Angel Sanchez

Self-consistent analysis of electric field effects on Si delta -doped GaAs

We theoretically study the subband structure of single Si delta -doped GaAs inserted in a quantum well and subject to an electric field applied along the growth direction. We use an efficient self-consistent procedure to solve simultaneously the Schrodinger and Poisson equations for different values of electric field and temperature. We thus find the confining potential, the subband energies and their corresponding envelope functions, the subband occupations, and the oscillator strength of intersubband transitions. Opposite to what is usually the case when dealing with the quantum-confined Stark effect in ordinary quantum wells, we observe an abrupt drop of the energy levels whenever the external field reaches a certain value. This critical value of the field is seen to depend only slightly on temperature. The rapid change in the energy levels is accompanied by the appearance of a secondary well in the confining potential and a strong decrease of the oscillator strength between the two lowest subbands. These results open the possibility to design devices for use as optical filters controlled by an applied electric field.

Enhanced suppression of localization in a continuous random-dimer model

We consider a one-dimensional Kronig-Penney model with randomly placed dimer impurities. We show that this model has infinitely many resonances (zeroes of the reflection coefficient) giving rise to extended states, instead of the one allowed resonance arising in random tight-binding models with paired correlated on-site energies. We present exact transfer-matrix numerical calculations supporting, both realizationwise and on average, the conclusion that the model has a very large number of extended states, which can be relevant in several physical contexts.

Dynamical phenomena in Fibonacci semiconductor superlattices

We present a detailed study of the dynamics of electronic wave packets in Fibonacci semiconductor superlattices, both in flat band conditions and subject to homogeneous electric fields perpendicular to the layers. Coherent propagation of electrons is described by means of a scalar Hamiltonian using the effective-mass approximation. We have found that an initial Gaussian wave packet is filtered selectively when passing through the superlattice. This means that only those components of the wave packer whose wave numbers belong to allowed subminibands of the fractal-like energy spectrum can propagate over the entire superlattice. The Fourier pattern of the transmitted part of the wave packet presents clear evidences of fractality reproducing those of the underlying energy spectrum. This phenomenon persists even in the presence of unintentional disorder due to growth-induced defects. Finally, we have demonstrated that periodic coherent-field-induced oscillations (Bloch oscillations), which we are able to observe in our simulations of periodic superlattices, are replaced in Fibonacci superlattices by more complex oscillations displaying quasiperiodic signatures, thus shedding more light onto the very peculiar nature of the electronic states in these systems.

Soliton ratchets in homogeneous nonlinear Klein-Gordon systems

We study in detail the ratchetlike dynamics of topological solitons in homogeneous nonlinear Klein-Gordon systems driven by a biharmonic force. By using a collective coordinate approach with two degrees of freedom, namely the center of the soliton, X(t), and its width, l(t), we show, first, that energy is inhomogeneously pumped into the system, generating as result a directed motion; and, second, that the breaking of the time shift symmetry gives rise to a resonance mechanism that takes place whenever the width l(t) oscillates with at least one frequency of the external ac force. In addition, we show that for the appearance of soliton ratchets, it is also necessary to break the time-reversal symmetry. We analyze in detail the effects of dissipation in the system, calculating the average velocity of the soliton as a function of the ac force and the damping. We find current reversal phenomena depending on the parameter choice and discuss the important role played by the phases of the ac force. Our analytical calculations are confirmed by numerical simulations of the full partial differential equations of the sine-Gordon and phi4 systems, which are seen to exhibit the same qualitative behavior. Our results show features similar to those obtained in recent experimental work on dissipation induced symmetry breaking.

Anomalous Resonance Phenomena of Solitary Waves with Internal Modes

We investigate the nonparametric, pure ac driven dynamics of nonlinear Klein-Gordon solitary waves having an internal mode of frequency Omega(i). We show that the strongest resonance arises when the driving frequency delta = Omega(i)/2, whereas when delta = Omega(i) the resonance is weaker, disappearing for nonzero damping. At resonance, the dynamics of the kink center of mass becomes chaotic. As we identify the resonance mechanism as an indirect coupling to the internal mode due to its symmetry, we expect similar results for other systems.

