ELECTRONIC STATES IN GRADED-GAP JUNCTIONS WITH BAND INVERSION

We theoretically study electronic states in graded-gap junctions of IV-VI compounds with band inversion. Using a two-band model within the ${\bf k}\cdot{\bf p}$ approximation and assuming that the gap and the gap centre present linear profiles, we demonstrate the existence of a set of localized states along the growth direction with a discrete energy spectrum. The envelope functions are found to be combination of harmonic oscillator eigenfunctions, and the corresponding energy levels are proportional to the square root of the quantum number. The level spacing can be directly controlled by varying the structure thickness.Comment: REVTEX 3.0, 7 pages, no figures, to appear in Phys. Lett.

Relativistic effects in Kronig-Penney models on quasiperiodic lattices

We study tunneling of Dirac particles through Kronig-Penney models on general lattices, computing their transmission coctTicient. We subsequently focus our attention on the Fibonacci lattice as a typical example of a quasi crystaL Wc compare our results to the non-relativistic ones, and find a shrinkage ofthe spectrum similar to that of per iodic systems.We want to thank c.L. Roy for calling our attention to some works on relativistic disordered systems. A.S. thanks L. Vazquez for encouragement, and the C.LC.y T. (Spain) for partial financial support under project MAT90-0544.Publicad

Anomalous scaling in a non local growth model in the Kardar-Parisi-Zhang universality class

We study the interface dynamics of a discrete model to quantitatively describe electrochemical deposition experiments. Extensive numerical simulations indicate that the interface dynamics is unstable at early times, but asymptotically displays the scaling of the Kardar-Parisi-Zhang universality class. During the time interval in which the surface is unstable, its power spectrum is anomalous; hence the behaviors at length scales smaller than or comparable with the system size are described by different roughness exponents. These results are expected to apply to a wide range of electrochemical deposition experiments.Comment: REVTEX (4 pages) and three figures (postscript), to be published in PRE (rapid communication, March, 1998

Optical absorption in Fibonacci lattices at finite temperature

We consider the dynamics of Frenkel excitons on quasiperiodic lattices, focusing our attention on the Fibonacci case as a typical example. We evaluate the absorption spectrum by solving numerically the equation of motion of the Frenkel-exciton problem on the lattice. Besides the main absorption line, satellite lines appear in the high-energy side of the spectra, which we have related to the underlying quasiperiodic order. The influence of lattice vibrations on the absorption line shape is also considered. We find that the characteristic features of the absorption spectra should be observable even at room temperature. Consequently, we propose that excitons act as a probe of the topology of the lattice even when thermal vibrations reduce their quantum coherence

Nonlinear Resonant Tunnelling Through Double Barrier Structures

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 $\delta$-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 nonlinearity 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.Comment: REVTeX 3.0. 8 pages, 4 figures (PostScript files available on request from ED [[email protected]]). Submitted to J Phys A. MA/UC3M/17/199

Delocalized vibrations in classical random chains

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.Comision Interministerial de Ciencia y Tecnologia of Spain for financial support under Project No. MAT90-0544Publicad

Dephasing effects induced by weak disorder in superlattices

We investigate the dephasing dynamics of Bloch oscillations in semiconductor superlattices by means of a very simple model including weak disorder and applied electric fields A thorough numerical study of our model allows us to claim that small, unintentional well width fluctuation can be responsible for fast dephasing of Bloch oscillations at low temperatures. We show that the lifetime of Bloch oscillations is controlled by a characteristic time which depends on the degree of disorder and is independent of the electric field This result is further supported by the excellent agreement between our model calculations and several recent experiments, and leads to specifi new predictions.This work has been supported by CICYT (Spain) under project MAT95-0325, and by DGES (Spain) PB96-0119Publicad

Effects of the electronic structure on the dc conductance of Fibonacci superlattices

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.Publicad