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 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. It is shown that 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 the same length. In the Kondo regime, the current scales as L 21͞2 , unlike the L 21 scaling of a perfect ring. We discuss the possibility of detection of the Kondo effect by means of a persistent current measurement.[S0031-9007(99)09451-X] PACS numbers: 73.23. Ra, 72.15.Qm, 73.20.Dx Electron transport through a quantum dot (QD) has been a subject of many experimental and theoretical studies in the past few years. These small devices contain several millions of real atoms, but behave as if they were single artificial atoms. Similar to real atoms, they have a discrete spectrum of energy, which has been measured [1] and theoretically understood on the basis of a confinement potential and a full many-body electron-electron interaction treatment [2]. Unlike real atoms, electronic transport can be realized through a single QD. Experimental results [3] show periodic oscillations of the conductance as a function of the electron density in the QD. These oscillations can be explained on the basis of a transport mechanism governed by Coulomb blockade and single-electron tunneling [4].Another manifestation of electron-electron interaction which was theoretically predicted [5] some years ago is the Kondo effect in a QD coupled to external leads. In this case, however, the effect is due to correlations between the electrons inside the QD and the conduction electrons in the leads. When the system operates in the so-called Kondo regime, a resonance in the vicinity of the Fermi level, localized at the QD, provides a new channel for the mesoscopic current to tunnel through, creating new phenomena which can be detected in a transport experiment. We have recently proposed [6] a current measurement on a ring connected to two leads having a QD inserted in one of its arms where the signature of the Kondo effect would be clearly reflected. Measurements of the current in a similar device have already demonstrated [7] the coherent character of the transport through the QD. However, this experiment was not appropriate to observe the Kondo effect. Its observation is a delicate task since it depends on several different energy scales and their relative sizes, such as the coupling constant between the QD and leads t 0 , the Kondo temperature T K , and the energy spacing between the dot levels De. For example, if t 02 ͞W . De, where W is the ring bandwidth, the charge and energy quantization in the QD is lost and the Kondo effect disappears. On the other hand, diminishing t 0 leads to an exponential reduction of T K [8]. So, in order to get simultane-ously accessible temperatures and charge-energy quantization De must be large enough, which implies small size QD. Very ...

This work proposes a new 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.

No abstract

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 Andersonimpurity 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. ͓S0163-1829͑97͒53012-8͔