9,881 research outputs found
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 {\em ideal}
values. As particular examples, we consider wide-gap
(GaAs-GaAlAs) 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.Comment: REVTeX 3.0, 20 pages, 7 uuencoded compressed PostScript figures as a
separate file. Submitted to IEEE J Quantum Elec
Transport in random quantum dot superlattices
We present a novel model to calculate single-electron states in random
quantum dot superlattices made of wide-gap semiconductors. The source of
disorder comes from the random arrangement of the quantum dots (configurational
disorder) as well as spatial inhomogeneities of their shape (morphological
disorder). Both types of disorder break translational symmetry and prevent the
formation of minibands, as occurs in regimented arrays of quantum dots. The
model correctly describes channel mixing and broadening of allowed energy bands
due to elastic scattering by disorder
Symmetry-Induced Tunnelling in One-Dimensional Disordered Potentials
A new mechanism of tunnelling at macroscopic distances is proposed for a wave
packet localized in one-dimensional disordered potential with mirror symmetry,
V(-x)=V(x). Unlike quantum tunnelling through a regular potential barrier,
which occurs only at the energies lower then the barrier height, the proposed
mechanism of tunnelling exists even for weak white-noise-like scattering
potentials. It also exists in classical circuits of resonant contours with
random resonant frequencies. The latter property may be used as a new method of
secure communication, which does not require coding and decoding of the
transmitting signal.Comment: 10 pages, 4 figure
Hybridization of multi-objective deterministic particle swarm with derivative-free local searches
The paper presents a multi-objective derivative-free and deterministic global/local hybrid algorithm for the efficient and effective solution of simulation-based design optimization (SBDO) problems. The objective is to show how the hybridization of two multi-objective derivative-free global and local algorithms achieves better performance than the separate use of the two algorithms in solving specific SBDO problems for hull-form design. The proposed method belongs to the class of memetic algorithms, where the global exploration capability of multi-objective deterministic particle swarm optimization is enriched by exploiting the local search accuracy of a derivative-free multi-objective line-search method. To the authors best knowledge, studies are still limited on memetic, multi-objective, deterministic, derivative-free, and evolutionary algorithms for an effective and efficient solution of SBDO for hull-form design. The proposed formulation manages global and local searches based on the hypervolume metric. The hybridization scheme uses two parameters to control the local search activation and the number of function calls used by the local algorithm. The most promising values of these parameters were identified using forty analytical tests representative of the SBDO problem of interest. The resulting hybrid algorithm was finally applied to two SBDO problems for hull-form design. For both analytical tests and SBDO problems, the hybrid method achieves better performance than its global and local counterparts
A multi-objective DIRECT algorithm for ship hull optimization
The paper is concerned with black-box nonlinear constrained multi-objective optimization problems. Our interest is the definition of a multi-objective deterministic partition-based algorithm. The main target of the proposed algorithm is the solution of a real ship hull optimization problem. To this purpose and in pursuit of an efficient method, we develop an hybrid algorithm by coupling a multi-objective DIRECT-type algorithm with an efficient derivative-free local algorithm. The results obtained on a set of “hard” nonlinear constrained multi-objective test problems show viability of the proposed approach. Results on a hull-form optimization of a high-speed catamaran (sailing in head waves in the North Pacific Ocean) are also presented. In order to consider a real ocean environment, stochastic sea state and speed are taken into account. The problem is formulated as a multi-objective optimization aimed at (i) the reduction of the expected value of the mean total resistance in irregular head waves, at variable speed and (ii) the increase of the ship operability, with respect to a set of motion-related constraints. We show that the hybrid method performs well also on this industrial problem
Pilot study of vegetation in the Alchichica-Perote region by remote sensing
A study of the application of satellite images to the identification of vegetation in a small area corresponding to the arid zone of Veracruz and part of Puebla is presented. This study is accomplished by means of images from the LANDSAT satellite obtained on January 19 and May 23, 1973. The interpretation of the different maps is made on the basis of information from the data bank of the Flora de Veracruz program, and various surveys made by land and air
Extended states and dynamical localization in semiconductor superlattices
We study the quantum dynamics of electronic wave packets in quantum-well based semiconductor
superlattices subject to an applied electric field. Using a high-accuracy numerical method, we
analyze the dynamical behavior of electronic wave packets in periodic, random and random dimer
superlattices. The spatial extent of electronic states is characterized by means of the time-dependent
inverse participation ratio. We show that the delocalized states recently found in random dimer
superlattices become spatially localized under the action of the applied field ~dynamical
localization! but wavepackets are much less localized than in purely random superlattices at
moderate field. We conclude that the resonant tunneling effects causing delocalization in dimer
superlattices play an important role even in the presence of moderate electric field.This work has been supported by CICYT (Spain) under project MAT95-0325.Publicad
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