R. Franco

Impurity states in a spherical GaAs-Ga 1-x Al x As quantum dots: Effects of hydrostatic pressure

We calculated the binding energies of shallow donors and acceptors in a spherical GaAs-Ga1-xAlxAs quantum dot under isotropic hydrostatic pressure for both a finite and an infinitely high barrier. We use a variational approach within the effective mass approximation. The binding energy is computed as a function of hydrostatic pressure, the dot sizes and the impurity position. The results show that the impurity binding energy increases with the pressure for any position of the impurity. We have also found that the binding energy depends on the location of the impurity and the pressure effects are less pronounced for impurities on the edge.

X-slave boson approach to the periodic Anderson model

Abstract The periodic anderson model (PAM) in the limit U=∞, can be studied by employing the Hubbard X operators to project out the unwanted states. In a previous work, we have studied the cumulant expansion of this Hamiltonian employing the hybridization as a perturbation, but probability conservation of the local states (completeness) is not usually satisfied when partial expansions like the “chain approximation (CHA)” are employed. To consider this problem, we use a technique similar to the one employed by Coleman to treat the same problem with slave-bosons in the mean-field approximation. Assuming a particular renormalization for hybridization, we obtain a description that avoids an unwanted phase transition that appears in the mean-field slave-boson method at intermediate temperatures.

Critical points of the anyon-Hubbard model

Anyons are particles with fractional statistics that exhibit a nontrivial change in the wave function under an exchange of particles. Anyons can be considered to be a general category of particles that interpolate between fermions and bosons. We determined the position of the critical points of the one-dimensional anyon-Hubbard model, which was mapped to a modified Bose-Hubbard model where the tunneling depends on the local density and the interchange angle. We studied the latter model by using the density-matrix renormalization-group method and observed that gapped (Mott insulator) and gapless (superfluid) phases characterized the phase diagram, regardless of the value of the statistical angle. The phase diagram for higher densities was calculated and showed that the Mott lobes increase (decrease) as a function of the statistical angle (global density). The position of the critical point separating the gapped and gapless phases was found using quantum information tools, namely the block von Neumann entropy. We also studied the evolution of the critical point with the global density and the statistical angle and showed that the anyon-Hubbard model with a statistical angle

Phase Evolution of the Electronic Transmission Through a Kondo Correlated Quantum Dot

ensuremath{theta}=ensuremath{pi}/4

Lateral Fano resonance and Kondo effect in the strong coupling regime of a quantum dot embedded in a quantum wire

is in the same universality class as the Bose-Hubbard model with two-body interactions.

Cálculo de la concurrencia para el modelo de Heisenberg

We study the scattering phase shift of the Kondo assisted transmission through a quantum dot (QD), considering a model that includes an additional non resonant channel transmission. To compute the phase evolution and the transmission amplitude of the QD, for different temperatures, we describe the QD employing the single Anderson impurity model in the limit of infinity Coulomb repulsion, within the X-boson approach. Our results are consistent with the development of an unusually large phase evolution at around p in the Kondo valley, observed in recent experiments, and is consistent with others theoretical treatments.

Linear conductance through parallel coupled quantum dots

We study the electronic transport through a quantum wire (QW), described by a tight-binding chain, with an embedded quantum dot (QD). We obtain the conductance with a strong Fano resonance; the density of states shows that this behavior is linked to a many-body renormalized QD resonant level Ẽf at the edge of the conduction band (CB), strongly hybridized with the Van Hove peaks of the one-dimensional density of states for the lead. The obtained Fano resonance is thermically activated, above the Kondo temperature; this is due to the quantum interference between the CB and a thermal activated channel created by the QD resonance at the vicinity of the bottom of the CB. Our results are in qualitative agreement with those reported for a T-coupled QD.

The Effective Charge Velocity of Spin-1/2 Superlattices

The concurrence is a quantity that allows us to measure the entanglement of a quantum system and can be calculated from the reduced density matrix. In this paper we show explicitly how to calculate the concurrence for a finite chain of spins s =1/2 described by the anisotropic Heisenberg model. We show that for finite chains the concurrence has a maximum at the critical point Δ = 1, which is the main feature in the thermodynamic limit. We observe that it is possible to obtain information about quantum phase transitions of the model by calculating the concurrence for chains of at least 12 sites

Thermopower and thermal conductance for a Kondo correlated quantum dot

We study the electronic transport through two parallel coupled quantum dots (QDs), employing the X-boson treatment for the single impurity Anderson model. We compute the linear conductance (LC) and transmission coefficient for different regimes of the system, as function of the QDs energy; our results show a suppression of the linear conductance at low temperatures; when the coupling between the QDs is significant, a drop in the transmission coefficient is evident, at the energy value of the side-coupled QD. We also obtain the temperature dependence of the LC, for different hybridizations between the QDs and the energy of one of them. Our results are consistent with those obtained by other theoretical treatments and recovers what is expected when the coupling between the QDs is weak.

Block entropy and quantum phase transition in the anisotropic Kondo necklace model

We calculate the spin gap of homogeneous and inhomogeneous spin chains, using the Whites density matrix renormalization group technique. We found that the spin gap is related to the ration between the spin velocity and the correlation exponent. We consider a spin superlattice, which is composed of a repeated pattern of two spin-1/2 XXZ chains with different anisotropy parameters. The behavior of the charge velocity as a function of the anisotropy parameter and the relative size of sub-chains was investigated. We found reasonable agreement between the bosonization results and the numerical ones.