J. Bobroff, S. Ouazi, H. Alloul, P. Mendels, N. BlanchardLaboratoire de Physique des Solides, Université Paris XI, Orsay

G. Collin J.F. Marucco, V. Guillen D. ColsonLLB, CEA-Saclay LEMHE, Université Paris XI SPEC, Saclay

Impurity effects in high TC superconductors

High TC cuprates

metallicity + correlations + exotic superconductivity

doping

T

Antiferromagnetism Superconductivity

pseudogap

• Impurity in more « simple » systems• in a metal• in a BCS superconductor

• in a correlated insulator

Impurity effects in cuprates

•Impurity in High TC cuprates • in the metallic « normal » state

• macroscopic properties• local magnetism

in underdoped state with increasing doping

• in the superconducting state• macroscopic properties• density of states• magnetism

A magnetic impurity in a metal

• RKKY spin polarization of the conduction carriers

• Kondo effect

0 100 200

χ

TemperatureMadhavan et al.

CuFe AuCo

A magnetic impurity in an isotropic BCS superconductor

• Decrease of TC only if it is magnetic (Abrikosov, Gork’ov)

• Possible local bond states

dI/dV(10-7Ω-1)

NbMn or NbAg

Yazdani et al.

14580 14600 14620 14640 146600.0

0.2

0.4

0.6

0.8

1.0

ν (kHz)

Y2 Ba Ni98% Zn2% O5 T = 200 K

Inte

nsité

(uni

tés a

rbitr

aire

s)

pur

Zn 2%

An impurity in an insulating correlated system

Ni Ni Ni Ni Ni Ni Ni Zn Ni Ni Ni Ni Ni

impurity

NMR spectrum

example : Y2BaNiO5 as a spin 1 Ni chain with Haldane gap

0 2 4 6 8

-4

0

4

Zn QMC computation

δ< S

zi >/<

Sz> u

position

T=100 K

< Si.Sj > ~ e-r/ξ<SZ(i)> ~ e-r/ξ(T) / T

Das et al., 2004Tedoldi et al., 2000

Cu

J’

Zn

Cu S=1/2

J=260 K

JJ’

J’ = 0.1 à 0.2 J

An impurity in an insulating correlated system

Some other low dimension spin systems

AF orderGapξ<3

2-leg ladder

AF order

Gap

Dimerisation

Spin-Peierls Chain

No orderξ~1/TSpin ½ Chain

AF ?Gapξ < 6

Spin 1 Chain

Common mechanism : breaking of a singlet

Non magnetic impurity effects in cuprates

The normal state

doping

T

Antiferromagnetism Superconductivity

pseudogap

Why choosing YBaCuO ?

YBaCuO7 92 12 kHz

Impurity effects on doping and pseudogap

0 100 200 300 400 5000.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

pure Zn : xplane = 3% Zn : xplane = 6 %

Ks (%

)

T (K)

YBaCuO6.6

• No effect on doping• No effect on pseudogap

• Effect on resistivity :

Zn : strong scattering center Ni : weaker

•Effect on macroscopic susceptibility :

Zn induces an effective paramagnetic moment S<1/2which decreases with increasing dopingNi induces a moment ≠ S=1 (Mendels, 1994,1999; Zagoulaiev, 1995 )

Impurity effects on macroscopic properties

(Chien, 1991)

100 150 200 250 300 350

0

-100

-200

-300

2èmes voisins

raie centrale

1ers voisins

89K

(ppm

)

T (K)

89Y

100 200 300 4000

200

400

600

800

7 K (p

pm)

T (K)

x = 0.85 % / plane x = 1.8 % / plane

7Li

100 200 3000

20

40

T (K)

17Δν

(kH

z)

17O

Nonmagnetic Impurity in Pseudogap regime

Alloul,91;Walstedt,93;Mahajan,94; Williams,95; Ishida,97; JB,97,99; Julien,2000; Itoh,2003; Ouazi, 2004

0 2 4 6 80.000

0.002

0.004

0.006

Distance to the impurity (unit cells)

Spatial dependence Extension

⟨Sz ( r

) ⟩

T = 85K

Ouazi, 2004

Multi-nuclei quantitative analysis of the induced staggered polarization

100 1500

2

4

6

8

10

ξ (c

ell u

nits

)

Temperature (K)

ξimpurity

• Analogy with spin chains• Theoreticall justifications :

t-J (Poilblanc)RVB (Khaliullin, Gabay, Nagaosa & Lee)

Nonmagnetic Impurity in Pseudogap regime

Spatial dependence

⟨Sz ( r

) ⟩

T = 85K

Effect of hole doping on the induced polarization

0 2 4 6 80.000

0.002

0.004

0.006

Distance to the impurity (unit cells)

100 1500

1

2

3

4

5

6

7

ξ imp (u

nit c

ells)

Température (K)

dopage

• persistence of corrélations at optimal doping

NAFL, SCR (Bulut, Ohashi)

Extension

dopage

100 200 300 400

200

300

400

YBa2Cu3O6.97

YBa2Cu3O6.6

7 K (p

pm)

T (K)

0

400

800 x = 0.85 % / plane x = 1.8 % / plane

6.4 6.6 6.8 7.0 7.20

50

100

150

200

250

Tem

pera

ture

(K)

oxygen doping

0

5

10

C (M

Hz.

