E. Diez

Electron transport across a Gaussian superlattice

We study the electron transmission probability in semiconductor superlattices where the height of the barriers is modulated by a Gaussian profile. Such structures act as efficient energy band-pass filters and, contrary to previous designs, it is expected to present a lower number of unintentional defects and, consequently, better performance. The j–V characteristic presents negative differential resistance with peak-to-valley ratios much greater than in conventional semiconductor superlattices.

Absence of localization and large dc conductance in random superlattices with correlated disorder

We study how the in8uence of structural correlations in disordered systems manifests itself in experimentally measurable magnitudes, focusing on dc conductance of semiconductor superlattices with general potential pro6les. We show that the existence of bands of extended states in these structures gives rise to very noticeable peaks in the 6nite-temperature dc conductance as the chem ical 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 eKects of disorder.

Quasi-ballistic-electron transport in random superlattices

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.

Transport in random quantum dot superlattices

We present a model based on the two-dimensional transfer matrix formalism to calculate single-electron states in a random wide-gap semiconductor quantum dot superlattice. With a simple disorder model both the random arrangement of quantum dots (configurational disorder) and the spatial inhomogeneities of their shape (morphological disorder) are considered. The model correctly describes channel mixing and broadening of allowed energy bands due to elastic electron scattering by disorder.

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.

Lump solitons in a higher-order nonlinear equation in 2+1 dimensions

We propose and examine an integrable system of nonlinear equations that generalizes the nonlinear Schrödinger equation to 2+1 dimensions. This integrable system of equations is a promising starting point to elaborate more accurate models in nonlinear optics and molecular systems within the continuum limit. The Lax pair for the system is derived after applying the singular manifold method. We also present an iterative procedure to construct the solutions from a seed solution. Solutions with one-, two-, and three-lump solitons are thoroughly discussed.

Nature of the extended states in random dimer-barrier superlattices

We theoretically study electron transmission in intentionally disordered GaAs–AlxGa1� xAs superlattices with structural short-range correlations in the Al mole fraction of the AlxGa1� xAs layers. The Al mole fraction in the equalwidth AlxGa1� xAs layers takes at random two different values, but with the constraint that one of them only appears at random in pairs, while GaAs layers are identical. We demonstrate that the superlattice supports two types of extended states, one of them comes from resonance effects at dimer barriers, as it was already reported for random dimer well superlattices, while the other type arises as a consequence of the binary nature of this heterostructure. Conditions for their observation in transport experiments are discussed. r 2002 Elsevier Science B.V. All rights reserved.

Coherent carrier dynamics in semiconductor superlattices

We investigate the coherent dynamics of carriers in semiconductor superlattices driven by ac-dc electric fields. We solve numerically the time-dependent effective-mass equation for the envelope function. We find that carriers undergo Rabi oscillations when the driving frequency is close to the separation between minibands.

Electron dynamics in intentionally disordered semiconductor superlattices

We study the dynamical behavior of disordered quantum well-based semiconductor superlattices where the disorder is intentional and short-range correlated. We show that, whereas the transmission time of a particle grows exponentially with the number of wells in an usual disordered superlattice for any value of the incident particle energy, for specific values of the incident energy this time increases linearly when correlated disorder is included. As expected, those values of the energy coincide with a narrow subband of extended states

Electron scattering on disordered double-barrier GaAs–AlxGa1−xAs heterostructures

Abstract We present a novel model to calculate vertical transport properties such as conductance and current in unintentionally disordered double-barrier GaAs–Al x Ga 1− x As heterostructures. The source of disorder comes from interface roughness at the heterojunctions (lateral disorder) as well as spatial inhomogeneities of the Al mole fraction in the barriers (compositional disorder). Both lateral and compositional disorder break translational symmetry along the lateral direction and therefore electrons can be scattered off the growth direction. The model correctly describes channel mixing due to these elastic scattering events. In particular, for realistic degree of disorder, we have found that the effects of compositional disorder on transport properties are negligible as compared to the effects due to lateral disorder.