 0.1

FeAs 1-γ

75


100

~T 3

(s -1 )

10

1

T c

Intensity (a.u.)

~T

T 1

-1

0,1

0,01

1E-3

~T 4.8 ~T

10 100

T (K)

LaO 0.9

F 0.1

FeAs

LaO 0.9

F 0.1

FeAs 1-γ

75


100

~T 3

As-deficiency

10

T c

~T

⇒

1

disorder

(s -1 )

T 1

-1

0,1

0,01

1E-3

~T 4.8 ~T

10 100

T (K)

LaO 0.9

F 0.1

FeAs

LaO 0.9

F 0.1

FeAs 1-γ

but:

higher T c

higher B c2 (0)

T < T c : T 1

-1

~ T 4.8

T

75


100

10

T c

~T 3

~T

1

T 1

-1

(s -1 )

0,1

0,01

1E-3

Scenarios:

~T

LaO 0.9

F 0.1

FeAs

LaO 0.9

F 0.1

FeAs 1-γ

~T 4.8 10 100

T (K)

Transition from s +/-

to conventional s +/+

state due to impurities

As deficiencies cause mainly intraband scattering and further protect

the unconventional s +/-

state (higher T c

, higher H c2

)

(„smart impurities“)

F. Hammerath, H.-J. Grafe, … S.L. Drechsler, I. Eremin et al.

75


100

10

T c

~T 3

~T

1

T 1

-1

(s -1 )

0,1

0,01

1E-3

Scenarios:

~T

LaO 0.9

F 0.1

FeAs

LaO 0.9

F 0.1

FeAs 1-γ

~T 4.8 10 100

T (K)

Fuchs et al, PRL 2008

Transition from s +/-

to conventional s +/+

state due to impurities

As deficiencies cause mainly intraband scattering and further protect

the unconventional s +/-

state (higher T c

, higher H c2

)

(„smart impurities“)

F. Hammerath, H.-J. Grafe, … S.L. Drechsler, I. Eremin et al.

SC transition in GdFeAsO 0.85 F 0.15 :

Strange and Puzzling behavior

10

α (10 -6 / K) C p

(J / mol K)

5

-2

-3

0T

0T

-4

-5

0 5 10 15 20 25 30

T (K)

SC transition in GdFeAsO 0.85 F 0.15

α (10 -6 / K) C p

(J / mol K)

10

5

-2

-3

0T

0T

specific heat c p

(J / mol K)

24

22

20

18

16

PrFeAsO 0.85 F 0.15

T C ~41K

B=0

B=9T

c p

/T (J / mol K 2 )

0.54

0.52

0.50

0.48

0.46

T (K)

34 36 38 40 42 44 46 48

36 38 40 42 44 46

T (K)

Δc p

=0.53 J/mol K

T c

= 41.2K

-4

-5

0 5 10 15 20 25 30

T (K)


α (10 -6 / K) C p

(J / mol K)

10

5

-2

-3

-4

•T C ≈ 20K

•T N

Gd

= 2.9K

0T

-5

0 5 10 15 20 25 30

T (K)

0T

SC transition visible

Δc p, SC = 2.0 J / (mol K)

BCS theory:

γ = Δc/(1.43 T c )

≈ 70 mJ/(molK)

dT

dp

C

= −TV

≈1.4

Δα

Δc

K

GPa

p


α (10 -6 / K) C p

(J / mol K)

10

5

-2

-3

•T C ≈ 20K

•T N

Gd

= 2.9K

0T

0T

Relaxation rate (μs -1 )

12

10

GdO 0.85

FeAsF 0.15

8

6

4

2

T C

0

0 50 100 150 200

T (K)

-4

-5

0 5 10 15 20 25 30

T (K)

GdFeAsO 0.85 F 0.15 : Specific heat

50

GdFeAsO 0.85

F 0.15

C p

(J / mol K)

40

30

20

10

B=9T

B=0

B dependence of c p

⇔

magnetic fluctuations?

0

0 20 40 60 80 100

T (K)

GdO 1-x F x FeAs – Gd HF-ESR: Width of the signal

Gd 3+ : 4f 7 , L = 0; J = S =7/2

no coupling to lattice, probes spin degrees of freedom

2

1

x=0.15

240K

160K

inhomogeneous broadening

absorption (a.u.)

0

-1

-2

-3

140K

120K

100K

80K

60K

40K

20K

Gd sees enhanced magnetism

-4

12 13

B (T)

Electron Spin Resonance of GdO 1-x F x FeAs

12.45

g=2

12.40

2

1

- undoped x=0.00

- doped x=0.15

240K

160K

B res

(T)

12.35

12.30

12.25

High frequency/field ESR, F = 328GHz

12.20

undoped x=0

doped x=0.15

12.15

0 50 100 150 200 250

absorption (arb. u.)

0

-1

-2

-3

140K

120K

100K

80K

60K

40K

20K

Temperature (K)

-4

11 12 13 14

Magnetic field (T)

Superconductor (T c

= 21 K): strong broadening and shift of the Gd-ESR response

⇒ onset of inhomogeneous quasi-static spin correlations at T < 80 K


8

Thermal Expansion

B = 0

Specific heat

Thermal expansion α (10 -6 /K)

9

8

7

16T

B=0

5T

9T

13T

14T

16T

5 10 15 20 25

T (K)

B = 14T

B = 0

9T

7T

5T

3T

0T

10 12 14 16 18 20 22

T (K)

14T

13T

12T

11T

10T

7

6

5

4

3

2

specific heat c p

(J / mol K)

GdFeAsO 0.85 F 0.15 : Magnetostriction

Magnetostriction β (10 -6 )

3

2

1

5K

20K

18K

15K

12K

10K

40K

8K

0

0 2 4 6 8 10 12 14 16

B (T)

GdFeAsO 0.85 F 0.15

20

B C2 ~20T

B C2 >60T

15

B (T)

10

5

T c

(B) thermal expansion

T c

(B) specific heat

B c

(T) magnetostriction

LaFeAsO 0.9 F 0.1

0

0 5 10 15 20

T(K)

Fuchs et al, PRL 2008

SC transition in GdFeAsO 0.85 F 0.15 :

Strange and Puzzling behavior

20

B C2 ~20T

Unusually small H C2 !

What causes pair breaking?

15

B (T)

10

5

Superconductivity

from thermal expansion

from specific heat

from magnetostriction

0

0 5 10 15 20 25 30

T (K)

Why large anomalies in c p , α?

Enhanced

magnetism ≤ 80K!

Intrinsic

inhomogeneities?

Spin lattice relaxation rate, T 1

-1

, normal state

0.25

0.20

(T 1

T) -1 (s -1 K -1 )

0.15

0.10

pseudogap ?

75

As NMR

H 0 || ab

0.05

LaO 0.9 F 0.1 FeAs

0.00

0 50 100 150 200 250 300 350 400 450 500

Temperature (K)

H.-J. Grafe et al., PRL 101, 047003 (2008)

H.-J. Grafe et al., New J. Phys. 2009


in FeAs Compounds

OUTLINE

• Introduction










Tuesday

Phase diagram of pnictide superconductors

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