Spinless Impurities in High-TcCuprates: Kondo-Like Behavior

Abstract

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...

Pseudogap Fermi-Bose Kondo Model

“…In contrast, spin fluctuations grow upon underdoping in all cuprates. Therefore, we propose the suppression of Kondo screening as observed in NMR [17] to be well explained within the pseudogap Fermi-Bose Kondo model with a doping-dependent host spin gap.…”

“…with Zn replacing Cu [16,17] -here the non-magnetic Zn induces a quasi-free magnetic moment in its vicinity, which can be related to the properties of the parent Mott insulator [18]. In particular, NMR experiments indicate Kondo screening of Zn-induced moments in optimally doped YBa 2 Cu 3 O 7−δ below the superconducting T c , but show that screening is suppressed in underdoped samples [17]. So far, theoretical analysis has been restricted to either a moment interacting with bulk spin fluctuations [10] or a moment interacting with Bogoliubov quasiparticles [6,13,19].…”

Multichannel Pseudogap Kondo Model: Large-NSolution and Quantum-Critical Dynamics

“…This is the case in so-called pseudogap systems with a power-law DOS ρ(ǫ) ∼ |ǫ| r (r > 0) which arises in one-dimensional interacting systems, in certain zero-gap semiconductors, and in systems with long-range order where the order parameter has nodes at the Fermi surface, e.g., p-and dwave superconductors (r = 2 and 1). Of special interest are high-T c cuprate superconductors, where indeed nontrivial Kondo-like behavior has been observed associated with the magnetic moments induced by non-magnetic Zn impurities [7].The pseudogap Kondo problem has attracted a lot of attention during the last decade. A number of studies [2,3] including the initial work by Withoff and Fradkin employed a slave-boson large-N technique; further progress and insight came from numerical renormalization group (NRG) calculations [4][5][6] and the local moment approach [8].…”

“…This is the case in so-called pseudogap systems with a power-law DOS ρ(ǫ) ∼ |ǫ| r (r > 0) which arises in one-dimensional interacting systems, in certain zero-gap semiconductors, and in systems with long-range order where the order parameter has nodes at the Fermi surface, e.g., p-and dwave superconductors (r = 2 and 1). Of special interest are high-T c cuprate superconductors, where indeed nontrivial Kondo-like behavior has been observed associated with the magnetic moments induced by non-magnetic Zn impurities [7].…”

Local Moment Formation in the Superconducting State of a Doped Mott Insulator

“…The local electronic structure near non-magnetic impurities in high-T c superconductors has attracted wide attention recently. A series of NMR experiments in YBa 2 Cu 3 O 7−x (YBCO) have shown that with the substitution of nonmagnetic Zn/Li for the Cu atoms in the CuO 2 plane, an S = 1/2 staggered magnetic moment is generated on the Cu ions in the vicinity of the impurity sites [1,2,3,4,5,6,7,8]. Below the superconducting transition temperature, the moment is partially screened in optimally doped YBCO, but remains unscreened down to the lowest temperatures in the underdoped regime [3,2].…”