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We present a design of two-dimensional periodic dielectric structures that gives rise to absolute band gaps common to E and H polarized waves. This structure is formed by an arrangement of parallel cylindrical rods centered at the vertices of regular hexagons. We show that two absolute band gaps exist for lattices of dielectric rods in air. Potential application of this structure at the submicrometer length scale is discussed.

We report a calculation of the electronic properties of GaAs/AlAs (311) superlattices through the Brillouin zone using realistic values of the parameters in a tight-binding Hamiltonian. The nature and the localization of the lowest conduction bands for these low-symmetry structures is investigated and its origin is studied with respect to the layer number. A direct-gap\char21{}to\char21{}indirect-gap transition is evidenced for L=3 nm which is equal to the thickness for which the crossing of \ensuremath{\Gamma}-X states occurs in the (100) superlattices. The in-plane anisotropy of the optical transitions and the relative intensity is found to be dependent on both the layer number and the nature of the states involved in the transition. Comparison with available experimental data is quite satisfactory.

We have studied the elastic and electronic properties of a semiconductor of cubic symmetry simultaneously subjected to a biaxial strain and a high hydrostatic pressure. Owing to the breakdown of the linear Hooke's law at high pressures, we have developed a nonlinear elastic model to describe this experimental situation. Our model is in accordance with Murnaghan's equation of state for hydrostatic pressure. It takes into account the significant pressure variation of the elastic stiffness constants in the high-pressure range. Electronic properties are obtained through a 20-band empirical tight-binding method using the atomic positions calculated from the nonlinear elastic model. The method is applied to a high-pressure experiment in a ZnSe epilayer biaxially strained on a GaAs substrate. We calculate the variation under high pressure of the electronic transitions in that system. Our approach gives satisfactory results and enables us to analyze the anomalous behavior of the experimental transitions in that system in terms of weak anisotropic components of the hydrostatic pressure applied in the diamond-anvil cell.

We present an approach that uses power moments of the Hamiltonian to calculate the electronic states in low-dimensional structures, such as quantum wells or superlattices. From a microscopic tight-binding description of the crystals, the orthogonalized moments of the Hamiltonian are used to obtain the coefficients of the continued fraction representing the density of the electronic states. This method allows us to calculate quite efficiently the electronic states of large-period systems and clusters. Application to a one-dimensional quantum-well model shows the ability of the method to determine the localized states with good accuracy. Then, we calculate the electronic structure of a strained CdTe/ZnTe superlattice and we study the dependence of the highest valence states at the center of the Brillouin zone, with respect to the valence-band offset. Existence of type-I--type-II transition for a critical value of the valence-band offset is brought out.

A self-consistent tight-binding (SCTB) calculation of the deformation potentials is performed for ZnTe, ZnSe, and ZnS. Tight-binding bulk parameters reproducing photoemission and reflectivity experiments are given for these compounds. Then the strain effects are considered in the SCTB model, which allows the calculation of the hydrostatic deformation potential and of the nonlinear variation of the fundamental ${\mathit{E}}_{0}$ gap with the relative change in the lattice constant \ensuremath{\Delta}a/${\mathit{a}}_{0}$. The method is applied to the determination of the offset in the strained ZnS-ZnSe system. Results are in good agreement with experimental and other theoretical works.

We study the effect of Coulomb interactions on the formation of polarons and bipolarons in polythiophene. First, the interaction of an ionized dopant is considered and these defects are found to be always energetically favored upon doping, bipolarons being the lowest-lying charged excitations. The influence of the Coulomb repulsion associated with the double occupation of the gap levels that occurs in bipolarons is then investigated within a phenomenological model. This interaction leads to a modification of the different equilibrium parameters and reduces appreciably the difference between the creation energy of a pair of polarons and that of a bipolaron. However, this contribution is not strong enough to destabilize bipolarons that remain the most stable excitation. Implications of these results for the existence of polarons are discussed depending on whether the excitations are photogenerated or created by doping.

On etudie les proprietes d'absorption optique et electronique des puits quantiques de GaSb/AlSb. On se place dans le cadre de l'approche de la fonction enveloppe multibande et en considerant la valeur de la discontinuite de la bande de valence qui a ete recemment proposee. Le desaccord du reseau modifie fortement la dispersion de la sous-bande de valence. On etudie aussi les effets d'une contrainte uniaxiale externe

We present in this paper the results of an energetic study of the excitation spectrum in doped polyselenophene. The calculations show that polarons and bipolarons are the lowest-lying energy charged states, the creation of bipolarons taking place more readily than that of two polarons, as in similar heterocyclic polymers. We compare the theoretical results with experimental data, such as the evolution of optical and magnetic properties upon doping. The presence of polarons and bipolarons is shown to be consistent with the main features of the spectra.