The present topical review focuses on recent advances concerning an intriguing phenomenon in condensed matter physics, the scattering of conduction electrons at the localized spin of a magnetic impurity: the Kondo effect. Spectroscopic signatures of this effect have been observed in the past by high-resolution photoemission which, however, has the drawback of averaging over a typical surface area of 1 mm(2). By combining the atomic-scale spatial resolution of the scanning tunneling microscope (STM) with an energy resolution of a few tens of mu eV achievable nowadays in scanning tunneling spectroscopy (STS), and by exposing the magnetic adatom to external magnetic fields, our understanding of the interaction of a single magnetic impurity with the conduction electrons of the nonmagnetic host has been considerably deepened. New insight has emerged by taking advantage of quantum size effects in the metallic support and by decoupling the magnetic adatom from the supporting host metal, for instance by embedding it inside a molecule or by separating it by an ultrathin insulating film from the metal surface. In this way, Kondo resonances and Kondo temperatures can be tailored and manipulated by changing the local density of states of the environment. In the weak coupling limit between a Kondo impurity and a superconductor only a convolution of tip and sample DOS is observed while for strongly coupled systems midgap states appear, indicating superconducting pair breaking. Magnetic impurities with co-adsorbed hydrogen on metallic surfaces show pseudo-Kondo resonances owing to very low-energy vibrational excitations detected by inelastic tunneling spectroscopy. One of the most recent achievements in the field has been the clarification of the role of magnetic anisotropy in the Kondo effect for localized spin systems with a spin larger than S = 1/2.

https://iopscience.iop.org/article/10.1088/0953-8984/21/5/053001/pdf

Spectroscopic manifestations of the Kondo effect on single adatoms.

This review presents systematically most of the experimental data which have been obtained from measurements of bulk properties of alloys and intermetallic compounds containing atoms of transition metals, rare earths or actinides which can be considered as non-interacting with each other. Criteria for the observation of effects due to non-interacting atoms are given. Data are reported from measurements of magnetic properties, electrical resistivity, specific heat, superconductivity, thermoelectric power, thermal conductivity, thermal expansion and compressibility, de Haas-van Alphen and Hall effects and optical properties. Systematic trends in experimental behaviour can be observed in many of these data, and using this observation experimental parameters are defined which are used to classify the data before attempting a comparison with theories.

https://iopscience.iop.org/article/10.1088/0034-4885/37/2/001/pdf

Formation of localized moments in metals: experimental bulk properties

The macroscopic and local properties of 3d transition metal impurities in normal metals are reviewed and compared with the theoretical situation in this field. The parameters of the Anderson and s-d exchange models are derived from direct and indirect experimental data using as a guide the Hartree-Fock approximation of the non-degenerate Anderson model. The basic observations about the magnetic-non-magnetic transition, and the behaviour of the magnetic, thermal and transport properties when going through the transition region are demonstrated for specific examples. A detailed comparison between the present status of theory and experiment is performed by inspecting the large body of experimental data of two typical alloys, which served as testing materials for the development of the existing theories. CuFe is often regarded as a typical ‘yes moment’ system, and the experiments are therefore compared with the predictions based on the s-d exchange model; in the case of AlMn, the spin-fluctuation con...

Experimental evidence for many-body effects in dilute alloys

Des solutions solides d'elements de transition dans des matrices de cuivre, de nickel, de cobalt, de fer et d'aluminium ont ete analysees par emission ionique secondaire. Un phenomene d'exaltation de la production des ions des elements solutes est mis en evidence. Cet effet est tout particulierement important pour le titane dans le cuivre et le chrome dans le nickel.

https://hal.archives-ouvertes.fr/jpa-00208146/document

Évolution des rendements de l'émission ionique des alliages avec la nature du soluté - Première partie : Résultats expérimentaux

The occurrence of localized magnetic moments in dilute alloys is a consequence of strong electron-electron correlation effects in the case of localized electronic states. The importance of this topic derives, at least in part, from its close relationship to the complicated problem of bulk magnetism in metals. Unfortunately, despite its intuitively simpler structure, the magnetic impurity problem continues to defy a satisfactory solution. Traditionally, the problem has been divided into two distinct parts. The first deals with the conditions under which a local moment can form; the second is concerned with the properties of the moment.

Magnetic hyperfine interaction studies of magnetic impurities in metals

A simple model is used to describe the magnetic properties of cobalt impurities in copper. Isolated impurities are nonmagnetic with a high spin-fluctuation temperature. Isolated pairs of neighbor cobalt atoms are very nearly magnetic: This gives rise to a large susceptibility and a large electronic specific heat, varying as the square of the concentration $C$. At low temperatures, only the group of three (or more) cobalt atoms carries a magnetic moment.

A. Blandin

Papers

Influence of Interactions Between Impurities on the Appearance of Magnetic Moments in Copper-Cobalt Dilute Alloys