National Institute Of Materials Physics - Romania

We model a small quantum dot with a magnetic impurity by the Anderson Hamiltonian with a supplementary exchange interaction term. The transport calculations are performed by means of the Green functions within the equation of motion scheme, in which two decoupling procedures are proposed, for high and low temperatures, respectively. The paper focuses on the charge fluctuations for such a system, an aspect not addressed before, as well as on the Kondo resonance. We show a specific role of the excited state, which can be observed in transport and in spin-spin correlations. Our studies show a many-body feature of the phase shift of transmitted electrons, which is manifested in a specific dip. In the Kondo regime, our calculations complement existing theoretical results. The system shows three Kondo peaks in the density of states: one at the Fermi energy and two side peaks, at a distance corresponding to the singlet-triplet level spacing. The existence of the central peak is conditioned by a degenerate state (the triplet) below the Fermi energy.

We study the Fano effect and the visibility of the Aharonov-Bohm oscillations for a mesoscopic interferometer with an embedded quantum dot in the presence of a nearby second dot. When the electron-electron interaction between the two dots is considered the nearby dot acts as a charge detector. We compute the currents through the interferometer and detector within the Keldysh formalism and the self-energy of the nonequilibrium Green's functions is found up to the second order in the interaction strength. The current formula contains a correction to the Landauer-Buttiker formula. Its contribution to transport and dephasing is discussed. As the bias applied on the detector is increased, the amplitude of both the Fano resonance and Aharonov-Bohm oscillations are considerably reduced due to controlled dephasing. This result is explained by analyzing the behavior of the imaginary part of the interaction self-energy as a function of energy and bias. We emphasize as well the role of the ring-dot coupling. Our theoretical results are consistent with the experimental observation of Buks [Nature 391, 871 (1998)].

We consider the superfluid-insulator transition for cold bosons under an effective magnetic field. We investigate how the applied magnetic field affects the Mott transition within mean-field theory and find that the critical hopping strength (t/U)(c) increases with the applied field. The increase in the critical hopping follows the bandwidth of the Hofstadter butterfly at the given value of the magnetic field. We also calculate the magnetization and superfluid density within mean-field theory.

Motivated by the pionieering experiments of Buks et al. [Nature 391, 871 (1998)] we investigate the visibility of the Fano effect in a single-dot Aharonov-Bohm interferometer which is Coulomb-coupled to a nearby quantum dot. The latter acts as a 'Which Path Detector' and is coupled to two leads on which a finite bias is applied. Using the non-equilibrium Keldysh-Green function formalism we compute the currents through the detector and the interferometer. We take into account the first two contributions to the interaction self energy and emphasize the correction to the Landauer formula which appears beyond the single-particle approximation. Particular attention is given to the coherence properties of the interferometer in the presence of the electron-electron interaction between the embedded dot and the detector. We show that when the detector is subjected to a finite bias the amplitude of Aharonov-Bohm oscillations of the current through the interferometer decreases. The Fano line is in turn rather stable under interaction. Our results generalize an earlier work of Silva and Levit [Phys. Rev. B 63, 201309 (2001)] and complement the existing description of the controlled dephasing. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Theoretical studies of the coherent electronic transport in a system of coupled quantum wires show that switching on the conducting channel in one wire can be manifested in the other coupled wires. Interference processes and electronic correlations are taken into account in our studies on the same footing. The conductance changes depend on the interference conditions of a transmitted wave with that one reflected from the wires that indirectly influence the transport. We show that electronic correlations lead to a dynamical Coulomb blockade effect, which changes the conductance response quantitatively but its shape is still kept the same. Our results are discussed in correspondence with an experiment recently performed by Morimoto [Appl. Phys. Lett. 82, 3952 (2003)] on a system of coupled quantum wires.

The dependence of the driven-current through a tunnel junction on the relative orientation of the magnetization in the electrodes deviates from the Julliere formula when the current is resonant through impurity levels. For asymmetric coupling of the impurities to the electrodes, the magnetoresistance can even be negative on the resonance. Such inversion in sign is due to the fact that not only the density of states in the electrodes is important (for both orientations of spins), but also the density of states at the impurity position, which has a specific behavior. We find the exact conditions under which the negative magnetoresistance occurs, as function of both polarization and coupling asymmetry. The resonant magnetoresistance may have single or double-dip aspect, depending on the mentioned parameters.

We show the role of the Fano interference on the magnetoresistance properties of a T-shaped double-dot placed between ferromagnetic electrodes. In the linear voltage regime, the magnetoresistance is a non-monotonic function of the gate voltage. We find high values of the magnetoresistance at the Fano dip and far from it. For resonant transmission, the magnetoresistance is lower and it can be negative for asymmetric couplings. Particularities of the system in nonequilibrium transport are also analysed. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We investigate theoretically in a tight-binding model the transport properties of the Aharonov-Bohm interferometer (ABI) with one dot embedded in each of its arms. For weak interdot coupling the model Hamiltonian describes the system considered in the experiments of Holleitner et al. [Phys. Rev. Lett. 87, 256802 (2001)]. The electronic transmittance of the interferometer is computed within the Landauer-Buttiker formalism while the coexistence of resonant and coherent transport is explicitly emphasized by using the Feschbach formula. The latter produces effective Hamiltonians whose spectral properties describe the tunneling processes through each dot. We reproduce numerically the stability charging diagrams reported in the experiments of Holleitner et al. When the magnetic flux is fixed and one dot is set to resonance the interferometer transmittance shows Fano, lineshapes as a function of the gate voltage applied to the other dot. Our model includes the effect of the magnetic field on the dot levels and explains the change of the asymmetric tail as the magnetic flux is varied. The transmittance assigned to the Fano, dips located in the almost crossing point of the charging diagrams shows Aharonov-Bohm oscillations.

We propose a general formalism for describing the coexistence of coherent and resonant transport in hybrid mesoscopic structures. The approach is based on Landauer- Buttiker formula for the electronic transmittance and on an old formula of Feshbach. The latter gives the complete Green function of coupled subsystems in terms of effective Green functions of the disconnected parts and provides informations about the individual contributions of each subsystem to transport. Motivated by the experiments of Kobayashi et al.(Phys. Rev. Lett. 88, 256806 (2002)) and Holleitner et al. (Phys. Rev. Lett. 87, 256802 (2001)) we apply the formalism to study transport in Aharonov-Bohm interferometers (ABI) containing one or two coupled two-dimensional quantum dots (QD). In the single dot case, we reproduce and explain the magnetic field control of the Fano interference and investigate the interaction effects in a self-consistent approach. In the double dot case, we obtain the charging diagrams and establish precise criteria for the observation of mesoscopic Fano effect and AB oscillations. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We use the Landauer-Buttiker formalism to study the mesoscopic Fano effect in Aharonov-Bohm rings with an embedded two-dimensional noninteracting dot. The magnetic field dependence of the dot levels leads to a global shift of the Fano lines which becomes important for small ring/dot area ratios. As the magnetic field is varied the Fano dips move periodically from one side of the peak to the other, as reported by Kobayashi [Phys. Rev. Lett. 88, 256806 (2002)]. We show that this effect appears due to a specific magnetic control of the difference between the phase of the single nonresonant path via the free arm of the ring and the global phase of all trajectories involving resonant tunnelings through the dot.