arXiv subjects

G. Chiappe

Publications and source records attributed to G. Chiappe.

34 records · Page 2Linked to original sources

arXiv2004

Effect of topology on the transport properties of two interacting dots

The transport properties of a system of two interacting dots, one of them directly connected to the leads constituting a side-coupled configuration (SCD), are studied in the weak and strong tunnel-coupling limits. The conductance behavior of the SCD structure has new and richer physics than the better studied system of two dots aligned with the leads (ACD). In the weak coupling regime and in the case of one electron per dot, the ACD configuration gives rise to two mostly independent Kondo states. In the SCD topology, the inserted dot is in a Kondo state while the side-connected one presents Coulomb blockade properties. Moreover, the dot spins change their behavior, from an antiferromagnetic coupling to a ferromagnetic correlation, as a consequence of the interaction with the conduction electrons. The system is governed by the Kondo effect related to the dot that is embedded into the leads. The role of the side-connected dot is to introduce, when at resonance, a new path for the electrons to go through giving rise to the interferences responsible for the suppression of the conductance. These results depend on the values of the intra-dot Coulomb interactions. In the case where the many-body interaction is restricted to the side-connected dot, its Kondo correlation is responsible for the scattering of the conduction electrons giving rise to the conductance suppression.

cond-mat.mes-hall↗

arXiv2003

Transport through an interacting system connected to leads

Keldysh formalism is used to get the current-voltage characteristic of a small system of interacting electrons described by a Hubbard model coupled to metallic wires. The numerical procedure is checked recovering well-known results for an Anderson impurity. When larger interacting regions are considered quite different results are obtained depending on whether the Hubbard part is half-filled or not. At half-filling the existence of a gap charge manifests itself making current exponentially small as a function both of the number of interacting sites and the value of U. The behavior changes at large voltages above the gap energy when activated charge transport takes place. On the contrary, for filling factors other than half, current goes through the interacting system suffering just a small amount of scattering at both connections. Conductance depends slightly on U and much more on the filling factor but not on the length of the interacting region.

cond-mat.str-el↗

arXiv2003

Classical trajectories in quantum transport at the band center of bipartite lattices with or without vacancies

Here we report on several anomalies in quantum transport at the band center of a bipartite lattice with vacancies that are surely due to its chiral symmetry, namely: no weak localization effect shows up, and, when leads have a single channel the transmission is either one or zero. We propose that these are a consequence of both the chiral symmetry and the large number of states at the band center. The probability amplitude associated to the eigenstate that gives unit transmission ressembles a classical trajectory both with or without vacancies. The large number of states allows to build up trajectories that elude the blocking vacancies explaining the absence of weak localization.

cond-mat.mes-hall↗

arXiv2003

Finite size effects and localization properties of disordered quantum wires with chiral symmetry

Finite size effects in the localization properties of disordered quantum wires are analyzed through conductance calculations. Disorder is induced by introducing vacancies at random positions in the wire and thus preserving the chiral symmetry. For quasi one-dimensional geometries and low concentration of vacancies, an exponential decay of the mean conductance with the wire length is obtained even at the center of the energy band. For wide wires, finite size effects cause the conductance to decay following a non-pure exponential law. We propose an analytical formula for the mean conductance that reproduces accurately the numerical data for both geometries. However, when the concentration of vacancies increases above a critical value, a transition towards the suppression of the conductance occurs. This is a signature of the presence of ultra-localized states trapped in finite regions of the sample.

cond-mat.mes-hall↗

arXiv2001

Interaction between Kondo impurities in a quantum corral

We calculate the spectral densities for two impurities inside an elliptical quantum corral using exact diagonalization in the relevant Hilbert subspace and embedding into the rest of the system. For one impurity, the space and energy dependence of the change in differential conductance $Δ= dI/dV$ observed in the quantum mirage experiment is reproduced. In presence of another impurity, $Δ= dI/dV$ is very sensitive to the hybridization between impurity and bulk. The impurities are correlated ferromagnetically between them. A hopping $\gtrsim 0.15$ eV between impurities destroy the Kondo resonance.

cond-mat.str-el↗

arXiv2001

Persistent currents in diffusive metallic cavities: Large values and anomalous scaling with disorder

The effect of disorder on confined metallic cavities with an Aharonov-Bohm flux line is addressed. We find that, even deep in the diffusive regime, large values of persistent currents may arise for a wide variety of geometries. We present numerical results supporting an anomalous scaling law of the average typical current $< I_{typ}>$ with the strength of disorder $w$, $< I_{typ}> \sim w^{- γ}$ with $γ< 2$. This is contrasted with previously reported results obtained for cylindrical samples where a scaling $< I_{typ}> \sim w^{-2}$ has been found. Possible links to, up to date, unexplained experimental data are finally discussed.

