Person:
Maciá Barber, Enrique Alfonso

First Name

Enrique Alfonso

Last Name

Maciá Barber

URI

https://hdl.handle.net/20.500.14352/75005

Affiliation

Universidad Complutense de Madrid

Faculty / Institute

Ciencias Físicas

Department

Física de Materiales

Area

Física de la Materia Condensada

Identifiers

Full item page

Search Results

Now showing 1 - 10 of 22

Suppression of localization in kronig-penney models with correlated disorder
(Physical Review B, 1994) Sáchez, A.; Maciá Barber, Enrique Alfonso; Domínguez-Adame Acosta, Francisco
We consider the electron dynamics and transport properties of one-dimensional continuous models with random, short-range correlated impurities. We develop a generalized Poincare map formalism to cast the Schrodinger equation for any potential into a discrete set of equations, illustrating its application by means of a specific example. We then concentrate on the case of a Kronig-Penney model with dimer impurities. The previous technique allows us to show that this model presents infinitely many resonances (zeroes of the reflection coefficient at a single dimer) that give rise to a band of extended states, in contradiction with the general viewpoint that all one-dimensional models with random potentials support only localized states. We report on exact transfer-matrix numerical calculations of the transmission coefFicient, density of states, and localization length for various strengths of disorder. The most important conclusion so obtained is that this kind of system has a very large number of extended states. Multifractal analysis of very long systems clearly demonstrates the extended character of such states in the thermodynamic limit. In closing, we brieBy discuss the relevance of these results in several physical contexts.
Excitation optical-absorption in self-similar aperiodic lattices
(Physical Review B, 1994) Maciá Barber, Enrique Alfonso; Domínguez-Adame Acosta, Francisco
Exciton optical absorption in self-similar aperiodic one-dimensional systems is considered, focusing our attention on Thue-Morse and Fibonacci lattices as canonical examples. The absorption line shape is evaluated by solving the microscopic equations of motion of the Frenkel-exciton problem on the lattice, in which on-site energies take on two values, according to the Thue-Morse or Fibonacci sequences. Results are compared to those obtained in random lattices with the same stoichiometry and size. We find that aperiodic order causes the occurrence of well-de6ned characteristic features in the absorption spectra, which clearly di8'er from the case of random systems, indicating a most peculiar exciton dynamics. The origin of all the absorption lines is assigned by considering the self-similar aperiodic lattices as composed of two-center blocks, within the same spirit of the renormalization group ideas.
Three-dimensional effects on the electronic structure of quasiperiodic systems
(Physica B, 1995) Maciá Barber, Enrique Alfonso; Domínguez-Adame Acosta, Francisco
We report on a theoretical study of the electronic structure of quasiperiodic, quasi-one-dimensional systems where fully three-dimensional interaction potentials are taken into account. In our approach, the actual physical potential acting upon the electrons is replaced by a set of nonlocal separable potentials, leading to an exactly solvable Schrodinger equation. By choosing an appropriate trial potential, we obtain a discrete set of algebraic equations that can be mapped onto a general tight-binding-like equation. We introduce a Fibonacci sequence either in the strength of the on-site potentials or in the nearest-neighbor distances, and we find numerically that these systems present a highly fragmented, self-similar electronic spectrum, which becomes singular continuous in the thermodynamical limit. In this way we extend the results obtained so far in one-dimensional models to the three-dimensional case. As an example of the application of the model we consider the chain polymer case.
Physical nature of critical wave functions in Fibonacci systems-Errata (vol 76, pg 2957, 1996)
(Physical review letters, 1997) Maciá Barber, Enrique Alfonso; Domínguez-Adame Acosta, Francisco
A transfer-matrix method for the determination of one-dimensional band structures
(Journal of Physics A-Mathematical and General, 1993) Méndez Martín, Bianchi; Domínguez-Adame Acosta, Francisco; Maciá Barber, Enrique Alfonso
