Persistence of Li Induced Kondo Moments in the Superconducting State of Cuprates

Abstract

Using7 Li NMR shift data, the anomalous local moment induced by spinless Li impurities persists below Tc in YBa2Cu3O6+y. In the underdoped regime, the moments retain their Curie law below Tc. In contrast, near optimal doping, the large Kondo screening observed above Tc (TK = 135 K) is strongly reduced below Tc as expected theoretically when the superconducting gap develops. The limited spatial extent of the induced moment (on 1 st near neighbour Cu) is not drastically modified below Tc, which allows a comparison with STM determination of the local density of states. Our results constrain theoretical models of the impurity electronic properties.The influence of impurities on superconductors has always been used as an effective probe of their actual properties. For example, while magnetic impurities are the most prominent s-wave pair breakers, any type of scattering is detrimental to d-wave superconductivity. To our knowledge, experimental investigation of the modifications of the magnetic properties of an impurity below the superconducting transition T c has never been performed. Macroscopic bulk magnetic experiments are unadapted since the various contributions to the susceptibility cannot easily be singled out below T c . Local measurements of the susceptibility of these moments using hyperfine techniques are in principle possible, but are usually prohibited by technical limitations, such as strong spin lattice relaxation effects for the impurity NMR.Despite this experimental void, an extensive theoretical work has been devoted to this question in classical Fermi liquids [1] [2]. The behavior of the moment below T c is predicted to depend both on the shape of the superconducting gap and on the Kondo temperature T K of the moment in the normal state. T K is a signature of the screening of the moment by the conduction electrons and is related to its coupling J to the carriers. The primary effect, anticipated, but not observed so far, is a reduction of the Kondo screening due to the pairing of the carriers. For small J, this results in a complete restoration at low T of the Curie susceptibility of the moment.In cuprate superconductors, which are correlated electronic systems, the magnetic properties of impurities are more intricate. In the normal state, spinless impurities like Zn We present here the first measurements of the induced moment properties below T c . they are performed using 7 Li NMR since the transferred hyperfine couplings are weak enough that relaxation effects do not prohibit NMR spectroscopy of the moment. We propose a method to extract the susceptibility χ loc probed at Li sites. This requires correcting the internal field seen by Li for screening and vortex effects due to superconductivity. It will then be shown that the Li induced moments survive below T c and that T K is strongly reduced. Furthermore, we will demonstrate that these induced moments remain confined primarily to the 1 st n.n. coppers below T c . Recent scanning tunneling microscopy (STM) experiments in the Zn substituted Bi2212 ...

Spin and Charge Order around Vortices and Impurities in High-TcSuperconductors

“…This indicates the stabilization of the magnetic moments. Our result of the strongest staggered magnetic moments on the four nearest-neighbor Cu sites of the impurity is consistent with the above experiment [25]. This leads us to speculate that the magnetism around vortices and around impurities may share a common origin.…”

“…However, SDW appears to be a robust feature induced by the impurity, which should be observable in neutron scattering experiments. Nuclear magnetic resonance (NMR) measurements have shown that when a Cu 2+ in the Cu-O plane is substituted by a strong nonmagnetic impurity, such as Zn 2+ , an effective magnetic moment can be induced on the Cu sites around the impurity site [22,23,24,25]. More recent NMR measurements [25] show for the first time that near optimal doping, the Kondo screening effect observed above the superconducting transition temperature, is strongly reduced in the superconducting state.…”

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

“…2c. It shows a staggered pattern localized around the impurity in good agreement with that derived from the NMR experiments [2,3]. The majority of the net moment resides on the four nearest neighbors of the impurity.…”

Orbital effect of Cooper pairs on the Kondo effect in unconventional superconductors

“…17 Similar behavior is also observed in Li-substituted YBa 2 Cu 3 O 7−δ . 18 In this case, the local magnetic susceptibility of the Li induced moment shows strong reduction of the Kondo screening below the superconducting transition temperature. At present the search for the influence of unconventional superconductivity on the Kondo effect is still in progress.…”