Phase diagram of the two-channel kondo lattice model in one dimension

Employing the density matrix renormalization group method and strong-coupling perturbation theory, we study the phase diagram of the SU(2)xSU(2) Kondo lattice model in one dimension. We show that, at quarter filling, the system can exist in two phases depending on the coupling strength. The weak-coupling phase is dominated by RKKY exchange correlations, while the strong-coupling phase is characterized by strong antiferromagnetic correlations of the channel degree of freedom. These two phases are separated by a quantum critical point. For conduction-band fillings of less than one-quarter, we find a paramagnetic metallic phase at weak coupling and a ferromagnetic phase at moderate to strong coupling.     

Formation of spin-polarons in the ferromagnetic Kondo lattice model away from half-filling

Even though realistic one-dimensional experiments in the field of half-metallic semiconductors are not at hand yet, we are interested in the underlying fundamental physics. In this regard we study a one-dimensional ferromagnetic Kondo lattice model, a model in which a conduction band is coupled ferromagnetically to a background of localized d?moments with coupling constant J(H), and investigate the T?=?0 phase diagram as a function of the antiferromagnetic interaction J between the localized moments and the band-filling n, since it has been observed that doping of the compounds has led to formation of magnetic domains. We explore the spin-polaron formation by looking at the nearest-neighbour correlation functions in the spin and charge regimes for which we use the density matrix renormalization group method, which is a highly efficient method to investigate quasi-one-dimensional strongly correlated systems.     

New type of charge and magnetic order in the ferromagnetic kondo lattice

We study numerically the one dimensional ferromagnetic Kondo lattice, a model widely used to describe nickel and manganese perovskites. By including a nearest-neighbor Coulomb interaction ( V) and a superexchange interaction between the localized moments ( K), we obtain the phase diagram in parameter space for several dopings at T = 0. Because of the competition between double and superexchange, we find a region where the formation of magnetic polarons induces a charge-ordered state which survives also for V = 0. This mechanism should be taken into account in theories of charge ordering involving spin degrees of freedom.     

Spin-selective Kondo insulator: cooperation of ferromagnetism and the Kondo effect

We propose the notion of a spin-selective Kondo insulator, which provides a fundamental mechanism to describe the ferromagnetic phase of the Kondo lattice model with antiferromagnetic coupling. This unveils a remarkable feature of the ferromagnetic metallic phase: the majority-spin conduction electrons show metallic while the minority-spin electrons show insulating behavior. The resulting Kondo gap in the minority-spin sector, which is due to the cooperation of ferromagnetism and partial Kondo screening, evidences a dynamically induced commensurability for a combination of minority-spin electrons and parts of localized spins. Furthermore, this mechanism predicts a nontrivial relation between the macroscopic quantities such as electron magnetization, spin polarization, and electron filling.     

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.     

Full counting statistics for the Kondo dot in the unitary limit

We calculate the charge transfer probability distribution function chi(lambda) for the Kondo dot in the strong-coupling limit within the framework of the Nozières-Fermi-liquid theory of the Kondo effect. At zero temperature, the ratio of the moments Cn of the charge distribution to the backscattering current Ibs follows a universal law Cn/2Ibs = (-1)n(1+2n)/6. The functional form of chi(lambda) is consistent with tunneling of electrons and, possibly, electron pairs. We then discuss the crossover behavior of chi(lambda) from weak to strong Coulomb repulsion in the underlying Anderson impurity model and relate this to the existing results. Finally, we extend our analysis to the case of finite temperatures.     

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     

Disorder-induced local magnetic moments in weakly correlated metallic systems

We consider the formation and the Kondo effect of local magnetic moments in the Anderson-Hubbard model with off-diagonal disorder. The existence of moments at sites weakly coupled to the environment is deduced in effective medium approximation. The distribution of moments is calculated both deep in the metallic phase and near the metal-insulator transition. We discuss the Kondo quenching of the moments and derive a distribution of local Kondo temperatures.     

Is the quantum dot at large bias a weak-coupling problem?

We examine the two-lead Kondo model for a dc-biased quantum dot in the Coulomb blockade regime. From perturbative calculations of the magnetic susceptibility, we show that the problem retains its strong-coupling nature, even at bias voltages larger than the equilibrium Kondo temperature. We give a speculative discussion of the nature of the renormalization group flows and the strong-coupling state that emerges at large voltage bias.     

Dimerization induced by the RKKY interaction

We report the presence of spin dimerization in the ground state of the one-dimensional Kondo lattice model at quarter filling. The emergence of this new phase of the Kondo lattice can be traced to the form of the RKKY interaction between the localized moments and provides the first example of dimerization induced indirectly by itinerant electrons. We propose this dimer ordering as the driving mechanism of the spin-Peierls phase observed in the quasi-one-dimensional organic compounds (Per)2M(mnt)(2) (M=Pt, Pd). Moreover, this suggests that a richer phase diagram than the Doniach paradigm may be needed to accommodate the physics of heavy fermion materials.     

Spin-Wave Excitations in Anisotropic Kondo Lattice Model

The spin-wave excitations in anisotropic Kondo lattice model are studied using the spin Green's function. Both the ferromagnetic and antiferromagnetic cases are considered. The equations to determine the spectrum of low-energy excitations are given. The anisotropy gaps are obtained, and the long-wavelength and strong-coupling limits are analyzed.     

