The spatial length of the Kondo screening is still a controversial issue related to Kondo physics. While renormalization-group and Bethe-Ansatz solutions have provided detailed information about the thermodynamics of magnetic impurities, they are insufficient to study the effect on the surrounding electrons, i.e., the spatial range of the correlations created by the Kondo effect between the localized magnetic moment and the conduction electrons. The objective of this work is to present a quantitative way of measuring the extension of these correlations by studying their effect directly on the local density of states ͑LDOS͒ at arbitrary distances from the impurity. The numerical techniques used, the embedded cluster approximation, the finite-U slave bosons, and numerical renormalization group, calculate the Green's functions in real space. With this information, one can calculate how the local density of states away from the impurity is modified by its presence, below and above the Kondo temperature, and then estimate the range of the disturbances in the noninteracting Fermi sea due to the Kondo effect, and how it changes with the Kondo temperature T K . The results obtained agree with results obtained through spin-spin correlations, showing that the LDOS captures the phenomenology of the Kondo cloud as well.

In this work we use the Slave Boson Mean Field Approximation at finite U to study the effects of spinspin correlations in the transport properties of two quantum dots coupled in series to metallic leads. Different quantum regimes of this system are studied in a wide range of parameter space. The main aspects related to the interplay between the half-filling Kondo effect and the antiferromagnetic correlation between the quantum dots are reviewed. Slave boson results for conductance, local density of states in the quantum dots, and the renormalized energy parameters, are presented. As a different approach to the Kondo physics in a double dot system, the Kondo cloud extension inside the metallic leads is calculated and its dependence with the inter-dot coupling is analyzed. In addition, the cloud extension permits the calculation of the Kondo temperature of the double quantum dot. This result is very similar to the corresponding critical temperature Tc, as a function of the parameters of the system, as obtained by using the finite temperature extension of the Slave Boson Mean Field Approximation.

A system of two interacting Cobalt atoms, at varying distances, has been studied in a recent Scanning Tunneling Microscope experiment by J. Bork et al., Nature Physics 7, 901 (2011). We propose a microscopic model that explains, for all the experimentally analyzed interatomic distances, the physics observed in these experiments. Our proposal is based on the two-impurity Anderson model, with the inclusion of a two-path geometry for charge transport. This many-body system is treated in the finite-U Slave Boson Mean Field Approximation and the Logarithmic-Discretization Embedded-Cluster Approximation. We physically characterize the different charge transport regimes of this system at various interatomic distances and show that, as in the experiments, the features observed in the transport properties depend on the presence of two impurities but also on the existence of two conducting channels for electron transport. We interpret the splitting observed in the conductance as the result of the hybridization of the two Kondo resonances associated to each impurity.

The transport properties of a linear structure of three quantum dots, with the central one connected to leads, are studied using the logarithmic discretization embedded cluster approximation. It is shown that the side dot spins can be ferromagnetically ͑F͒ or antiferromagnetically ͑AF͒ correlated between them, depending on the charge at the central dot. The system possesses a regime of coexistence of a two stage Kondo effect and the F phase. Ferromagnetism destroys the Kondo ground state when the system is driven to the molecular regime by increasing the interdot interaction above the largest Kondo temperature. Instead, an AF ground state does not compete with the Kondo regime. This remarkable behavior indicates that the measurement of the conductance can be an efficient readout procedure when the system operates as a quantum gate.

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We analyze the transport properties of a double quantum dot device with both dots coupled to perfect conducting leads and to a finite chain of N noninteracting sites connecting both of them. The interdot chain strongly influences the transport across the system and the local density of states of the dots. We study the case of a small number of sites, so that Kondo box effects are present, varying the coupling between the dots and the chain. For odd N and small coupling between the interdot chain and the dots, a state with two coexisting Kondo regimes develops: the bulk Kondo due to the quantum dots connected to leads and the one produced by the screening of the quantum dot spins by the spin in the finite chain at the Fermi level. As the coupling to the interdot chain increases, there is a crossover to a molecular Kondo effect, due to the screening of the molecule (formed by the finite chain and the quantum dots) spin by the leads. For even N the two Kondo temperatures regime does not develop and the physics is dominated by the usual competition between Kondo and antiferromagnetism between the quantum dots. We finally study how the transport properties are affected as N is increased. For the study we used exact multiconfigurational Lanczos calculations and finite-U slave-boson mean-field theory at T = 0. The results obtained with both methods describe qualitatively and also quantitatively the same physics.

Resumo Neste trabalho o problema clássico de uma partícula que descreve um arco de círculo ao deslizar pela superfície polida de um iglu esférico é estendido para o caso de um iglu elipsoidal. Neste caso, a altura na qual a partícula perde contato com a superfície do iglu passa a obedecer a uma equação cúbica com coeficientes que dependem dos parâmetros que determinam a trajetória elíptica. A solução analítica dessa equação é então obtida a partir do método de Cardano-Tartaglia.

The paper studies the electronic current in a one-dimensional lead under the effect of spin–orbit coupling and its injection into a metallic conductor through two contacts, forming a closed loop. When an external potential is applied, the time reversal symmetry is broken and the wave vector k of the circulating electrons that contribute to the current is spin-dependent. As the wave function phase depends upon the vector k, the closed path in the circuit produces spin-dependent current interference. This creates a physical scenario in which a spin-polarized current emerges, even in the absence of external magnetic fields or magnetic materials. It is possible to find points in the system’s parameter space and, depending upon its geometry, the value of the Fermi energy and the spin–orbit intensities, for which the electronic states participating in the current have only one spin, creating a high and totally spin-polarized conductance. For a potential of a few tens of meV, it is possible to obtain a spin-polarized current of the order of μA. The properties of the obtained electronic current qualify the proposed device as a potentially important tool for spintronics applications.