Person: Maciá Barber, Enrique Alfonso
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First Name
Enrique Alfonso
Last Name
Maciá Barber
Affiliation
Universidad Complutense de Madrid
Faculty / Institute
Ciencias Físicas
Department
Física de Materiales
Area
Física de la Materia Condensada
Identifiers
6 results
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Now showing 1 - 6 of 6
Item Delocalized vibrations in classical random chains(Physical Review B, 1993) Domínguez-Adame Acosta, Francisco; Maciá Barber, Enrique Alfonso; Sánchez, AngelNormal 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.Item Optical absorption in paired correlated random lattices(Physical Review B, 1994) Domínguez-Adame Acosta, Francisco; Maciá Barber, Enrique Alfonso; Sánchez, AngelOptical 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.Item Erratum: Suppression of localization in kronig-penney models with correlated disorder (Vol. 49, PG 147, 1994)(Physical review B, 1994) Sánchez, Angel; Maciá Barber, Enrique Alfonso; Domínguez-Adame Acosta, FranciscoWe 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.Item Energy-spectra of quasi-periodic systems via information entropy(Physical Review E, 1994) Maciá Barber, Enrique Alfonso; Domínguez-Adame Acosta, Francisco; Sánchez, AngelWe study the relationship between the electronic spectrum structure and the configurational order of one-dimensional 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.Item Incoherent exciton trapping in self-similar aperiodic lattices(Physical review B, 1995) Domínguez-Adame Acosta, Francisco; Maciá Barber, Enrique Alfonso; Sánchez, AngelIncoherent 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 di8'ers 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.Item Effects of the electronic structure on the dc conductance of Fibonacci superlattices(Physical Review B, 1994) Maciá Barber, Enrique Alfonso; Domínguez-Adame Acosta, Francisco; Sánchez, AngelWe derive a discrete Hamiltonian describing a Fibonacci superlattice in which the electronic potential is taken to be an array of equally spaced delta 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 Lyapunov coefficient, 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.