C. A. Büsser

Transport in coupled quantum dots: Kondo effect versus antiferromagnetic correlation

The interplay between the Kondo effect and the interdot 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 antiferromagnetic state as the interdot interaction is increased. The conductance, the charge at the dots, and the spin-spin correlation are obtained as a function of the gate potential.

Transport properties and Kondo correlations in nanostructures: Time-dependent DMRG method applied to quantum dots coupled to Wilson chains

We apply the adaptive time-dependent density-matrix renormalization-group method tDMRG to the study of transport properties of quantum-dot systems connected to metallic leads. Finite-size effects make the usual tDMRG description of the Kondo regime a numerically demanding task. We show that such effects can be attenuated by describing the leads by Wilson chains, in which the hopping matrix elements decay exponentially away from the impurity tn n/2. For a given system size and in the linear-response regime, results for 1 show several improvements over the undamped =1 case: perfect conductance is obtained deeper in the strongly interacting regime and current plateaus remain well defined for longer time scales. Similar improvements were obtained in the finite-bias regime up to bias voltages of the order of the Kondo temperature. These results show that with the proposed modification, the tDMRG characterization of Kondo correlations in the transport properties can be substantially improved, while it turns out to be sufficient to work with much smaller system sizes. We discuss the numerical cost of this approach with respect to the necessary system sizes and the entanglement growth during the time evolution.

Conductance dip in the Kondo regime of linear arrays of quantum dots

Using exact-diagonalization of small clusters and Dyson equation embedding techniques, the conductance

Numerical results indicate a half-filling SU(4) Kondo state in carbon nanotubes

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Method to study highly correlated nanostructures: The logarithmic-discretization embedded-cluster approximation

of linear arrays of quantum dots is investigated. The Hubbard interaction induces Kondo peaks at low temperatures for an odd number of dots. Remarkably, the Kondo peak is split in half by a deep minimum, and the conductance vanishes at one value of the gate voltage. Tentative explanations for this unusual effect are proposed, including an interference process between two channels contributing to

Interference effects in the conductance of multilevel quantum dots

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Electron transport through a molecular conductor with center-of-mass motion.

, with one more and one less particle than the exactly-solved cluster ground-state. The Hubbard interaction and fermionic statistics of electrons also appear to be important to understand this phenomenon. Although most of the calculations used a particle-hole symmetric Hamiltonian and formalism, results also presented here show that the conductance dip exists even when this symmetry is broken. The conductance cancellation effect obtained using numerical techniques is potentially interesting, and other many-body techniques should be used to confirm its existence.

Kondo versus indirect exchange: the role of the lattice and the actual range of RKKY interactions in real materials

Numerical calculations simulate transport experiments in carbon nanotube quantum dots (P. Jarillo-Herrero et al., Nature 434, 484 (2005)), where a strongly enhanced Kondo temperature T_K ~ 8K was associated with the SU(4) symmetry of the Hamiltonian at quarter-filling for an orbitally double-degenerate single-occupied electronic shell. Our results clearly suggest that the Kondo conductance measured for an adjacent shell with T_K ~ 16K, interpreted as a singlet-triplet Kondo effect, can be associated instead to an SU(4) Kondo effect at half-filling. Besides presenting spin-charge Kondo screening similar to the quarter-filling SU(4), the half-filling SU(4) has been recently associated to very rich physical behavior, including a non-Fermi-liquid state (M. R. Galpin et al., Phys. Rev. Lett. 94, 186406 (2005)).

Unscreened Coulomb interactions and the quantum spin Hall phase in neutral zigzag graphene ribbons.

This work proposes an approach to study transport properties of highly correlated local structures. The method, dubbed the logarithmic discretization embedded cluster approximation LDECA, consists of diagonalizing a finite cluster containing the many-body terms of the Hamiltonian and embedding it into the rest of the system, combined with Wilson s idea of a logarithmic discretization of the representation of the Hamiltonian. The physics associated with both one embedded dot and a double-dot side coupled to leads is discussed in detail. In the former case, the results perfectly agree with Bethe ansatz data, while in the latter, the physics obtained is framed in the conceptual background of a two-stage Kondo problem. A many-body formalism provides a solid theoretical foundation to the method. We argue that LDECA is well suited to study complicated problems such as transport through molecules or quantum dot structures with complex ground states.

Electrostatic control over polarized currents through the spin-orbital Kondo effect

Using exact-diagonalization techniques supplemented by a Dyson equation embedding procedure, the transport properties of multilevel quantum dots are investigated in the Kondo regime. The conductance can be decomposed into the contributions of each level. It is shown that these channels can carry a different phase, and destructive interference processes are observed when the phase difference between them is