Phase effects on the conductance of a double-dot system in a ring structure threaded by a magnetic flux are studied. The Aharonov-Bohm effect combined with the dot many-body charging effects determine the phases of the currents going through each arm of the ring. The cases for zero magnetic flux or half a quantum of flux are discussed in detail. It is shown that, depending upon the magnetic flux and the state of charge of the dots, controlled by gate potentials, the dephasing of the upper and lower arm current gives rise to a $S=1∕2$ or $S=1$ Kondo regime.

The electronic properties of two interacting dots connected to leads are studied in the Kondo regime. The configuration is such that one dot is inserted into the lead while the other is side-connected to it. The situation, where both dots are in the Kondo regime is investigated. We find that the development of a Kondo state related to the connected dot is mediated by the inserted dot. In this case, an anti-resonance appears in the density of states of the inserted dot, at the Fermi level. The equation of motion method is used to calculate the Green's functions of the system.

The effect of inter-dot many body interactions on the transport properties of coupled dots connected to leads is studied. Results are obtained by exactly diagonalizing a cluster composed by the double-dot and its vicinity, which is then connected to the leads. We analyse two configurations: coupled dots aligned and perpendicular to the leads. We show that in the weak coupling limit they present quite different conductance features as a function of gate potential. In the strong coupling limit they show qualitatively similar behaviour.

We study the persistent current circulating along a mesoscopic ring with a dot side-coupled to it when threaded by a magnetic field. A cluster including the dot and its vicinity is diagonalized and embedded into the rest of the system. The result is numerically exact. We show that in the Kondo regime, the current can be a smooth or a strongly dependent function of the gate potential according to the structure of occupation of the highest energetic electrons of the system.

A numerically exact calculation of the T=0 transport properties of a quantum wire interacting with a lateral two-level quantum dot is presented. The wire conductance is calculated for all different states of charge and spin of the quantum dot. For a dot with two electrons we obtain an enhancement of the Kondo temperature at the singlet-triplet transition and a nonuniversal scaling law for its dependence upon the dot energy spacing. We find that the Kondo correlation is stronger for a dot spin S(D)similar to1 than for S(D)similar to1/2. In both cases the wire current is totally quenched by the Kondo effect. When the dot is in the mixed-valence regime and 1/2less than or similar toS(D)less than or similar to1 the wire conductance is partially quenched except in a very small region of gate potential where it reaches the maximum value e(2)/h.

The transport properties of a quantum wire interacting with a lateral two-level quantum dot are presented. A cluster including the dot and its vicinity is diagonalized and embedded into the res of the system. The result is numerically exact for T = 0. The wire conductance is calculated for all different states of charge and spin of the quantum dot. We find that the Kondo correlation is. stronger for a dot spin S-D approximate to 1 than for S-D approximate to 1/2. In both cases the wire current is totally quenched by the Kondo effect. When the dot is in the mixed-valence regime and 1/2; SD; 1 the wire conductance is partially quenched except in a very small region of the gate potentia where it reaches the maximum value e(2)/h.

We study the persistent current circulating along a mesoscopic ring with a dot side coupled to it when threaded by a magnetic field. A cluster including the dot and its vicinity is diagonalized and embedded into the rest of the system. The result is numerically exact. We show that a ring of any size can have the properties of a Kondo ground state, although for small size it depends upon the configuration of the highest occupied ring levels. In the Kondo regime, the states in the vicinity of the Fermi level do not contribute to the persistent current, although it gets the contribution of the states below ${E}_{f}.$ This clarifies previous interpretations. When the dot is near resonance, the current can depend smoothly or strongly on the gate potential, according to the structure of occupation of the highest energetic electrons of the system.

The competition between the Kondo effect and the antiferromagnetic interaction is studied in an asymmetric coupled-dot system as a function of the inter-dot coupling. A numerical result for the transport properties at zero temperature is obtained by diagonalizing a cluster, composed by the double-dot and its vicinity, which is connected to leads. It is shown that the system goes from the Kondo regime to an antiferromagnetic state as the inter-dot interaction is increased.

