Ferromagnetic ground state in the Kondo lattice model

We present a series of rigorous examples of the Kondo lattice model that exhibit full ferromagnetism in the ground state. The models are defined in one-, two- and three-dimensional lattices, and are characterized by a range of hopping terms, specific electron filling, and large ferromagnetic coupling. Our examples show that a sufficient strong but finite exchange coupling between conduction electrons and localized spins could overcome the competition from mobility of a finite density of electrons and drive the system from a paramagnetic phase to a ferromagnetic phase. We also establish a relation of ferromagnetism between the Hubbard model and Kondo lattice model. Meanwhile some rigorous results on ferromagnetism in the corresponding Hubbard model are presented. Received: 10 September 1997 / Revised: 15 October 1997 / Accepted: 17 October 1997     

Magnetic field effect on doped Kondo insulators

In this work we study the effect of an applied magnetic field on the metal–insulator transition in Kondo insulators doped with non-rare earth element. In the theoretical formulation of the problem we use a two band model, in which a narrow correlated band is hybridized with a wide band of conduction electrons. The obtained phase diagrams show that, depending on the type of impurities introduced into the Kondo insulators, the external magnetic field and the impurities can either cooperate or compete to the stabilization of the metallic phase.     

On the competition between magnetic order and local Kondo effect in Kondo lattice

We investigate the competition between magnetic order and local Kondo effect in a Kondo lattice model (i.e. the Coqblin-Schrieffer Hamiltonian extended to a lattice) in a mean-field approximation, taking account of the spin-orbit degeneracy of each localized f level. This leads to the definition of a dependent Kondo temperature. We study the Kondo phase and compare its energy with the energies of magnetic phases, when the number of the conduction band electron per site is near one. We present a phase diagram which shows the occurrence of three phases: Kondo, antiferromagnetic and paramagnetic phases. Our model in the mean-field approximation also shows a somewhat flat Kondo temperature, for large values of , as a function of the exchange coupling J between conduction and localized f electrons. Finally we show some scaling effects between and J and we define a corresponding Kondo temperature. Received 21 September 1998 and Received in final form 8 February 1999     

Double-exchange ferromagnetism in the partially-filled one-dimensional Kondo lattice model

We investigate the incompletely saturated ferromagnetic phase which occurs at strong-coupling in the partially-filled one-dimensional (1D) Kondo lattice model. The double-exchange interaction responsible for the ferromagnetic ordering is absent in dilute Kondo systems, and is a missing element in nearly all theoretical treatments of the model. We discuss how: 1) double-exchange arises in the system, even though the Kondo coupling is antiferromagnetic, and show that at strong-coupling it favors an alignment of the spins of unpaired localized moments; and 2) how this determines the ground-state phase diagram, and properties of the localized moments.     

Kondo tunneling through real and artificial molecules

When an asymmetric double dot is hybridized with itinerant electrons, its singlet ground state and lowly excited triplet state cross, leading to a competition between the Zhang-Rice mechanism of singlet-triplet splitting in a confined cluster and the Kondo effect (which accompanies the tunneling through quantum dot under a Coulomb blockade restriction). The rich physics of an underscreened S = 1 Kondo impurity in the presence of low-lying triplet-singlet excitations is exposed and estimates of the magnetic susceptibility and the electric conductance are presented, together with applications for molecule chemisorption on metallic substrates.     

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.     

Instability of Non—Fermi Liquid Behavior in the Two—Channel Kondo Model

The effects of interchannel scattering of conduction electrons by the impurity and repulsion of conduction electrons at the impurity site on the two-channel Kondo model are simultaneously considered in this paper,It is shown that these two perturbations will substantially modify the usual local non-Fermi liquid behavior of the two-channel Kondo model.With bosonization and unitary transformations we find that the system can be transformed into a single channel Kondo model with anisotropy between longitudinal and transverse exchange couplings,Whatever for originally antiferromagnetic or ferromagnetic isotropic coupling,the system always flows to strong-coupling limit,which exhibits local Fermi liquid behavior at low temperatures.     

Spin order in one-dimensional Kondo and Hund lattices

We study numerically the one-dimensional Kondo and Hund lattices consisting of localized spins interacting antiferromagnetically or ferromagnetically with the itinerant electrons, respectively. Using the density-matrix renormalization group we find, for both models and in the small coupling regime, the existence of new magnetic phases where the local spins order forming ferromagnetic islands coupled antiferromagnetically. Furthermore, by increasing the interaction parameter |J| we find that this order evolves toward the ferromagnetic regime through a spiral-like phase with longer characteristic wavelengths. These results shed new light on the zero temperature magnetic phase diagram for these models.     

Hybridized nature of pseudogap in Kondo insulators CeRhSb and CeRhAs

We studied the electronic structure of Kondo insulators CeRhSb and CeRhAs using high-resolution photoemission spectroscopy. We found that the 4f-derived density of states shows a depletion (pseudogap) at E(F) in contrast to metallic Kondo materials. It was found that the size of the f pseudogap is smaller than that of conduction electrons ( c pseudogap) while both scale well with the Kondo temperature. The present results indicate that the hybridization between 4f and conduction electrons near E(F) is essential for the Kondo gap in the Ce-based compounds.     