K)

Fit inC/(T+Θ)

Susceptibility on 1st near neighbor Cu C

Θ

Effect of hole doping on the induced polarization

JB, 1999

Impurity effects in cuprates

The superconducting state

doping

T

Antiferromagnetism Superconductivity

pseudogap

• Effect on TC :

only dependent on scattering potential (d-wave)

(Rullier-Albenque, 2000)

• Effect on penetration depth :

λ increases with impurity content,ns decreases with impurity content (Bonn, 1993;Bernhard, 1996)

Impurity effects in the superconducting state

Pan, Nature 2000 Hudson, Nature 2001

Impurity effects in the superconducting state

Bi2212 + Zn T = 4.2 K

Sidis, 1996

purZn 2%

Effect on local Density Of States

Effect on magneticresponse χ’’(QAF,ω)

close to Zn

far from Zn

500

1000

Li 1% Li 2%

T C1%T C

2%

Y Ba2Cu3O 6.97

7 K (p

pm)

1000

2000

3000

T C1%

Y Ba2C u3O 6.67 K

(ppm

)

0 40 80 120

400

600

T (K )

T C1%

Y Ba2Cu3O 7 Ca20%

7 K (p

pm)

JB, 2001

Impurity effects in the superconducting stateMagnetic effect on 1st near neighbor Cu of non magnetic Li

40350 40400 40450 40500 405500.0

0.5

1.0

xp=0 %

xp=6%xp=3%

xp=1.5%

Inte

nsity

(arb

itrar

y un

its)

ν (kHz)

Impurity effects in the superconducting state

Induced polarization by non Magnetic Zn or magnetic Ni

In-plane 17O NMR in YBaCuO7 (optimal doping) with Zn at T=10K

Ouazi et al., 2005

Impurity effects in the superconducting state

Induced polarization by non Magnetic Zn or magnetic Ni

In-plane 17O NMR in YBaCuO7 (optimal doping) with Zn at H = 7 Tesla

0 1 2 3 4 5 6 70

50

100

150

200

250

Zn in-plane concentration xP (%)

17O

full

wid

th a

t hal

f max

imum

(kH

z) field-independent contribution field-dependent contribution μSR vortex lattice field distribution

0 20 40

0

1000

2000

Temperature (K)

Zn : 1.5 %Zn : 3 %Zn : 6 %Ni : 2.25 %

ΔνH -

ΔνL (p

pm)

0 10 20 30 40 50 1000

1000

2000 Zn : 1.5 %Zn : 3 %Zn : 6 %Ni : 2.25 %

Temperature (K)

ΔνL (p

pm)

4 0 3 5 0 4 0 4 0 0 4 0 4 5 0 4 0 5 0 0 4 0 5 5 0

0 .5

1 .0

x p = 3 %

Inte

nsity

(arb

itrar

y un

its)

ν (k H z )

ΔνL ΔνH

ΔνH - ΔνLΔνL

Induced polarization by non Magnetic Zn or magnetic Ni below TC

0 10 20 30 40 50 1000

1000

2000 Zn : 1.5 %Zn : 3 %Zn : 6 %Ni : 2.25 %

Temperature (K)

Δν L (p

pm)

Induced polarization by non Magnetic Zn or magnetic Ni below TC

ΔνL

Staggered magnetization survives below TC for Zn and Ni

• Zn : no T-dependence – saturation of ξ ∼ 3 cells• Ni : Curie-Weiss T-dependence – local moment on Ni in the Ni case at least, coexistence between superconductivity and staggered magnetism

xp=3%

ΔνL ΔνH

Induced polarization by non Magnetic Zn or magnetic Ni below TC

New low temperature asymetry for Zn, not for Ni

0 20 40

0

1000

2000

Temperature (K)

Zn : 1.5 %Zn : 3 %Zn : 6 %Ni : 2.25 %

ΔνH -

ΔνL (p

pm)

ΔνH - ΔνL

Ouazi et al., 2005

• Local Space-Varying Density of States effect near EF

• Confirmation of STM measurements, in YBaCuO bulk

• T-dependence : sharp decrease when increasing T

• BCS + unitary scattering (Hirschfeld, Balatsky, Salkola, Flatté ...)

• RPA (Ohashi)

• t-J + d-wave superconductivity (Wang & Lee)

• Kondo (Povkolnikov, Vojta, Sachdev)

Some models for an impurity in the superconducting state

Normal state of cuprates : • Pseudogap analogous to low dimension

insulating spin systems : strong correlations• Optimal doping : still some correlations• Quantitative estimate of the polarization :

strong constraint for any microscopic model

Superconducting state : • Many features typical of an anisotropic BCS

superconductivity, especially DOS effects• Zn and Ni Induced moments very similar to

normal state• Coexistence of staggered moments and

superconductivity (Ni case)

Conclusions