Impurity effects in high T C superconductors

Transcription

1 Impurity effects in high T C superconductors J. Bobroff, S. Ouazi, H. Alloul, P. Mendels, N. Blanchard Laboratoire de Physique des Solides, Université Paris XI, Orsay G. Collin J.F. Marucco, V. Guillen D. Colson LLB, CEA-Saclay LEMHE, Université Paris XI SPEC, Saclay

2 High T C cuprates T pseudogap Antiferro magnetism Superconductivity doping metallicity + correlations + exotic superconductivity

3 Impurity effects in cuprates Impurity in more «simple» systems in a metal in a BCS superconductor in a correlated insulator Impurity in High T C 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

4 A magnetic impurity in a metal RKKY spin polarization of the conduction carriers Kondo effect χ CuFe AuCo 1 2 Temperature Madhavan et al.

5 A magnetic impurity in an isotropic BCS superconductor Decrease of T C only if it is magnetic (Abrikosov, Gork ov) Possible local bond states NbMn or NbAg di/dv (1-7 Ω -1 ) Yazdani et al.

6 An impurity in an insulating correlated system example : Y 2 BaNiO 5 as a spin 1 Ni chain with Haldane gap Ni Ni Ni Ni Ni Ni Ni Zn Ni Ni Ni Ni Ni impurity Intensité (unités arbitraires) Y 2 Ba Ni 98% Zn 2% O 5 Zn 2% pur ν (khz) T = 2 K NMR spectrum

7 An impurity in an insulating correlated system 4 Zn QMC computation Cu Zn J=26 K δ< S z i >/< S z > u -4 T=1 K Cu S=1/ position J J J J =.1 à.2 J <S Z (i)> ~ e -r/ξ(t) / T < S i.s j > ~ e -r/ξ Das et al., 24 Tedoldi et al., 2

8 Some other low dimension spin systems Spin 1 Chain Spin ½ Chain Gap ξ < 6 ξ~1/t AF? No order Spin-Peierls Chain 2-leg ladder Gap Dimer isation Gap ξ<3 AF order AF order Common mechanism : breaking of a singlet

9 Non magnetic impurity effects in cuprates The normal state T pseudogap Antiferro magnetism Superconductivity doping

10 Why choosing YBaCuO? YBaCuO khz

11 Impurity effects on doping and pseudogap.14 YBaCuO 6.6 K s (%) pure Zn : x plane = 3% Zn : x plane = 6 % T (K) No effect on doping No effect on pseudogap

12 Impurity effects on macroscopic properties Effect on resistivity : Zn : strong scattering center Ni : weaker Effect on macroscopic susceptibility : (Chien, 1991) Zn induces an effective paramagnetic moment S<1/2 which decreases with increasing doping Ni induces a moment S=1 (Mendels, 1994,1999; Zagoulaiev, 1995 )

13 Nonmagnetic Impurity in Pseudogap regime K (ppm) x =.85 % / plane 7 Li x = 1.8 % / plane T (K) 89 K (ppm) ers voisins 2 èmes voisins 89 Y 17 Δν (khz) O raie centrale T (K) T (K) Alloul,91;Walstedt,93;Mahajan,94; Williams,95; Ishida,97; JB,97,99; Julien,2; Itoh,23; Ouazi, 24

14 Nonmagnetic Impurity in Pseudogap regime Multi-nuclei quantitative analysis of the induced staggered polarization S z (r).6.4 Spatial dependence T = 85K ξ (cell units) Extension ξ impurity Distance to the impurity (unit cells) 1 15 Temperature (K) Analogy with spin chains Theoreticall justifications : t-j (Poilblanc) RVB (Khaliullin, Gabay, Nagaosa & Lee) Ouazi, 24

15 Effect of hole doping on the induced polarization dopage S z (r) Spatial dependence T = 85K ξ imp (unit cells) Distance to the impurity (unit cells) Extension Température (K) persistence of corrélations at optimal doping NAFL, SCR (Bulut, Ohashi)

16 Effect of hole doping on the induced polarization dopage 7 K (ppm) Susceptibility on 1 st near neighbor Cu YBa 2 Cu 3 O 6.97 x =.85 % / plane x = 1.8 % / plane YBa 2 Cu 3 O T (K) 8 4 Fit in C/(T+Θ) C (MHz.K) Temperature (K) C Θ oxygen doping JB, 1999

17 Impurity effects in cuprates The superconducting state T pseudogap Antiferro magnetism Superconductivity doping

18 Impurity effects in the superconducting state Effect on T C : only dependent on scattering potential (d-wave) (Rullier-Albenque, 2) Effect on penetration depth : λ increases with impurity content, n s decreases with impurity content (Bonn, 1993;Bernhard, 1996)

19 Impurity effects in the superconducting state Effect on local Density Of States close to Zn far from Zn Bi Zn T = 4.2 K Pan, Nature 2 Hudson, Nature 21 Effect on magnetic response χ (Q AF,ω) Zn 2% pur Sidis, 1996

20 Impurity effects in the superconducting state Magnetic effect on 1 st near neighbor Cu of non magnetic Li 3 T C 1% YBa 2 Cu 3 O 6.6 K (ppm) Li 1% Li 2% K (ppm) % T C 1% T C YBa 2 Cu 3 O 6.97 K (ppm) YBa 2 Cu 3 O 7 Ca 2% T C 1% T (K) JB, 21

21 Impurity effects in the superconducting state Induced polarization by non Magnetic Zn or magnetic Ni 1. Intensity (arbitrary units).5. x p = % x p =1.5% x p =6% x p =3% ν (khz) In-plane 17 O NMR in YBaCuO 7 (optimal doping) with Zn at T=1K Ouazi et al., 25

22 Impurity effects in the superconducting state Induced polarization by non Magnetic Zn or magnetic Ni 17 O full width at half maximum (khz) field-independent contribution field-dependent contribution μsr vortex lattice field distribution Zn in-plane concentration x P (%) In-plane 17 O NMR in YBaCuO 7 (optimal doping) with Zn at H = 7 Tesla

23 Induced polarization by non Magnetic Zn or magnetic Ni below T C Intensity (arbitrary units) 1..5 Δν L Δν H x p =3% ν (khz) Δν L Δν H - Δν L Δν L (ppm) 2 1 Zn : 1.5 % Zn : 3 % Zn : 6 % Ni : 2.25 % Δν H - Δν L (ppm) 2 1 Zn : 1.5 % Zn : 3 % Zn : 6 % Ni : 2.25 % Temperature (K) 2 4 Temperature (K)

24 Induced polarization by non Magnetic Zn or magnetic Ni below T C 2 Δν L Zn : 1.5 % Zn : 3 % Zn : 6 % Ni : 2.25 % Δν L Δν H x p =3% Δν L (ppm) Temperature (K) Staggered magnetization survives below T C 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

25 Induced polarization by non Magnetic Zn or magnetic Ni below T C Δν H - Δν L Δν H - Δν L (ppm) 2 1 Zn : 1.5 % Zn : 3 % Zn : 6 % Ni : 2.25 % 2 4 Temperature (K) Ouazi et al., 25 New low temperature asymetry for Zn, not for Ni Local Space-Varying Density of States effect near E F Confirmation of STM measurements, in YBaCuO bulk T-dependence : sharp decrease when increasing T

26 Some models for an impurity in the superconducting state BCS + unitary scattering (Hirschfeld, Balatsky, Salkola, Flatté...) RPA (Ohashi) t-j + d-wave superconductivity (Wang & Lee) Kondo (Povkolnikov, Vojta, Sachdev)

27 Conclusions 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)