Resonances in the dynamics of f 4 kinks perturbed by ac forces

We study the dynamics of φ 4 kinks perturbed by an ac force, both with and without damping. We address this issue by using a collective coordinate theory, which allows us to reduce the problem to the dynamics of the kink center and width. We carry out a careful analysis of the corresponding ordinary differential equations, of Mathieu type in the undamped case, finding and characterizing the resonant frequencies and the regions of existence of resonant solutions. We verify the accuracy of our predictions by numerical simulation of the full partial differential equation, showing that the collective coordinate prediction is very accurate. Numerical simulations for the damped case establish that the strongest resonance is the one at half the frequency of the internal mode of the kink. In the conclusion we discuss on the possible relevance of our results for other systems, especially the sine-Gordon equation. We also obtain additional results regarding the equivalence between different collective coordinate methods applied to this problem.

EXPLANATION OF DELOCALIZATION IN THE CONTINUOUS RANDOM-DIMER MODEL

We propose an explanation of the bands of extended states appearing in random one dimensional models with correlated disorder, focusing on the Continuous Random Dimer model (A. Sanchez, E. Macia, and F. Dom´onguez-Adame, Phys. Rev. B 49, 147 (1994)). We show exactly that the transmission coefficient at the resonant energy is independent of the number of host sites between two consecutive dimers. This allows us to understand why are there bands of extended states for every realization of the model as well as the dependence of the bandwidths on the concentration. We carry out a perturbative calculation that sheds more light on the above results. In the conclusion we discuss generalizations of our results to other models and possible applications which arise from our new insight of this problem.

Intentionally disordered superlattices with high-DC conductance

We study disordered quantum-well-based semiconductor superlattices where the disorder is intentional and short-range correlated. Such systems consist of quantum wells of two different thicknesses randomly distributed along the growth direction, with the additional constraint that wells of one kind always appears in pairs. Imperfections due to interface roughness are considered by allowing the quantum-well thicknesses to fluctuate around their ideal values. As particular examples, we consider wide-gap (GaAs-Ga/sub 1-x/Al/sub x/As) and narrow-gap (InAs-GaSb) superlattices. We show the existence of a band of extended states in perfect correlated disordered superlattices, giving rise to a strong enhancement of their finite-temperature dc conductance as compared to usual random ones whenever the Fermi level matches this band. This feature is seen to survive even if interface roughness is taken into account. Our predictions can be used to demonstrate experimentally that structural correlations inhibit the localization effects of disorder, even in the presence of imperfections. This effect might be the basis of new, filter-like or other specific-purpose electronic devices. >

Kink stability, propagation, and length-scale competition in the periodically modulated sine-Gordon equation.

We have examined the dynamical behavior of the kink solutions of the one-dimensional sine-Gordon equation in the presence of a spatially periodic parametric perturbation. Our study clarifies and extends the currently available knowledge on this and related nonlinear problems in four directions. First, we present the results of a numerical simulation program that are not compatible with the existence of a radiative threshold predicted by earlier calculations. Second, we carry out a perturbative calculation that helps interpret those previous predictions, enabling us to understand in depth our numerical results. Third, we apply the collective coordinate formalism to this system and demonstrate numerically that it reproduces accurately the observed kink dynamics. Fourth, we report on the occurrence of length-scale competition in this system and show how it can be understood by means of linear stability analysis. Finally, we conclude by summarizing the general physical framework that arises from our study.

Disorder and fluctuations in nonlinear excitations in DNA

We study the effects of the sequence on the propagation of nonlinear excitations in simple models of DNA, and how those effects are modified by noise. Starting from previous results on soliton dynamics on lattices defined by aperiodic potentials, [F. Dominquez-Adame et al., Phys. Rev. E 52, 2183 (1995)], we analyze the behavior of lattices built from real DNA sequences obtained from human genome data. We confirm the existence of threshold forces, already found in Fibonacci sequences, and of stop positions highly dependent on the specific sequence. Another relevant conclusion is that the effective potential, a collective coordinate formalism introduced by Salerno and Kivshar [Phys. Lett. A 193, 263 (1994)] is a useful tool to identify key regions that control the behaviour of a larger sequence. We then study how the fluctuations can assist the propagation process by helping the excitations to escape the stop positions. Our conclusions point out to improvements of the model which look promising to describe mechanical denaturation of DNA. Finally, we also consider how randomly distributed energy focus on the chain as a function of the sequence.