The analysis of the impact of spin–orbit coupling (SOC) on the Kondo state has generated considerable controversy, mainly regarding the dependence of the Kondo temperature TK on SOC strength. Here, we study the one-dimensional (1D) single impurity Anderson model (SIAM) subjected to Rashba (α) and Dresselhaus (β) SOC. It is shown that, due to time-reversal symmetry, the hybridization function between impurity and quantum wire is diagonal and spin independent (as it is the case for the zero-SOC SIAM), thus the finite-SOC SIAM has a Kondo ground state similar to that for the zero-SOC SIAM. This similarity allows the use of the Haldane expression for TK, with parameters renormalized by SOC, which are calculated through a physically motivated change of basis. Analytic results for the parameters of the SOC-renormalized Haldane expression are obtained, facilitating the analysis of the SOC effect over TK. It is found that SOC acting in the quantum wire exponentially decreases TK while SOC at the impurity exponentially increases it. These analytical results are fully supported by calculations using the numerical renormalization group (NRG), applied to the wide-band regime, and the projector operator approach, applied to the infinite-U regime. Literature results, using quantum Monte Carlo, for a system with Fermi energy near the bottom of the band, are qualitatively reproduced, using NRG. In addition, it is shown that the 1D SOC SIAM for arbitrary α and β displays a persistent spin helix SU(2) symmetry similar to the one for a 2D Fermi sea with the restriction α = β.

In this work, using the finite-U slave boson mean-field approximation to solve the single-impurity Anderson model, the authors apply two different strategies to study the Kondo cloud, by analyzing quantities that are dependent on the distance to the magnetic impurity and then finding a universal distance scale xi(K) through the collapse of the results into an universal function. The first method is based on the analysis of the local density of states of the conduction electrons (denoted as xi(L)(K)), while the second relies on the analysis of spin correlations (xi(Sigma)(K)). Our calculations show that there is exact quantitative agreement, in the way xi(K) depends on U/Gamma, between xi(Sigma)(K) and the results obtained through the heuristic expression xi(K) proportional to v(F)/T-K, while there is very close quantitative agreement between xi(Sigma)(K) and xi(Sigma)(K). The use of the slave boson technique to calculate the spin correlations, which eliminates finite size effects, allowed us to study very large Kondo clouds, something that is very difficult using other techniques, like the density matrix renormalization Group method, for example. In addition, the very smooth curves obtained for the spin correlations allowed us to qualitatively identify a region in the Kondo cloud, adjacent to the impurity, that had been connected to the free orbital fixed point in previous numerical renormalization group calculations [Phys. Rev. B 84, 115120 (2011)].

In this paper, the metal-insulator transition is studied in polyaniline (PANI) as the doping increases. The study incorporates a polymerization process that produces a cross-linking of the PANI chains, yielding an extended disordered branched lattice. The presence of the phenazine structure is fundamental and allows the polymer to acquire extra dimensions, which permits a transition to the metallic phase when doping is increased as the Fermi level goes through the mobility edge into a region of extended states. This PANI system is described assuming that the polymerization process gives rise to a structure that could be represented by a Bethe lattice. It is observed, as indicated by the experiments, that the conductivity increases as a function of the pH of the acid solution when agents like bipolarons are introduced.

Themetal-insulator transition taking place in Polyaniline (PAN) is investigated in a one-dimensional configuration when impurities are introduced by doping. The electronic transport is numerically analyzed, representing the system by a Huckel tight-binding Hamiltonian and using the non-equilibrium Green's function formalism to obtain the conductance of the system under the presence of polaron and bipolaron doping. In a detailed analysis, we highlight the importance of studying extensive N sites chains, evaluating the conductance as a function of the de-coherence parameter., decreasing it to the limit of 0(+).; this procedure allows us to study the existence of delocalized states in this system. It was possible to verify that although the bipolaron/polaron doping produces a displacement of the Fermi energy into a region of states outside the gap of the pure polymer, the conductance in the limit of zero inelastic scattering is zero, showing the existence of localized states at the Fermi level, due to disorder. This result indicates that the description of PAN as a linear one-dimensional object analyzed in a extensive N sites disordered chain does notmanifest correlated disorder as proposed by the Random Dimer Model (RDM), where transport could occur because N of the electronic states are extended. The lack of charge diffusion at the Fermi level in this one-dimensional description of the system shows that it is not an adequate model to study the metal-insulator transition when the polymer is doped. The incorporation of the nature of the polymerization process, introducing higher dimensional effects in its early stages, is possibly an essential ingredient to derive an appropriate model to describe the conducting behavior of the PAN system.

