In this work, we use three different numerical techniques to study the charge transport properties of a system in the two-level SU(2) (2LSU2) regime, obtained from an SU(4) model Hamiltonian by introducing orbital mixing of the degenerate orbitals via coupling to the leads. SU(4) Kondo physics has been experimentally observed, and studied in detail, in Carbon Nanotube Quantum Dots. Adopting a two molecular orbital basis, the Hamiltonian is recast into a form where one of the molecular orbitals decouples from the charge reservoir, although still interacting capacitively with the other molecular orbital. This basis transformation explains in a clear way how the charge transport in this system turns from double-to single-channel when it transitions from the SU(4) to the 2LSU2 regime. The charge occupancy of these molecular orbitals displays gate-potential-dependent occupancy oscillations that arise from a competition between the Kondo and Intermediate Valence states. The determination of whether the Kondo or the Intermediate Valence state is more favorable, for a specific value of gate potential, is assessed by the definition of an energy scale T0, which is calculated through DMRG. We speculate that the calculation of T0 may provide experimentalists with a useful tool to analyze correlated charge transport in many other systems. For that, a current work is underway to improve the numerical accuracy of its DRMG calculation and explore different definitions.

Electron tunneling through quantum-dots side coupled to a quantum wire, in equilibrium and nonequilibrium Kondo regime, is studied. The mean-field finite-U slave-boson formalism is used to obtain the solution of the problem. We have found that the transmission spectrum shows a structure with two anti-resonances localized at the renormalized energies of the quantum dots. The DOS of the system shows that when the Kondo correlations are dominant there are two Kondo regimes with its own Kondo temperature. The above behavior of the DOS can be explained by quantum interference in the transmission through the two different resonance states of the quantum dots coupled to common leads. This result is analogous to the Dicke effect in optics. We investigate the many body Kondo states as a function of the parameters of the system.

We study theoretically the out-of-equilibrium transport properties of a double quantum dot system in the Kondo regime. We model the system by means of a two-impurity Anderson Hamiltonian. The transport properties are characterized by Kondo effect properties, however, superimposed them, the system possesses novel non-linear bistability behavior.Recently experiments on quantum dots (QDs) at temperatures (T ) below the Kondo temperature (T k ) have shown that new physics emerges when their transport properties are studied. 1,2,? These experiments confirm that many of the phenomena that characterize strongly correlated metals and insulators, as it is the case of the Kondo effect, are present in QDs. The advantage of studying the quantum-coherent many-body Kondo state in QDs, in comparison with natural compounds, consists in the possibility of continuous tuning the relevant parameters governing the properties of this state, in an equilibrium and out-of-equilibrium situation. In particular, the problem of electrons tunnelling through doublequantum-dots (DQDs) in the Kondo regime has received much attention in recent years. [4][5][6][7][8][9] The DQD is the simplest system where it is possible to study the competition between the inter-dot antiferromagnetic spin-spin correlation and the dot-conduction spin-spin correlation present in its ground state. The type of coupling between the QDs determines the character of the electronic states and the transport properties of the artificial molecule. In the tunnelling regime, the electronics states are extended across the entire system and form a coherent state based on the bonding or anti-bonding levels of the QDs. In this context, recently Aguado and Langreth 4 studied the out-of-equilibrium transport properties of a DQD in the Kondo regime. They found that for inter-dot-coupling greater than the level broadening, there is a critical voltage above which the coherent configuration is unstable. This instability is reflected as a drastic drop of the current leading to a singular region of negative differential conductance. This behavior resembles the I-V characteristic of a double barrier structure in the accumulation of charge regime. 10 This system, due to non-linearities introduced by the Coulomb interactions, has a bistable behavior, characterized by two solutions for the current. In this work we report the existence of a similar bistable behavior in an out-of-equilibrium double quantum-dot in the Kondo regime.

Electron tunneling through a double quantum dot molecule side attached to a quantum wire, in the Kondo regime, is studied. The mean-field finite-U slave-boson formalism is used to obtain the solution of the problem. We found conductance cancelations when the molecular energies of the side attached double quantum-dot cross the Fermi energy. We investigate the many body molecular Kondo states as a function of the parameters of the system.Comment: 12 pages, 7 figures. Submitted to Solid State Com

Electron tunneling through a two stage Kondo system constituted by a double quantum-dot molecule side coupled to a quantum wire, under the effect of a finite external potential is studied. We found that I-V characteristic shows a negative differential conductance region induced by the electronic correlation. This phenomenon is a consequence of the properties of the two stage Kondo regime under the effect of an external applied potential that takes the system out of equilibrium. The problem is solved using the mean-field finite-U slave-boson formalism.