Kondo Resonance Splitting in a Quantum Dot with Perpendicular Magnetic Fields

Using the nonequilibrium Green＇s function technique, we investigate the Kondo effect in the quantum dot with perpendicular magnetic fields, in which one is the Zeeman splitting lies in the z-direction and the other is the spin flip points at the x-direction. It is found whatever one or two magnetic fields are applied, the local density of states （LDOS） will split into two peaks. The positions of two Kondo resonance peaks are determined by Zeeman energy △ when J = 0, and by √△^2＋J^2 when J≠0.     

Towards a description of the Kondo effect using time-dependent density-functional theory

We demonstrate that the zero-temperature conductance of the Anderson model can be calculated within the Landauer formalism combined with static density-functional theory. The proposed approximate functional is based on finite-temperature density-functional theory and yields the exact Kohn-Sham potential at the particle-hole symmetric point. Furthermore, in the limit of zero temperature it correctly exhibits a derivative discontinuity which is shown to be essential to reproduce the conductance plateau. On the other hand, at the Kondo temperature the exact Kohn-Sham conductance overestimates the real one by an order of magnitude. To understand the failure of density-functional theory, we resort to its time-dependent version and conclude that the suppression of the Kondo resonance must be attributed to dynamical exchange-correlation corrections.     

Hidden Caldeira-Leggett dissipation in a Bose-Fermi Kondo model

We show that the Bose-Fermi Kondo model (BFKM), which may find applicability both to certain dissipative mesoscopic qubit devices and to heavy-fermion systems described by the Kondo lattice model, can be mapped exactly onto the Caldeira-Leggett model. This mapping requires an ohmic bosonic bath and an Ising-type coupling between the latter and the impurity spin. This allows us to conclude unambiguously that there is an emergent Kosterlitz-Thouless quantum phase transition in the BFKM with an ohmic bosonic bath. By applying a bosonic numerical renormalization group approach, we thoroughly probe physical quantities close to the quantum phase transition.     

Splitting and Restoration of Kondo Peak in a Deformed Molecule Quantum Dot Coupled to Ferromagnetic Electrodes

We adopt the nonequilibrium Green＇s function method to theoretically study the Kondo effect in a deformed molecule, which is treated as an electron-phonon interaction （EPI） system. The self-energy for phonon part is calculated in the standard many-body diagrammatic expansion up to the second order in EPI strength. We find that the multiple phonon-assisted Kondo satellites arise besides the usual Kondo resonance. In the antiparallel magnetic configuration the splitting of main Kondo peak and phonon-assisted satellites only happen for asymmetrical dot-lead couplings, but it is free from the symmetry for the parallel magnetic configuration. The EPI strength and vibrational frequency can enhance the spin splitting of both main Kondo and satellites. It is shown that the suppressed zero-bias Kondo resonance can be restored by applying an external magnetic field, whose magnitude is dependent on the phononic effect remarkably. Although the asymmetry in tunnel coupling has no contribution to the restoration of spin splitting of Kondo peak, it can shrink the external field needed to switch tunneling magnetoresistance ratio between large negative dip and large positive peak.     

Pseudogap Fermi-Bose Kondo model

We consider a magnetic impurity coupled to both fermionic quasiparticles with a pseudogap density of states and bosonic spin fluctuations. Using renormalization group and large-N calculations we investigate the phase diagram of the resulting Fermi-Bose Kondo model. We show that the Kondo temperature is strongly reduced by low-energy spin fluctuations, and make connections to experiments in cuprate superconductors. Furthermore, we derive an exact exponent for the critical behavior of the conduction electron T matrix, and propose our findings to be relevant for certain scenarios of local quantum criticality in heavy-fermion metals.     