The response of an asymmetric double barrier resonant device to the passage of terahertz radiation is discussed. Within the bistable region the radiation is able to turn the current flowing through the system on or off, with an onset that depends on the bias, the strength of the incoming radiation and its frequency.

The response of an asymmetric double barrier resonant device to the passage of terahertz radiation is discussed. Within the bistable region the radiation is able to turn the current flowing through the system on or off, with an onset that depends on the bias, the strength of the incoming radiation, and its frequency.

We show that the addition of a magnetic field parallel to the current induces self sustained intrinsic current oscillations in an asymmetric double barrier structure. The oscillations are attributed to the nonlinear dynamic coupling of the current to the charge trapped in the well, and the effect of the external field over the local density of states across the system. Our results show that the system bifurcates as the field is increased, and may transit to chaos at large enough fields.

We have studied resonant tunneling through an asymmetric double barrier in the presence of a magnetic field in the direction of the current flow. The electron-electron interaction as well as coupling to a longitudinal-optical phonon field are included. The main resonance peak in the I-V characteristic is followed by a phonon-induced peak, both showing a structure at finite magnetic field. We find a bistable region whose width oscillates with magnetic field, in accordance with experiment. This region is expected in the main resonance, yet, as we show, it may also be present in the phonon peak in the limit of large asymmetry. \textcopyright{} 1996 The American Physical Society.

Since the pioneering ideas of Mott regarding the metal-insulator transition a system goes through due to the electronic interaction, there has been a great amount of effort developed to propose model hamiltonians which retain what is thought to be the essential features of this phenomena.

The intrinsic bistability of an asymmetric double barrier device in the presence of a magnetic field in the direction of the current flow is discussed. A simple tight binding formalism with a nonlinear Hartree term that represents the electron-electron interaction is shown to produce the magnetic field-dependent hysteresis loops characteristic of these systems. The width of the resulting bistable region oscillates with magnetic field, as observed in experiment.

The nonequilibrium Green-function Keldysh formalism is used to analyze resonant interband tunneling in double-barrier structures and nonresonant interband transport in polytype heterostructures of InAs, GaSb, and AlSb. The systems are modeled by a multiband tight-binding Hamiltonian that incorporates mixing of electron, light-hole, and heavy-hole states. The model is solved by the real-space renormalization technique, which is very rapid and numerically stable for any size of the system. The large difference in effective masses and the opposite curvature of the energy dispersion of the conduction band in InAs and valence bands in GaSb are reflected in the transport properties. The I-V characteristics of double-barrier structures show quite different features according to whether the well is InAs or GaSb. For the latter case, the current intensity peaks and the peak-to-valley current ratios are much larger than for the former case. The calculated I-V characteristics are generally in very good agreement with the experimental data. The density of states and the dispersion relation of the resonant states as a function of the in-plane wave vector are also discussed.

We extended a previously developed diagrammatic formulation for the calculation of the effective dielectric response of composites prepared as a random, homogeneous, and isotropic distribution of small spherical inclusions in an otherwise homogeneous matrix. This is done within the long-wavelength, dipolar approximation in the low-density regime of inclusions. We propose a new diagrammatic summation and we compare our results with two recently reported computer simulations.

We extend a previously developed diagrammatic formalism for the calculation of the effective dielectric response of composites, prepared as a collection of small spherical inclusions embedded in an otherwise homogeneous matrix. This is done within the long wavelength, dipolar approximation for a low filling fraction of spheres. We propose a new diagrammatic approximation and we compare our results with recently reported numerical simulations.

A theoretical model has been developed to investigate the process of resonant interband tunneling in semiconductor single and double-barrier structures. The heterostructure is described by a two-states tight-binding Hamiltonian and the current is calculated by application of the Keldysh nonequilibrium Green function diagramatic technique. The model is applied to InAs/AlSb/GaSb heterostructures in which interband tunneling has been recently been observed. The calculated current-voltage characteristics show different behavior according to having GaSb or InAs as the quantum well. Also, in some cases, they are very asymmetric with respect to change of bias polarization, in agreement with experimental results.

We formulate a diagrammatic approach for the calculation of the effective dielectric response of a collection of spheres located at random positions and embedded in a homogeneous medium. Through infinite summations of specific classes of diagrams we derive Maxwell Garnett's theory and an expression given by Felderhof, Ford and Cohen [1]. Extending the diagrammatic procedure developed by Matsubara and Toyozawa [2] in their theory of impurity band conduction in semiconductors we obtain an improved version of the Clausius-Mossotti relation with a renormalized polarizability instead of the bare polarizability of the spheres. We compare our results with other theories and with experiment. We also find that the simple theory of a renormalized polarizability in the Maxwell Garnett theory developed by us [3] corresponds to the choice of a specific class of diagrams.