Suchergebnisse

Incoherent transport of excitations in one-dimensional disordered lattices with pairs of traps placed at random is studied by numerically solving the corresponding master equation. Results are compared to the case of lattices with the same concentration of unpaired traps, and it is found that pairing of traps causes a slowdown of the decay rate of both the mean square displacement and the survival probability of excitations. We suggest that this result is due to the presence of larger trap-free segments in the lattices with paired disorder, which implies that pairing of traps causes less disruption on the dynamics of excitations. In the conclusion we discuss the implications of our work, placing it in a more general context. ; This work is partially supported by Universidad Complutense through project PR161/93-4811. A.S. is partially supported by DGICyT (Spain) Grant No. PB92- 0248, by MEC (Spain)/Fulbright, and by the European Union Network ERBCHRXCT930413. Work at Los Alamos is performed under the auspices ofthe U.S. DOE. ; Publicado

We study electronic transport properties of disordered polymers in a quasi-one-dimensional model with fully three-dimensional interaction potentials. We consider such quasi-one-dimensionallattices in the presence of both uncorrelated and short-range correlated impurities. In our procedure, the actual physical potential acting upon the electrons is replaced by a set of nonlocal separable potentials, leading to a Schrodinger equation that is exactly solvable in the momentum representation. By choosing an appropriate potential with the same spectral structure as the physical one, we obtain a discrete set of algebraic equations that can be mapped onto a tight-binding-like equation. We then show that the reflection coefficient of a pair of impurities placed at neighboring sites (dimer defect) vanishes for a particular resonant energy. When there is a finite number of such defects randomly distributed over the whole lattice, we find that the transmission coefficient is almost unity for states close to the resonant energy, and that those states present a very large localization length. Multifractal analysis techniques applied to very long systems demonstrate that these states are truly extended in the thermodynamic limit. These results are obtained with parameters taken from actual physical systems such as polyacetylene, and thus reinforce the possibility of verifying experimentally theoretical predictions about the absence of localization in quasi-one-dimensional disordered systems. ; Work at Madrid is supported by UCM through Project No. PR161/93-4811. Work at Leganes is supported by the DGICyT (Spain) through Project No. PB92-0248, and by the European Union Human Capital and Mobility Programme through Contract No. ERBCHRXCT930413. ; Publicado

We study resonant tunneling in B-8-doped diodes grown by Si-molecular beam epitaxy. A Thomas-Fermi approach is used to obtain the conduction-band modulation. Using a scalar Hamiltonian within the effective-mass approximation we demonstrate that the occurrence of negative differential resistance (NDR) only involves conduction-band states, whereas interband tunneling effects seem to be negligible. Our theoretical results are in very good agreement with recent experimental observations of NDR in this type of diode. ; F. D.-A. acknowledges support from UCM through project PR 161/93-4811. A. S. acknowledges partial support from c.I.c. y T. (Spain) through project PB92-0248 and by the European Union Human Capital and Mobility Programme through contract ERBCHRXCT930413. ; Publicado

We study resonant tunnelling through double-barrier structures under an applied bias voltage, in which nonlinearities due to self-interaction of electrons in the barrier regions are included. As an approximation, we concern ourselves with thin barriers simulated by 8-function potentials. This approximation allows for an analytical expression of the transmission probability through the structure. We show that the typical peaks due to resonant tunneling decrease and broaden as non linearity increases. The main conclusion is that nonlinear effects degrade the peak-to-valley ratio but improve the maximum operation frequency of the resonant tunnelling devices. ; FD.-A. acknowledges support from UCM through project PR161193-4811. A.S. acknowledges partial support from e.1.e. y T. (Spain) through project PB92-0248 and from the European Union Human Capital and Mobility Programme through contract ERBCHRXCT930413. ; Publicado

Incoherent exciton dynamics in one-dimensional perfect lattices with traps at sites arranged according to aperiodic deterministic sequences is studied. We focus our attention on Thue-Morse and Fibonacci systems as canonical examples of self-similar aperiodic systems. Solving numerically the corresponding master equation we evaluate the survival probability and the mean-square displacement of an exciton initially created at a single site. Results are compared to systems of the same size with the same concentration of traps randomly as well as periodically distributed over the whole lattice. Excitons progressively extend over the lattice on increasing time and, in this sense, they act as a probe of the particular arrangements of traps in each system considered. The analysis of the characteristic features of their time decay indicates that exciton dynamics in self-similar aperiodic arrangements of traps is quite close to that observed in periodic ones, but differs significantly from that corresponding to random lattices. We also report on characteristic features of exciton motion suggesting that Fibonacci and Thue-Morse orderings might be clearly observed by appropriate experimental measurements. In the conclusions we comment on the implications of our work on the way towards a unified theory of the ordering of matter. ; This work is partially supported by Universidad Complutense through Project No. PRI61/93-4811. A.S. is partially supported by DGICyT (Spain) Grant No. PB92-0248, by MEC (Spain)/Fulbright, and by the European Union Network ERBCHRXCT930413. Work at Los Alamos is performed under the auspices of the U .S. DOE. ; Publicado