Temperature dependence of the Kondo resonance and its satellites in CeCu2Si2

We present high-resolution photoemission spectroscopy studies on the Kondo resonance of the strongly correlated Ce system CeCu2Si2. By exploiting the thermal broadening of the Fermi edge we analyze position, spectral weight, and temperature dependence of the low-energy 4f spectral features, whose major weight lies above the Fermi level E(F). We also present theoretical predictions based on the single-impurity Anderson model using an extended noncrossing approximation, including all spin-orbit and crystal field splittings of the 4f states. The excellent agreement between theory and experiment provides strong evidence that the spectral properties of CeCu2Si2 can be described by single-impurity Kondo physics down to T approximately 5 K.     

Effective Kondo model for a trimer on a metallic surface

I consider a Hubbard-Anderson model which describes localized orbitals in three different atoms hybridized both among themselves and with a continuum of extended states. Using a generalized Schrieffer-Wolf transformation, I derive an effective Kondo model for the interaction between the doublet ground state of the isolated trimer and the extended states. For an isoceles trimer with distances a, l, l between the atoms, the Kondo temperature is very small for la when a is small. The results agree with experiments for a Cr trimer on Au(111).     

A fractional-spin phase in the power-law Kondo model

We consider a Kondo impurity coupled to a fermionic host with a power-law density of states near the Fermi level, ρ(ε) ∼ |ε|^r, with exponent r < 0. Using both perturbative renormalization group (poor man's scaling) and numerical renormalization group methods, we analyze the phase diagram of this model for ferromagnetic and antiferromagnetic Kondo coupling. Both sectors display non-trivial behavior with several stable phases separated by continuous transitions. In particular, on the ferromagnetic side there is a stable intermediate-coupling fixed point with universal properties corresponding to a fractional ground-state spin. Received 18 February 2002 Published online 31 July 2002     

Scaling and decoherence in the nonequilibrium Kondo model

We study the Kondo effect in quantum dots in an out-of-equilibrium state due to an applied dc-voltage bias. Using the method of infinitesimal unitary transformations ("flow equations"), we develop a perturbative scaling picture that naturally contains both equilibrium coherence and nonequilibrium decoherence effects. This framework allows one to study the competition between Kondo effect and current-induced decoherence, and it establishes a large regime dominated by single-channel Kondo physics for asymmetrically coupled quantum dots.     

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     

Intermediate valence: Phase diagram and Kondo behaviour in a simple model

We have studied an infinitely narrow band version of the Anderson Hamiltonian. Including a Coulomb repulsion between localized and band states in the mean field approximation, we have been able to describe metal-insulator phase transitions and different properties characteristic of intermediate valence in both phases. These include: non-integer occupation, specific heat, saturating static magnetic succeptibility and dynamical properties such as Mössbauer spectra and spin flip neutron spectra. We discuss for what range of values of the parameters some results can be reproduced by a narrow band version of the Kondo Hamiltonian.     

Magnetocaloric effect in the ferromagnetic Kondo lattice model

The Kondo lattice model describes a lattice of localized spins S_i interacting with the conduction electrons via a local exchange coupling J. Assuming a ferromagnetic Hund's rule coupling J>0, the model can be used to describe some itinerant magnetocaloric materials such as Gd(Si_xGe_1-x)₄, La(Fe_1-xSi_x)₁₃, and LaCa_1-xMn_xO₃, which are important for magnetic refrigeration near room temperature. The localized magnetic moments are described in the model Hamiltonian by spin operators, and the conduction electrons by fermionic operators. To study the magnetocaloric effect, a uniform external magnetic field is added through a Zeeman term. By averaging the fermionic degrees of freedom, one obtains an indirect exchange coupling between spins at sites i and j, which corresponds to the RKKY interaction. The self-consistent mean value is evaluated in the effective Heisenberg Hamiltonian within the random phase approximation (RPA). The conduction electron magnetization for a given value of is obtained from the corresponding Green's functions through the equation of motion method. The pressure and doping dependence of the Curie temperature are taken into account in the evaluation of . The magnetocaloric effect is characterized by the isothermal entropy change ΔS and the adiabatic temperature change ΔT_ad upon magnetic field variations in the neighborhood of the ferromagnetic phase transition. The results are obtained for and compared to measurements with Gd compounds.     

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