cond-mat.mes-hall↗

arXiv2001

Effect of a Variable-Spin Quantum Dot on a Wire Conductance

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 non-universal scaling law for its dependence upon the dot energy spacing. We find that the Kondo correlation is stronger for a dot spin $S_D \sim 1$ than for $S_D \sim 1/2$. In both cases the wire current is totally quenched by the Kondo effect. When the dot is in the mixed-valence regime and $1/2 < S_D < 1$ the wire conductance is partially quenched except in a very small region of gate potential where it reaches the maximum value $e^2/h$.

cond-mat.str-el↗

arXiv2001

Kondo Effect and Persistent Currents in a Mesoscopic Ring: Numerically Exact Results

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 be in the Kondo regime, although for small sizes it depends upon the magnetic flux. 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.

cond-mat.str-el↗

arXiv2000

Enhancement of persistent currents due to confinement in metallic samples

Confinement and surface roughness (SR) effects on the magnitude of the persistent current are analized for ballistic bidimensional metallic samples. Depending on the particular geometry, localized border states can show up at half filling. These border states contribute coherently to the persistent current and its magnitude is enhanced with respect to their value in the absent of confinement. A linear scaling of the typical current $I_{typ}$ with the number of conduction channels M is obtained. This result is robust with respect to changes in the relevant lengths of the samples and to the SR. Possible links of our results to experiments are also discussed.

cond-mat.mes-hall↗

arXiv1999

Transport in Coupled Quantum Dots: Kondo Effect Versus Anti-Ferromagnetic Correlation

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.

cond-mat.str-el↗

arXiv1999

Persistent currents in mesoscopic cavities with surface roughness

We consider a bidimensional discrete annular cavity with surface roughness (SR) threaded by a magnetic flux. In the ballistic regime and at half filling, localized border-states show up. These border states contribute coherentely to the persistent current and the magnitude of the typical current $I_{typ}$ is enhaced with respect to their value in the absence of confinement. This result is robust with respect to changes in the relevant lengths of the sample.

cond-mat.mes-hall↗

arXiv1996

Bistability of persistent currents in mesoscopic rings

We study the persistent currents flowing in a mesoscopic ring threaded by a magnetic flux and connected to a stub of finite length. Multistability processes and Coulomb blockade are demonstrated to be present in this system. These properties are functions of the magnetic flux crossing the ring which plays the role that the external applied potential fulfills in the multistability behaviour of the standard mesoscopic heterostructures.

cond-mat↗

arXiv1996

Fractional Aharonov-Bohm effect in mesoscopic rings

We study the effects of correlations on a one dimensional ring threaded by a uniform magnetic flux. In order to describe the interaction between particles, we work in the framework of the U $\infty$ Hubbard and $t$-$J$ models. We focus on the dilute limit. Our results suggest the posibility that the persistent current has an anomalous periodicity $ϕ_{0}/p$, where $p$ is an integer in the range $2\leq p\leq N_{e}$ ($N_{e}$ is the number of particles in the ring and $ϕ_{0}$ is the flux quantum). We found that this result depends neither on disorder nor on the detailed form of the interaction, while remains the on site infinite repulsion.

cond-mat↗

arXiv1996

Recovery of the persistent current induced by the electron-electron interaction in mesoscopic metallic rings

Persistent currents in mesoscopic metallic rings induced by static magnetic fields are investigated by means of a Hamiltonian which incorporates diagonal disorder and the electron-electron interaction through a Hubbard term ($U$). Correlations are included up to second order perturbation theory which is shown to work accurately for $U$ of the order of the hopping integral. If disorder is not very strong, interactions increase the current up to near its value for a clean metal. Averaging over ring lengths eliminates the first Fourier component of the current and reduces its value, which remains low after interactions are included.

cond-mat↗

arXiv1994

Capacitance spectroscopy in quantum dots: Addition spectra and decrease of tunneling rates

A theoretical study of single electron capacitance spectroscopy in quantum dots is presented. Exact diagonalizations and the unrestricted Hartree-Fock approximation have been used to shed light over some of the unresolved aspects. The addition spectra of up to 15 electrons is obtained and compared with the experiment. We show evidence for understanding the decrease of the single electron tunneling rates in terms of the behavior of the spectral weight function. (To appear in Phys. Rev. B (Rapid Comm.))

cond-mat↗

arXiv1994

The wavefunction renormalization constant for the one- and two- band Hubbard hamiltonians in two dimensions

The wavefunction renormalization in the Hubbard model is studied, by using well tested mean field tecniques. The 'orthogonality catastrophe' is shown to exist, even in 2D, for doping levels sufficiently close to half filling. The results are extended beyond mean field, by constructing delocalized wavefunctions, with the correct symmetries of the exact solutions. The physics are still well described by the assumption of the existence of the orthogonality catastrophe.

cond-mat↗