We show that the discretized forms of the Schrodinger and the Dirac equations for an arbitrary potential in one dimension are equivalent to the Poincare map of the corresponding wave equation for an array of delta-function potentials. Therefore, the dynamics of particles in general periodic potentials may be studied by means of an equivalent generalized Kronig-Penney model, in which there exist several delta-function potentials in each unit cell. Taking into account the techniques of dynamical systems, the transfer matrix method is then used in a simple form to compute the energy band edges and the dispersion law inside the allowed bands.
Can fractal-like spectra be experimentally observed in aperiodic superlattices?
(Semiconductor science and technology, 1996) Maciá Barber, Enrique Alfonso; Domínguez-Adame Acosta, Francisco
We numerically investigate the effects of inhomogeneities in the energy spectrum of aperiodic semiconductor superlattices, focusing our attention on Thue-Morse and Fibonacci sequences. In the absence of disorder, the corresponding electronic spectra are self-similar. The presence of a certain degree of randomness, due to imperfections occurring during the growth processes, gives rise to a progressive loss of quantum coherence, smearing out the finer details of the energy spectra predicted for perfect aperiodic superlattices and spurring the onset of electron localization. However, depending on the degree of disorder introduced, a critical size for the system exists, below which peculiar transport properties, related to the pre-fractal nature of the energy spectrum, may be measured.
Delocalized vibrations in classical random chains
(Physical Review B, 1993) Domínguez-Adame Acosta, Francisco; Maciá Barber, Enrique Alfonso; Sánchez, Angel
Normal modes of one-dimensional disordered chains with two couplings, one of them assigned at random to pairs in an otherwise perfect chain, are investigated. We diagonalize the dynamical matrix to find the normal modes and to study their spatial extent. Multifractal analysis is used to discern clearly the localized or delocalized character of vibrations. In constrast to the general viewpoint that all normal modes in one dimensional random chains are localized, we find a set of extended modes close to a critical frequency, whose number increases with the system size and becomes independent of the defect concentration.
Physical nature of critical wave functions in Fibonacci systems
(Physical Review Letters, 1996) Maciá Barber, Enrique Alfonso; Domínguez-Adame Acosta, Francisco
We report on a new class of critical states in the energy spectrum of general Fibonacci systems. By introducing a transfer matrix renormalization technique, we prove that the charge distribution of these states spreads over the whole system, showing transport properties characteristic of electronic extended states. Our analytical method is a first step to find out the link between the spatial structure of critical wave functions and their related transport properties.
Optical absorption in paired correlated random lattices
(Physical Review B, 1994) Domínguez-Adame Acosta, Francisco; Maciá Barber, Enrique Alfonso; Sánchez, Angel
Optical absorption in a random one-dimensional lattice in the presence of paired correlated disorder is studied. The absorption line shape is evaluated by solving the microscopic equations of motion of the Frenkel-exciton problem in the lattice. We find that paired correlation causes the occurrence of well-defined characteristic lines in the absorption spectra which clearly differ from the case of unpaired correlation. The behavior of the absorption lines as a function of defect concentration is studied in detail. We also show how exciton dynamics can be inferred from experimental data by deriving an analytical expression relating the energy and intensity of lines to the model parameters.
Fluorescence decay in aperiodic Frenkel lattices
(Physical Review B, 1996) Domínguez-Adame Acosta, Francisco; Maciá Barber, Enrique Alfonso
We study motion and capture of excitons in self-similar linear systems in which interstitial traps are arranged according to an aperiodic sequence, focusing our attention on Fibonacci and Thue-Morse systems as canonical examples. The decay of the fluorescence intensity following a broadband pulse excitation is evaluated by solving the microscopic equations of motion of the Frenkel exciton problem. We find that the average decay is exponential and depends only on the concentration of traps and the trapping rate. In addition, we observe small-amplitude oscillations coming from the coupling between the low-lying mode and a few high-lying modes through the topology of the lattice. These oscillations are characteristic of each particular arrangement of traps and they are directly related to the Fourier transform of the underlying lattice. Our predictions then can be used to determine experimentally the ordering of traps.