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

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

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