Resonant states in GaAs/Ga1-xAlxAs Multi-Quantum-Wells

The effect of buffer layers on resonant states in a Multi-Quantum-Well (MQW) sandwiched between two substrates is investigated here theoretically. These resonances appear as well-defined peaks in the density of states (DOS). The local and total densities of states are obtained from an analytical determination of the Green functions. Due to the substrate/buffer layer/ MQW /substrate interaction, different kinds of resonant states are found and their properties are investigated. We show in particular that an incident electron in the left-hand side substrate is transmitted in the right hand side substrate of the structure with large time delays in the phase time. The peaks in the phase time associated with the transmission coefficient are found to be similar to those corresponding to the DOS. The intensity of these peaks associated with extended states in MQW’s and Tamm like states lying at the MQW/buffer layer interface, strongly depends on the width of the buffer layer.The effect of buffer layers on resonant states in a Multi-Quantum-Well (MQW) sandwiched between two substrates is investigated here theoretically. These resonances appear as well-defined peaks in the density of states (DOS). The local and total densities of states are obtained from an analytical determination of the Green functions. Due to the substrate/buffer layer/ MQW /substrate interaction, different kinds of resonant states are found and their properties are investigated. We show in particular that an incident electron in the left-hand side substrate is transmitted in the right hand side substrate of the structure with large time delays in the phase time. The peaks in the phase time associated with the transmission coefficient are found to be similar to those corresponding to the DOS. The intensity of these peaks associated with extended states in MQW’s and Tamm like states lying at the MQW/buffer layer interface, strongly depends on the width of the buffer layer

Explicit asymptotic modelling of transient Love waves propagated along a thin coating

The official published version can be obtained from the link below.An explicit asymptotic model for transient Love waves is derived from the exact equations of anti-plane elasticity. The perturbation procedure relies upon the slow decay of low-frequency Love waves to approximate the displacement field in the substrate by a power series in the depth coordinate. When appropriate decay conditions are imposed on the series, one obtains a model equation governing the displacement at the interface between the coating and the substrate. Unusually, the model equation contains a term with a pseudo-differential operator. This result is confirmed and interpreted by analysing the exact solution obtained by integral transforms. The performance of the derived model is illustrated by numerical examples.This work is sponsored by the grant from Higher Education of Pakistan and by the Brunel University’s “BRIEF” research award

Phonons in Slow Motion: Dispersion Relations in Ultra-Thin Si Membranes

We report the changes in dispersion relations of hypersonic acoustic phonons in free-standing silicon membranes as thin as \sim 8 nm. We observe a reduction of the phase and group velocities of the fundamental flexural mode by more than one order of magnitude compared to bulk values. The modification of the dispersion relation in nanostructures has important consequences for noise control in nano and micro-electromechanical systems (MEMS/NEMS) as well as opto-mechanical devices.Comment: 5 page

Electronic properties in heterostructures

Using the interface response, the effect of internal surface of Single Quantum Well (SQW) and of Superlattice (SL) (Substrate / Single Quantum Well and of Substrate / Superlattice interfaces) are studied in term of changing the heterostructures parameters and keeping constant the parameters of substrate. Surface and interface considered as well as planar defects are associated with the states which are localised in mini gaps (MG). Electronic properties are touched in this way and their investigation is made by band structures and variational density of states (DOS).Using the interface response, the effect of internal surface of Single Quantum Well (SQW) and of Superlattice (SL) (Substrate / Single Quantum Well and of Substrate / Superlattice interfaces) are studied in term of changing the heterostructures parameters and keeping constant the parameters of substrate. Surface and interface considered as well as planar defects are associated with the states which are localised in mini gaps (MG). Electronic properties are touched in this way and their investigation is made by band structures and variational density of states (DOS)

Surface and interface shear horizontal acoustic waves in piezoelectric superlattices

7 pages, 7 figures, 1 table.-- PACS: 68.35.Gy; 68.35.Ja; 63.22.+m; 73.20.DxThe propagation of acoustic waves of shear horizontal polarization in infinite and semi-infinite superlattices made of two piezoelectric media is studied within a Green's function method. Localized modes induced by a free surface of the superlattice or a superlattice/substrate interface are investigated theoretically. These modes appear as well-defined peaks of the total density of states inside the minigaps of the superlattice. The spatial localization of the different modes is studied by means of the local density of states. The surface of the superlattice and the superlattice/substrate interface are considered to be either metallized or nonmetallized. We show the possibility of the existence of interface modes, which are without analogue in the case of the interface between two homogeneous media (the so-called Maerfeld–Tournois modes). We also generalize to piezoelectric superlattices a rule about the existence and number of surface states, namely when one considers two semi-infinite superlattices together obtained by the cleavage of an infinite superlattice, one always has as many localized surface modes as minigaps, for any value of the wave vector k (parallel to the interfaces). Specific applications of these results are given for CdS–ZnO superlattices with a free surface or in contact with a BeO substrate.The work of A.B., E.E., D.B., and A.N. has been supported by the Program-in-Aid for Scientific Research (PARS). The work of V.R.V. has been supported by the Dirección General de Enseñanza Superior (Spain) through Grant No. PB96-0916. E.E., A.N. and V.R.V. thank the CNCPRST (Morocco) and CSIC (Spain) for a cooperation project.Peer reviewe

Vibrational Properties in heterostructures

This is a comprehensive path about theoretical approaches to the vibrational properties in periodic and multilayered structures. The phonons modes taking a large part of our work are detailed beyond the limit offered by the area targeted by this sort of paper. In fact, in many cases of this type of materials, metallic, semiconductor or insulator, these modes are investigated. Although, the electronic properties are devoted, generally, to the semiconductor ones as well as the optical properties will be derived in the second paper of this volume. Another topic can be touched which concerns the coupling effect between phonons and electrons or photons and electrons giving rise to interesting results; but it is not addressed here. Throughout this paper, we bring forth the theoretical model currently used by us, and the results obtained in consequence. This model is called the response function based on the Green function and its properties are stressed as well as the results obtained.This is a comprehensive path about theoretical approaches to the vibrational properties in periodic and multilayered structures. The phonons modes taking a large part of our work are detailed beyond the limit offered by the area targeted by this sort of paper. In fact, in many cases of this type of materials, metallic, semiconductor or insulator, these modes are investigated. Although, the electronic properties are devoted, generally, to the semiconductor ones as well as the optical properties will be derived in the second paper of this volume. Another topic can be touched which concerns the coupling effect between phonons and electrons or photons and electrons giving rise to interesting results; but it is not addressed here. Throughout this paper, we bring forth the theoretical model currently used by us, and the results obtained in consequence. This model is called the response function based on the Green function and its properties are stressed as well as the results obtained

Absolute acoustic band gap in coupled multilayer structures

We show theoretically that by combining two or more finite superlattices, it is possible to realize an omnidirectional acoustic mirror that prevents propagation of acoustic waves over a wide range of frequencies. An appropriate choice of the elastic parameters in the superlattices enables us to propose a structure in which any incident wave launched from any type of substrate (or vacuum) falls inside a minigap of the coupled SLs and therefore undergoes a total reflection

Elastic Vibrations of Planar and Deterministic Rough Surfaces

We present a few recent or new theoretical results about surface acoustic localized and resonant modes associated with planar or deterministic rough surfaces. The following composite systems are considered: one or two adlayers deposited on a semi-infinite substrate and wires or grooves integrated near a planar surface. The surface modes can be obtained as well-defined features of the density of states resulting from a calculation of the Green functions in these heterostructures. In this work, the materials are described in the framework of the elasticity theory