The interplay between the Kondo effect and the inter-dot magnetic interaction in a coupled-dot system is studied. An exact result for the transport properties at zero temperature is obtained by diagonalizing a cluster, composed by the double-dot and its vicinity, which is connected to leads. It is shown that the system goes continuously from the Kondo regime to an anti-ferromagnetic state as the inter-dot interaction is increased. The conductance, the charge at the dots and the spin-spin correlation are obtained as a function of the gate potential.

The possibility of detection of the Kondo effect by means of the measurement of Bohm-Aharonov oscillations of the current going through a quantum dot embedded in one of the arms of a mesoscopic ring connected to two leads is demonstrated. The ring is threaded by a magnetic flux and the dot is subjected to a gate voltage V0 which controls its charge content. Whenever the dot has an odd number of electrons and, as a consequence, a net spin the system is in the Kondo regime. Thus, the Kondo effect can be turned on and off as the two dot levels and their associate Coulomb blockade peaks are tuned to the Fermi level by changing V0. Since the Kondo resonance provides a new channel for the electrons to tunnel through the dot it allows the flowing of current along the arm of the ring which contains the dot even though the dot levels are out of resonance. The interference between the waves going along the upper and lower arms of the ring gives rise to the Bohm-Aharonov oscillations of the current and is a clear sign of the Kondo effect.

The persistent current through a quantum dot inserted in a mesoscopic ring of length L is studied. A cluster representing the dot and its vicinity is exactly diagonalized and embedded into the rest of the ring. The Kondo resonance provides a new channel for the current to flow. It is shown that due to scaling properties, the persistent current at the Kondo regime is enhanced relative to the current flowing either when the dot is at resonance or along a perfect ring of same length. In the Kondo regime the current scales as $L^{-1/2}$, unlike the $L^{-1}$ scaling of a perfect ring. We discuss the possibility of detection of the Kondo effect by means of a persistent current measurement.

We study the effect of the Kondo resonance on the tunneling current oscillations in a mesoscopic ring with a quantum dot, threaded by an external magnetic flux. Two situations are considered: the isolated ring and the ring connected to two external contacts. The system is described by an Anderson impurity tight-binding Hamiltonian where the electron-electron interaction is restricted to the dot. It is shown that at low temperature, even with the system out of resonance, the Kondo peak provides a channel for the electron to tunnel through the dot giving rise to persistent currents, for the isolated ring and to the Bohm-Aharonov oscillations, for the ring electrically connected.

We present an analysis of the Kondo effect on the Bohm-Aharonov oscillations of the tunneling currents in a mesoscopic ring with a quantum dot inserted in one of its arms. The system is described by an Anderson impurity tight-binding Hamiltonian where the electron-electron interaction is restricted to the dot. The currents are obtained using nonequilibrium Green functions calculated through a cumulant diagrammatic expansion in the chain approximation. It is shown that at low temperature, even with the system out of resonance, the Kondo peak provides a channel for the electron to tunnel through the dot, giving rise to the Bohm-Aharonov oscillations of the current. At high temperature these oscillations are important only if the dot level is aligned to the Fermi level, when the resonance condition is satisfied.

The Bohm-Aharonov effect in a mesoscopic ring containing a double-barrier structure in its upper part is investigated. The interference between electrons propagating along different sides of the ring is modified by the presence of a double barrier. A local Coulomb interaction, U, is included to describe the highly localized electrons at the well and an Abrikosov-Suhl (AS) resonance appears at low temperatures. So, in this energy range the current is flux dependent. Above the Kondo temperature TK the AS resonance disappears and the flux-independent behavior is recovered. The current is studied as a function of magnetic flux, gate potential and position of the Fermi level.

