In materials with strong local Coulomb interactions, simple defects such as atomic substitutions strongly affect both macroscopic and local properties of the system. A nonmagnetic impurity, for instance, is seen to induce magnetism nearby. Even without disorder, models of such correlated systems are generally not soluble in 2 or 3 dimensions, and so few exact results are known for the properties of such impurities. Nevertheless, some simple physical ideas have emerged from experiments and approximate theories. Here, we first review what we can learn about this problem from 1D antiferromagnetically correlated systems. We then discuss experiments on the high Tc cuprate normal state which probe the effect of impurities on local charge and spin degrees of freedom, and compare with theories of single impurities in correlated hosts, as well as phenomenological effective Kondo descriptions. Subsequently, we review theories of impurities in d-wave superconductors including residual quasiparticle interactions, and compare with experiments in the superconducting state. We argue that existing data exhibit a remarkable similarity to impurity-induced magnetism in the 1D case, implying the importance of electronic correlations for the understanding of these phenomena, and suggesting that impurities may provide excellent probes of the still poorly understood ground state of the cuprates.Comment: 66 pages, 48 figures, review articl

We report NMR shift AK and T\ data of 89 Y taken from 77 to 300 K in YBa 2 Cu 3 0 6 +x for 0.35 < x < 1, from the insulating to the metallic state. A Korringa law and therefore a Fermi-liquid picture is found to apply for the spin part K 5 of AK. The spin contribution x*ix 9 T) to Xm is singled out, as the T variation of AK scales linearly with the macroscopic susceptibility # m . This implies that Cu(3

Rullier-Albenque, Alloul, and Tourbot Reply: Openov [1] agrees that the Abrikosov-Gor'kov (AG) theory for d-wave superconductivity does not apply to our accurate data on the T c depression induced by controlled disorder [2]. He suggests that inclusion of an s-wave contribution can explain the quasilinear decrease of our data. As already detailed in the references 8, 12, and 13 of [2], the AG theory fails to explain the initial slope of the T c decay. This slope is fixed by the value of the plasma frequency ! p and a consensus is now well established from the numerous optical data for ! p 1:2 0:2 eV for YBCO 7 [3]. To obtain a quasilinear decay one needs to introduce a significant s-wave component which is maintained when the d-wave one has been killed, assuming a weak spin scattering. Although the orthorhombic structure of YBCO 7 favors s d wave admixture, the upper limit of the s-wave component quoted is 20% [4] (corresponding to 0:93). Thus to get 0:9, Openov is forced to use a value of ! p smaller than that given by optical data. Obviously, it is possible to play again with such mathematical fits to also explain the data for YBCO 6:6 . But, as the fraction of s-wave component should be reduced for this hole content, it seems to us even more difficult to find for YBCO 6:6 a set of physically sound parameters compatible with those taken above and resulting as well in a quasilinear decrease of T c .Therefore alternative approaches must be considered in the case of cuprates. Let us mention that, as nonmagnetic impurities induce a magnetic response [5], the relative contributions of spin and potential scattering in these correlated electron systems cannot be treated classically. Also, as noted in [2], the upturns in the ab T curves is not taken into account in the AG approach.Let us come now to the final point of Openov, who doubts the actual validity of our analysis of the transition width T c , which is a good estimate of the macroscopic homogeneity of the samples. For a chemical substitution such as Zn, T c increases much faster [6] than in [2] (see the inset of Fig. 1). This is natural, as increasing Zn content brings us towards the solubility limit, hence a macroscopic inhomogeneity of the sample. For our electron irradiation experiment, as explained in [2], the spatial distribution of defects is particularly homogeneous. We proposed that the width of the transition is given by T c ÿa ab dT c =d ab [7]. Openov questions our model as, for him, the T c maximum is not compatible with the linear variation of T c with ab . He probably overlooked our data which do curve slightly when T c approaches zero. The two data sets are totally consistent since as shown in [2] the Emery-Kivelson (EK) equation (Ref. 10 in [2]) fits reasonably both variations of T c and T c with ab :. We demonstrate in Fig. 1 this consistency without any model for T c ab , just by integrating numerically the data for T c , using the equation above with the single adjustable parameter a 0:06.

