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 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...