We theoretically study electron transport in disordered, quantum-well-based, semiconductor superlattices with structural short-range correlations. Our system consists of equal-width square barriers and quantum wells with two different thicknesses. The two kinds of quantum wells are randomly distributed along the growth direction. Structural correlations are introduced by adding the constraint that one of the wells always appears in pairs. We show that such correlated disordered superlattices exhibit a strong enhancement of their dc conductance as compared to usual random ones, giving rise to quasi-ballistic-electron transport. Interestingly, this phenomenon is also detected in superlattices with random fluctuations of the well thicknesses. Our predictions can be used to demonstrate experimentally that structural correlations inhibit the localization effects of disorder and, most important, that it should be clearly observed even in the presence of imperfections ; Work at Madrid was supported by UCM through Project No. PR161/93-4811. Work at Leganes was supported by the DGICyT (Spain) through Project No. PB92-0248, and by the European Union Human Capital and Mobility Programme through Contract No. ERBCHRXCT930413. ; Publicado

We study the relationship between the electronic spectrum structure and the configurational order of onedimensional quasiperiodic systems. We take the Fibonacci case as a specific example, but the ideas outlined here may be useful to accurately describe the energy spectra of general quasiperiodic systems of technological interest. Our main result concerns the minimization of the information entropy as a characteristic feature associated with quasiperiodic arrangements. This feature is shown to be related to the ability of quasiperiodic systems to encode more information, in the Shannon sense, than periodic ones. In the conclusion we comment on interesting implications of these results on further developments on the issue of quasiperiodic order. ; This work is partially supported by UCM under Project No. PR161/93-481l. A.S. is partially supported by DGICyT (Spain) through Project No. PB92-0248, and by the European Union Human Capital and Mobility Programme through Contract No. ERBCHRXCT930413 ; Publicado

We study how the influence of structural correlations in disordered systems manifests itself in experimentally measurable magnitudes, focusing on dc conductance of semiconductor supedattices with general potential profiles. We show that the existence of bands of extended states in these structures gives rise to very noticeable peaks in the finite-temperature dc conductance as the chemical potential is moved through the bands or as the temperature is increased from zero. On the basis of these results we discuss how dc conductance measurements can provide information on the location and width of the bands of extended states. Our predictions can be used to demonstrate experimentally that structural correlations inhibit the localization effects of disorder. ; Work at Leganes is supported by the Direccion General de Investigacion Cientifica y Tecnica (Spain) through project PB92-0248, and by the European Union Human Capital and Mobility Programme through Contract No. ERBCHRXCT930413. Work at Madrid is supported by U niversidad Complutense through Project No. PR161/93-4811. ; Publicado

We derive a discrete Hamiltonian describing a Fibonacci superlattice in which the electronic potential is taken to be an array of equally spaced 0 potentials, whose strengths modulate the chemical composition in the growth direction. In this model both diagonal and off-diagonal elements of the Hamiltonian matrix become mutually related through the potential strengths. The corresponding energy spectrum and related magnitudes, such as the Lyapunovcoefficient, transmission coefficient, and Landauer resistance, exhibit a highly fragmented, self-similar nature. We investigate the influence of the underlying spectrum structure on the dc conductance at different temperatures obtaining analytical expressions which relate special features of the dc conductance with certain parameters that characterize the electronic spectrum of Fibonacci superlattices. ; A.S. is partially supported by DGICyT (Spain) through Project No. PB92-0248, and by European Union through NETWORK nonlinear Spatio-Temporal Structures in Semiconductors, Fluids, and Oscillator Ensembles. ; Publicado

We have examined the dynamical behavior of the kink solutions of the one-dimensional sineGordon 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 interpr~t 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. ; A.S. is supported by the Ministerio de Educaci6n y Ciencia (Spain) and the Fulbright Fund, by Direcci6n General de Investigaci6n Cientffica y Tecnica (Spain) through Project No. PB92-0378, and by the European Union (Network on Nonlinear Spatio-Temporal Structures in Semiconductor, Fluids, and Oscillator Ensembles). He also thanks the Los Alamos National Laboratory for warm hospitality and a productive atmosphere. Work at Los Alamos is performed under the auspices of the U.S. Department of Energy. ; Publicado

Dirac materials are characterized by energy-momentum relations that resemble those of relativistic massless particles. Commonly denominated Dirac cones, these dispersion relations are considered to be their essential feature. These materials comprise quite diverse examples, such as graphene and topological insulators. Band-engineering techniques should aim to a full control of the parameter that characterizes the Dirac cones: the Fermi velocity. We propose a general mechanism that enables the fine-tuning of the Fermi velocity in Dirac materials in a readily accessible way for experiments. By embedding the sample in a uniform electric field, the Fermi velocity is substantially modified. We first prove this result analytically, for the surface states of a topological insulator/semiconductor interface, and postulate its universality in other Dirac materials. Then we check its correctness in carbon-based Dirac materials, namely graphene nanoribbons and nanotubes, thus showing the validity of our hypothesis in different Dirac systems by means of continuum, tight-binding and ab-initio calculations. ; Tis work was supported by the Spanish MINECO under grants MAT2013-46308, MAT2016-75955, FIS2015-64654-P, FIS2013-48286-C02-01-P and FIS2016- 76617-P, by the Basque Government through the ELKARTEK project (SUPER) and the University of the Basque Country (Grant No. IT-756-13). ; Peer Reviewed

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. Dominguez-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 there are 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 insight into this problem. ; AS. was partially supported by MEC (Spain)/Fulbright, by DGICyT (Spain) through project PB92-0248, and by the European Union Human Capital and Mobility Programme through Contract No. ERBCHRXCT930413. ED.-A was supported by Universidad Complutense through project PRI61193-4811. G.P.B. gratefully acknowledges partial support from Linkage Grant No. 93-1602 from NATO Special Programme Panel on Nanotechnology. Work at Los Alamos is performed under the auspices of the U.S. Department of Energy. ; Publicado