Exciton effects in a scaling theory of intermediate valence and Kondo systems

An interplay of the Kondo scattering and exciton effects (d-f Coulomb interaction) in intermediate valence systems and Kondo lattices is demonstrated to lead to an essential change of the scaling behavior in comparison with the standard Anderson model. In particular, a marginal regime can occur where characteristic fluctuation rate is proportional to flow cutoff parameter. In this regime, the “Kondo temperature” itself is strongly temperature-dependent, which may give a key to the interpretation of controversial experimental data for heavy fermions and related systems.     

On the theory of superconductivity in Kondo lattice systems

An attempt is made to explain the occurrence of superconductivity in Kondo lattice systems with special reference to CeCu₂Si₂. Starting point is the Fermi liquid approach. It is generalized from a Kondo impurity to the Kondo lattice by means of the Korringa-Kohn-Rostocker method. From it a hybridization model is derived and discussed in detail. Two electron-phonon mechanisms are investigated which appear in Kondo lattices. One results from the additional phase shifts caused by the Kondo ions while the other is responsible for the so-called Kondo volume collapse. It is shown that the latter is sufficiently strong in order to explain why CeCu₂Si₂ is a superconductor while LaCu₂Si₂ is not. An estimate for the superconducting transition temperatureT _c produces the right order of magnitude.     

Determination of the orbital moment and crystal-field splitting in LaTiO3

Utilizing a sum rule in a spin-resolved photoelectron spectroscopic experiment with circularly polarized light, we show that the orbital moment in LaTiO3 is strongly reduced from its ionic value, both below and above the Ne el temperature. Using Ti L2,3 x-ray absorption spectroscopy as a local probe, we found that the crystal-field splitting in the t2g subshell is about 0.12-0.30 eV. This large splitting does not facilitate the formation of an orbital liquid.     

Spin current Kondo effect in frustrated Kondo systems

Magnetic frustrations can enhance quantum fluctuations in spin systems and lead to exotic topological insulating states.When coupled to mobile electrons,they may give rise to unusual non-Fermi liquid or metallic spin liquid states whose nature has not been well explored.Here,we propose a spin current Kondo mechanism underlying a series of non-Fermi liquid phases on the border of Kondo and magnetic phases in a frustrated three-impurity Kondo model.This mechanism is confirmed by renormalization group analysis and describes movable Kondo singlets calledholonsinduced by an effective coupling between the spin current of conduction electrons and the vector chirality of localized spins.Similar mechanisms may widely exist in all frustrated Kondo systems and be detected through spin current noise measurements.     

Variational local moment approach: From Kondo effect to Mott transition in correlated electron systems

The variational local moment approach (VLMA) solution of the single impurity Anderson model is presented. It generalizes the local moment approach of Logan et al. by invoking the variational principle to determine the lengths of local moments and orbital occupancies. We show that VLMA is a comprehensive, conserving and thermodynamically consistent approximation and treats both Fermi and non-Fermi liquid regimes as well as local moment phases on equal footing. We tested VLMA on selected problems. We solved the single- and multi-orbital impurity Anderson model in various regions of parameters, where different types of Kondo effects occur. The application of VLMA as an impurity solver of the dynamical mean-field theory, used to solve the multi-orbital Hubbard model, is also addressed.     

Local moment spin resonance in a Kondo system

The ESR of the system $\text{Yb:}\text{Au}{}_{\mathrm{x}}\text{Ag}{}_{\mathrm{1?x}}$ has been investigated for 0.7 ? x ? 1. The g-shift (Δg) and the temperature derivative of the linewidth $\text{(}\text{ΔH}\text{ΔT}\text{)}$ increase with decreasing Au concentration. For T ? 2°K effects in $\text{(}\text{ΔH}\text{ΔT}\text{)}$ are observed, which are attributed to the Kondo effect.     

Superconductivity in charge Kondo systems

We present a theory for superconductivity and charge Kondo fluctuations, i.e., resonant quantum valence fluctuations by two charge units, for Tl-doped PbTe. We show that Tl is very special as it first supplies a certain amount of charge carriers to the PbTe-valence band and then puts itself into a self-tuned resonant state to yield a new, robust pairing mechanism for these carriers.     

Magnetic excitations in crystal-field split 4f systems

A survey is given of magnetic excitations in metallic 4f systems when the crystal-field splitting is comparable to or larger than the magnetic interaction between different sites. Particular emphasis is put on the interaction of crystal-field split rare-earth ions with conduction electrons. A number of physical effects is discussed which result from that interaction. The modifications of the magnetic excitations are described which result when the coupling of phonons to the crystal-field levels is so strong that a bound state between the two types of excitations is formed. For a study of these phenomena the method of neutron scattering is an indispensable tool. For that reason it plays a central part in this review.     