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We compare the effects of in-plane non magnetic Li + and Zn 2+ impurities on the normal state of high-Tc cuprates. 89 Y NMR shows that the extra hole introduced by Li is not localized in its vicinity. The Tc depression and induced moments on near neighbour Cu sites of Zn or Li are found identical. These effects of spinless impurities establish the major influence of the spin perturbation with respect to the charge defect. The susceptibility of the induced moment measured by 7 Li NMR displays a 1/(T+Θ) behavior. Θ increases with doping up to about 200 K in the overdoped regime. We attribute this to a "Kondo like" effect.Increasingly, impurities are used to probe the magnetic properties of correlated systems. For instance, in cuprates, substitution of the Cu sites of the CuO 2 planes directly reveals the existence of magnetic correlations in the planes and probes their interplay with superconductivity. In particular, Zn 2+ substitution has been studied thoroughly because it was unexpectedly found to strongly affect both the normal and superconducting states. Above T c , in a metallic picture, Zn 2+ should only weakly affect both magnetism and transport properties; the former because it is a spinless impurity and the latter because it has the same charge as Cu 2+ . In contrast, Zn acts as a very strong scattering center [1]. The fact that T c is depressed by this scattering is primarily a consequence of the now well established d-wave anisotropy of the superconducting order parameter [2]. Furthermore, Zn induces local magnetic moments on its near neighbor (n.n.) coppers, as shown by NMR [3] and macroscopic SQUID measurements [4]. Zn, as a spin vacancy, creates indeed a perturbation of the local antiferromagnetic correlations, as also observed in undoped low dimensional spin chains or ladders [5]. Such effects were anticipated on theoretical grounds [6] [7]. However, until now no experiment could clearly expose the relation between the magnetic correlations and the scattering effects on T c . Another interesting problem is the evolution of these anomalies with hole doping. Recent macroscopic experiments showed that the local moment susceptibility falls rapidly, though it still exists at optimal doping [4]. Such local moments have also been found in Al 3+ substituted LaSrCuO at optimal doping, despite some qualitative differences with Zn [8]. In this later work, NMR of 27 Al itself was used to probe locally the susceptibility of its n.n. Cu sites. However, no experiment has yet been dedicated to probing the evolution of this moment into the overdoped regime. Such an experiment should help to clarify wether the cuprates exhibit an uncorrelated Fermi Liquid behavior at high doping.In order to address both problems, we have undertaken a study of Li in YBaCuO which substitutes within the CuO 2 planes [9]. Li + is not magnetic like Zn 2+ but has a different valence. Comparing the local magnetism and the effect of T c between Li + and Zn 2+ will elucidate the respective roles of charge and spin in the impurity response of...

TV and T/v (Neel) have been measured for a series of YBa2(Cuo.96Zno.o4)306+x samples. The T variations of the homogeneous susceptibility Xs of the Cu02 planes, given by the shift of the 89 Y NMR line, are found to be nearly unchanged with respect to pure samples for x > 0.5, which implies that the charge transfer is negligibly modified by Zn, and that the magnetic pseudogap is not associated with superconducting pairing. Detection of an unusual Curie contribution to the 89 Y NMR width for x > 0.5 provides evidence that Zn induces magnetic moments in the CuC>2 planes, which play a role in the depression of 7V. PACS numbers: 74.70.Hk, 75.20.Hr, 75.30.Kz, 76.60.Cq The sizable reduction of T c induced by Zn substitutions in cuprates has stimulated a large number of experimental studies [1] to try to understand the changes of the electronic properties induced by Zn. A major problem is to clarify whether the hole content of the conducting CUO2 planes is modified in the presence of zinc. We insist here on the use of microscopic magnetic probes [2,3], such as the 89 Y NMR, which allows a separation of the modifications of the 1:2:3 phase from those associated with the existence of minute amounts of impurity phases. In the course of a detailed study on the phase diagram of pure YBa2Cu3C>6+jc we have evidenced that Zn does not induce an appreciable reduction of Tj\ in YBa2Cu306 [2]. The present work is an attempt to compare the modifications of the magnetic properties over the whole range of oxygen content. We shall see here that both T c and T/v are very sensitive to oxygen doping. This will allow us, among other things, to resolve an apparently conflicting experimental report from Mossbauer data [4] indicating that JT/V is largely reduced by Zn substitution, in YBa2Cu 3 06. But the main concern of this Letter is to provide indications of the changes of the magnetic properties in the metallic state. We show here that the phase diagram versus oxygen content and the determination of the uniform susceptibility % s of the CuC>2 planes from 89 Y NMR data give direct evidence that the charge-transfer processes are not markedly modified by Zn in the metallic state. We rather clearly demonstrate for the first time on microscopic grounds that Zn induces local moments which are certainly responsible for a pair-breaking mechanism or an electronic localization. The fact that the moments are localized in the Q1O2 planes is demonstrated here by comparison with data taken on Co-substituted samples.

The synthesis of a unique isotope engineered system, double-wall carbon nanotubes with natural carbon outer and highly 13C enriched inner walls, is reported from isotope enriched fullerenes encapsulated in single-wall carbon nanotubes (SWCNTs). The material allows the observation of the D line of the highly defect-free inner tubes that can be related to a curvature induced enhancement of the electron-phonon coupling. Ab initio calculations explain the inhomogeneous broadening of inner tube Raman modes due to the distribution of different isotopes. Nuclear magnetic resonance shows a significant contrast of the isotope enriched inner SWCNTs compared to other carbon phases and provides a macroscopic measure of the inner tube mass content. The high curvature of the small diameter inner tubes manifests in an increased distribution of the chemical shift tensor components.

59 Co NMR spectra in oriented powders of the superconducting (HSC) Na0.35CoO2,1.3H2O and Na0.35CoO2 compounds reveal a single electronic Co state with identical T independent NMR shift tensor. These phases differ markedly from Na0.7CoO2, in which we resolve 3 types of Co sites. The large T variation of their spin susceptibilities χs and the anisotropy of the orbital susceptibility χ orb allow us to conclude that charge disproportionation occurs, in a non magnetic Co 3+ and two magnetic sites with about 0.3 and 0.7 holes in the t2g multiplet. The data are consistent with those for the single Co site in the anhydrous and HSC phase assuming the expected Co 3.65+ charge.