半金属磁性材料研究进展

半金属材料是一种新型的功能自旋电子学材料，是一种具有特殊能带结构的物质，近年来13益受到人们的关注。半金属材料从微观上具有导体和绝缘体双重性质：对一种自旋取向的电子其能带结构呈现金属性，而另一自旋取向的电子其能带结构呈现绝缘体性．文章着重对Half—Heusler结构半金属材料、CrO2铁磁半金属、Fe3O4亚铁磁材料半金属、反铁磁材料半金属和钙钛矿及双钙钛矿半金属的结构特性进行分析和综述，并对半金属材料的应用原理和应用前景作了阐述。     

Coexistence of antiferromagnetic and ferromagnetic phase for ferromagnetic Kondo lattice model

To study the proposed phase separations in doped manganites, we performed Monte-Carlo calculations for the ferromagnetic Kondo lattice model with strong Hund's coupling between conduction electrons and localized spins. For the practical calculations, we adopted a one dimensional lattice and treated the spins of the localized t_2g electrons semi-classically. A direct evidence of the phase separation is observed from a snapshot of the spatial dependence of localized spins. No indication of the canted or spiral phases is found in the results of simulations. Further, the calculated results of the spin structure factor in the phase separation region are well compared with recent experiments. Received: 1st September 1998 / Revised: 30 October 1998 / Accepted: 27 November 1998     

Quantum-tunneling-induced Kondo effect in single molecular magnets

We consider transport through a single-molecule magnet strongly coupled to metallic electrodes. We demonstrate that, for a half-integer spin of the molecule, electron and spin tunneling cooperate to produce both quantum tunneling of the magnetic moment and a Kondo effect in the linear conductance. The Kondo temperature depends sensitively on the ratio of the transverse and easy-axis anisotropies in a nonmonotonic way. The magnetic symmetry of the transverse anisotropy imposes a selection rule on the total spin for the occurrence of the Kondo effect which deviates from the usual even-odd alternation.     

Spin-resolved splitting of Kondo resonances in the presence of RKKY-type coupling

We have performed spin-resolved measurements on a Kondo impurity in the presence of RKKY-type exchange coupling. By placing manganese phthalocyanine (MnPc) molecules on Fe-supported Pb islands, a Kondo system is devised which is exchange coupled to a magnetic substrate via conduction electrons in Pb, inducing spin splitting of the Kondo resonance. The spin-polarized nature of the split Kondo resonance and a spin filter effect induced by spin-flip inelastic electron tunneling are revealed by spin-polarized scanning tunneling microscopy and spectroscopy.     

Spin-dependent thermoelectric effect in polaronic Kondo transport through a Rashba quantum dot coupled with ferromagnetic leads

We investigate the spin-dependent thermoelectric effect of a Rashba molecular quantum dot coupled with both ferromagnetic leads and a phonon bath in the Kondo regime. A transport formula is derived to deal with the strong electron–electron and electron–phonon interaction with the spin–orbit coupling of arbitrary intensity simultaneously. The numerical results show that only strengthening the electron–phonon coupling can improve the charge thermopower, while even very small spin–orbit coupling can suppress both the thermocharge figure of merit and the thermospin one at the Kondo temperature greatly. It is also found that the electron–phonon coupling in conjunction with the spin–orbit coupling can rebuild Fermi liquid state in the Kondo regime.     

Interplay between the mesoscopic Stoner and Kondo effects in quantum dots

We consider electrons confined to a quantum dot interacting antiferromagnetically with a spin-1 / 2 Kondo impurity. The electrons also interact among themselves ferromagnetically with a dimensionless coupling J , where J =1 denotes the bulk Stoner transition. We show that as J approaches 1 there is a regime with enhanced Kondo correlations, followed by one where the Kondo effect is destroyed and impurity is spin polarized opposite to the dot electrons. The most striking signature of the first, Stoner-enhanced Kondo regime is that a Zeeman field increases the Kondo scale, in contrast to the case for noninteracting dot electrons. Implications for experiments are discussed.     

On the Optical Conductivity of Kondo Insulators

The temperature-dependent optical conductivity of a Kondo insulator for different temperatures with impurity doping on the conduction band is studied within the U = ∞ Anderson lattice model in the framework of a slave-boson mean-field theory under the coherent potential approximation. The results show that the depletion of the optical conductivity in low-frequency region decreases with increasing of the temperature, and the system exhibits a transition between a low-temperature insulating phase and a high-temperature metallic phase. With increasing of the impurity concentration, the effect of the impurity scattering could result in a nearly temperature-independent gap behavior. The numerical results on the localization of the electron and its variation with temperature are also discussed with the help of optical sum rule.     

The Phase Diagram of Kondo Problem in Luttinger Liquid

The problem of Kondo effect in a Luttinger liquid which consists of right-moving, spin-up electrons and left-moving, spin-down electrons is investigated in the bosonized form, and the phase diagram at zero temperature is obtained by a yavefunctional method.     

Non-Kondo mechanism for resistivity minimum in spin ice conduction systems

We present a mechanism of resistivity minimum in conduction electron systems coupled with localized moments, which is distinguished from the Kondo effect. Instead of the spin-flip process in the Kondo effect, electrons are elastically scattered by local spin correlations which evolve in a particular way under geometrical frustration as decreasing temperature. This is demonstrated by the cellular dynamical mean-field theory for a spin-ice-type Kondo lattice model on a pyrochlore lattice. Peculiar temperature dependences of the resistivity, specific heat, and magnetic susceptibility in the non-Kondo mechanism are compared with the experimental data in metallic Ir pyrochlore oxides.