Line width of crystal-field excitations in metallic rare-earth systems

A theory is developed for the line width of crystal-field excitations in metallic rare-earth systems were the damping is due to conduction electron-hole excitations. First the single ion or dilute alloy case is studied. As an example the theory is applied to Ce in LaAl₂ and compared with recent experiments. Then the case of a regular lattice of rare-earth ions is considered. The theory is applied to the ₁ – ₄ system. Special attention is paid to the behaviour of the central peak near an induced-moment phase transition. It is shown that the dynamics of the conduction electrons leads to observable effects in the centralpeak intensity and can not be neglected as it is usually done. The theoretical predictions are in agreement with recent experimental findings on Pr₃Tl.     

Two Kondo impurities in nanoscopic systems

We study the problem of two Kondo impurities in a small system. Using a slave boson approach we investigate the effect on the electron confinement of the Kondo physics of the two impurity problem. We show that the confinement splits the symmetric and antisymmetric channels and for small systems weakly coupled to a reservoir this gives two well defined behaviors: For the Fermi energy lying at a resonant state, the two impurities are Kondo screened with two characteristic energy scales. For the Fermi energy between two resonances, the inter-impurity interaction destroys the Kondo effect.Received: 2 February 2004, Published online: 28 May 2004PACS: 72.10.Fk Scattering by point defects, dislocations, surfaces, and other imperfections (including Kondo effect) - 72.15.Qm Scattering mechanisms and Kondo effect     

The Kondo effect in periodic narrow-band systems

The Kondo divergences owing to interaction of current carriers with local moments in highly correlated electron systems are considered within the Hubbard and s-d exchange models with infinitely strong on-site interaction, the many-electron Hubbard representation being used. The picture of density of states containing a peak at the Fermi level is obtained. Various forms of the self-consistent approximation are used. The problem of the violation of analytical properties of the Green's function is discussed. Smearing of the “Kondo” peak owing to spin dynamics and finite temperatures is investigated. Received 25 November 1999 and Received in final form 31 January 2000     

Critical theory of overscreened Kondo fixed points

The low-temperature fixed point of the Kondo model, for k bands and a spin-s impurity, is well understood by Nozières' Fermi liquid theory for k 2s. However when k > 2s, a new type of non-trivial fixed point is known to occur. We study this fixed point using higher-level Kac-Moody conformal field theory and Cardy's approach to boundary critical phenomena. The specific heat and magnetization are shown to be determined by the leading irrelevant operator and the corresponding critical exponents are obtained exactly. The Wilson ratio is argued to be universal and its exact value is also calculated. The asymptotic finite-size spectrum is determined. Thermodynamic exponents agree precisely with the Bethe ansatz; for k = 2, S = 1/2, the Wilson ratio also agrees well with the approximate value obtained from the Bethe ansatz; the slope of the β-function agrees with the perturbative result in the large-k limit and the finite-size spectrum agrees excellently with approximate results obtained previously by Wilson's numerical renormalization group method in the case k = 2, S = 1/2.     

Kondo impurities in nanoscopic and mesoscopic systems

We present results for Kondo impurities in nanoscopic systems. Using Wilson's numerical renormalization group we analyze two different situations: an isolated system with a discrete spectrum of well-defined energy levels and a fixed number of electrons and a nanoscopic system weakly coupled to a macroscopic reservoir. In the latter case, new regimes not observed in macroscopic homogeneous systems are induced by the confinement of conduction electrons. These new confinement-induced regimes are very different depending on whether the Fermi energy is at resonance or between two quasi-bound states.     

Scattering theory of Kondo mirages and observation of single Kondo atom phase shift

We explain the origin of the Kondo mirage seen in recent quantum corral scanning tunneling microscope experiments with a scattering theory of electrons on the surfaces of metals. Our theory, combined with experimental data, provides a direct observation of a single Kondo atom phase shift. The Kondo mirage observed at the empty focus of an elliptical quantum corral is shown to arise from multiple electron bounces off the corral wall adatoms. We demonstrate our theory with direct quantitive comparison to experimental data.     

Kondo insulators in the periodic Anderson model: a local moment approach

The symmetric periodic Anderson model is well known to capture the essential physics of Kondo insulator materials. Within the framework of dynamical mean-field theory, we develop a local moment approach to its single-particle dynamics in the paramagnetic phase. The approach is intrinsically non-perturbative, encompasses all energy scales and interaction strengths, and satisfies the low-energy dictates of Fermi liquid theory. It captures in particular the strong coupling behaviour and exponentially small quasiparticle scales characteristic of the Kondo lattice regime, as well as simple perturbative behaviour in weak coupling. Particular emphasis is naturally given to strong coupling dynamics, where the resultant clean separation of energy scales enables the scaling behaviour of single-particle spectra to be obtained. Received 19 December 2002 Published online 14 March 2003