Room-temperature ferromagnetism of Cu-doped ZnO films probed by soft X-ray magnetic circular dichroism

We report direct evidence of room-temperature ferromagnetic ordering in O-deficient ZnO:Cu films by using soft x-ray magnetic circular dichroism and x-ray absorption. Our measurements have revealed unambiguously two distinct features of Cu atoms associated with (i) magnetically ordered Cu ions present only in the oxygen-deficient samples and (ii) magnetically disordered regular Cu2+ ions present in all the samples. We find that a sufficient amount of both oxygen vacancies (V(O)) and Cu impurities is essential to the observed ferromagnetism, and a non-negligible portion of Cu impurities is uninvolved in the magnetic order. Based on first-principles calculations, we propose a microscopic "indirect double-exchange" model, in which alignments of localized large moments of Cu in the vicinity of the V(O) are mediated by the large-sized vacancy orbitals.     

Experimental investigation on transmission properties of lossy single-negative metamaterials

We studied the ground state of alkaline-earth-metal atoms confined in one-dimensional optical lattices modeled by the Kondo lattice model plus a quadratic confining potential. We considered the half-filling case and both ferromagnetic and antiferromagnetic interaction between the localized and delocalized atoms. We found Kondo insulator domains that always coexist with metallic and/or band insulator regions. We observed that the on-site delocalized-localized spin correlation remains constant in the insulating regions and used it to determine the state diagrams. Metallic regions were found for both ferromagnetic and antiferromagnetic couplings.     

The Charged and the Spin-Excitation Gaps in the Double-Exchange Model: a Rigorous Result

We extend a previous result of ours [G.S. Tian, Phys. Rev. B58 (1998) 76121 on the charged gap and the spin-excitation gap of the half-filled Kondo lattice model to the doubleexchange model. In our original approach, this model cannot be dealt with since its localized spins have a large spin number S=3/2. By following a construction argument due to Zener and rewriting the double-exchange Hamiltonian, we are able to overcome this difficulty and re-establish the same relation for this model.     

Kondo effect in quantum dots coupled to ferromagnetic leads

We study the Kondo effect in a quantum dot coupled to ferromagnetic leads and analyze its properties as a function of the spin polarization of the leads. Based on a scaling approach, we predict that for parallel alignment of the magnetizations in the leads the strong-coupling limit of the Kondo effect is reached at a finite value of the magnetic field. Using an equation of motion technique, we study nonlinear transport through the dot. For parallel alignment, the zero-bias anomaly may be split even in the absence of an external magnetic field. For antiparallel spin alignment and symmetric coupling, the peak is split only in the presence of a magnetic field, but shows a characteristic asymmetry in amplitude and position.     

Dynamical mean-field theory of a simplified double-exchange model

Simplified double-exchange model including transfer of the itinerant electrons with spin parallel to the localized spin in the same site and the indirect interaction J of kinetic type between localized spins is comprihensively investigated. The model is exactly solved in infinite dimensions. The exact equations describing the main ordered phases (ferromagnetic and antiferromagnetic) are obtained for the Bethe lattice with (z is the coordination number) in analytical form. The exact expression for the generalized paramagnetic susceptibility of the localized-spin subsystem is also obtained in analytical form. It is shown that temperature dependence of the uniform and the staggered susceptibilities has deviation from Curie-Weiss law. Dependence of Curie and Néel temperatures on itinerant-electron concentration is discussed to study instability conditions of the paramagnetic phase. Anomalous temperature behaviour of the chemical potential, the thermopower and the specific heat is investigated near the Curie point. It is found for J=0 that the system is unstable towards temperature phase separation between ferromagnetic and paramagnetic states. A phase separation connected with antiferromagnetic and the paramagnetic phases can occur only at . Zero-temperature phase diagram including the phase separation between ferromagnetic and antiferromagnetic states is given. Received 28 May 1999 and Received in final form 14 July 1999     

CPA density of states and conductivity in a double-exchange system containing impurities

We study density of states and conductivity of the doped double-exchange system, treating interaction of charge carriers both with the localized spins and with the impurities in the coherent potential approximation. It is shown that under appropriate conditions there is a gap between the conduction band and the impurity band in paramagnetic phase, while the density of states is gapless in ferromagnetic phase. This can explain metal-insulator transition frequently observed in manganites and magnetic semiconductors. Activated conductivity in the insulator phase is numerically calculated. Received 13 June 2000 and Received in final form 5 January 2001     

Understanding the heavy fermion phenomenology from a microscopic model

We solve the 3D periodic Anderson model using a two impurity cluster dynamical mean field theory. We obtain the temperature versus hybridization phase diagram. Approaching the quantum critical point (QCP) both the Néel and lattice Kondo temperatures decrease and they do not cross at the lowest temperature we reached. While strong ferromagnetic spin fluctuation on the Kondo side is observed, our result suggests the critical static spin susceptibility is local in space at the QCP. We observe in the crossover region a logarithmic temperature dependence in the specific heat coefficient and spin susceptibility.