Persistent currents and currents circulating along an imperfect mesoscopic ring threaded by an external magnetic field are investigated. We study the effect of the Coulomb interaction on a ring with a double-barrier structure within it for two situations: an isolated ring and a ring connected to two reservoirs at different chemical potentials. For the case of the isolated ring the magnetic flux plays the role of the electric field in the standard mesoscopic heterostructures. For the isolated ring the ground state is obtained using a Lanczos algorithm. The persistent current, the total spin of the ground state and the charge distribution are calculated. The current exhibits a bistable behavior. The Coulomb blockade is, to some extent, masked by the Kondo effect. For the ring connected to leads, the presence of the double-barrier structure modifies substantially the interference phenomena shown by perfect rings due to the Bohm-Aharonov effect. The behavior of the current as a function of the magnetic flux is also affected by the Kondo effect.

We investigate the effect of differences between the quantum well and the electrode carrier effective masses on the current–voltage characteristics of double-barrier interband tunneling structures under an applied magnetic field parallel to the current. The system is described by a two-band tight-binding Hamiltonian that incorporates electron and light-hole interaction and the current is calculated using the Keldysh nonequilibrium Green’s-function diagrammatic technique. The formalism is applied to InAs/AlSb/GaSb double-barrier structures considering either the InAs or the GaSb as the quantum well. The behavior of the I–V characteristics as a function of the magnetic-field strength is strongly influenced by the mass differences and opposite curvatures of the conduction and the valence bands. The features we obtain are quite different for InAs and GaSb. They are both also different from the more traditional magneto-tunneling results for GaAs/AlGaAs, where the electrodes and the quantum well electronic masses are equal. The behavior of the I–V characteristics with the applied magnetic field is in good qualitative agreement with available experimental data.

We calculate the current fluctuation of a mesoscopic ring in the ballistic-quantum-transport regime and in the presence of a magnetic field as a function of the Fermi energy, the applied external magnetic field, and the frequency. The system is represented by a tight-binding Hamiltonian, and nonequilibrium Green functions are obtained by the Keldysh formalism. The behavior of the shot noise as a function of the magnetic flux is very sensitive to the value of the Fermi energy and frequency. We find that for zero frequency and Fermi level such that the transmission is maximum the shot noise has oscillations with period hc/2e and vanishes when the current is maximum. This behavior holds for frequencies up to ${10}^{9}$--${10}^{10}$ Hz, for a ring of length of about 1500 \AA{} and Fermi energy of 5 meV. On the other hand, for a Fermi level corresponding to a minimum in the transmission the period of the oscillations is hc/e, as for the current.

The nonequilibrium Green-function Keldysh formalism is used to analyze resonant interband tunneling in double-barrier structures and nonresonant interband transport in polytype heterostructures of InAs, GaSb, and AlSb. The systems are modeled by a multiband tight-binding Hamiltonian that incorporates mixing of electron, light-hole, and heavy-hole states. The model is solved by the real-space renormalization technique, which is very rapid and numerically stable for any size of the system. The large difference in effective masses and the opposite curvature of the energy dispersion of the conduction band in InAs and valence bands in GaSb are reflected in the transport properties. The I-V characteristics of double-barrier structures show quite different features according to whether the well is InAs or GaSb. For the latter case, the current intensity peaks and the peak-to-valley current ratios are much larger than for the former case. The calculated I-V characteristics are generally in very good agreement with the experimental data. The density of states and the dispersion relation of the resonant states as a function of the in-plane wave vector are also discussed.

We comment on the nature of the K-Si interface bond.

A theoretical model has been developed to investigate the process of resonant interband tunneling in semiconductor single and double-barrier structures. The heterostructure is described by a two-states tight-binding Hamiltonian and the current is calculated by application of the Keldysh nonequilibrium Green function diagramatic technique. The model is applied to InAs/AlSb/GaSb heterostructures in which interband tunneling has been recently been observed. The calculated current-voltage characteristics show different behavior according to having GaSb or InAs as the quantum well. Also, in some cases, they are very asymmetric with respect to change of bias polarization, in agreement with experimental results.