Navegando por Autor "Albuquerque, E. L."
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Artigo Band gaps of acoustic waves propagating in a solid/liquid phononic fibonacci structure(Elsevier, 2010-09-01) Sesion Junior, Paulo Dantas; Albuquerque, E. L.We study the acoustic–phonon transmission spectra in quasiperiodic (Fibonacci type) superlattices made up from the solid crystal quartz and the liquid mercury (Hg). The phonon dynamics is described by a coupled elastic equations within the static field approximation model. We use a transfer-matrix treatment to simplify the algebra, which would be otherwise quite complicated, allowing a neat analytical expression for the phonon transmission coefficients. Numerical results is presented and discussed for both the transmittance spectra as well as the localization factor derived from the Lyapunov exponent, showing that the Fibonacci quasiperiodic structure acts as a filter for the phonon’s transmission spectraArtigo Low - temperature interface specific heat due to localized modes(ELSEVIER, 1982) Carriço, Artur da Silva; Albuquerque, E. L.Green functions are derived for the displacement associated with acoustic vibrations in isotropic elastic media, and used to evaluate the surface specific heat in the harmonic approximation. The contribution of surface elastic waves of the Rayleigh and love types are pointed out and their features discussed. The nature of the results and their relations to previous works in this field is also presented and discussed.Artigo Structural, electronic and optical properties of ilmenite and perovskite CdSnO3 from DFT calculations(IOP Publishing, 2010-10-12) Sesion Junior, Paulo Dantas; Henriques, J. M.; Barboza, C. A.; Albuquerque, E. L.; Freire, V. N.; Caetano, E. W. S.CdSnO3 ilmenite and perovskite crystals were investigated using both the local density and generalized gradient approximations, LDA and GGA, respectively, of the density functional theory (DFT). The electronic band structures, densities of states, dielectric functions, optical absorption and reflectivity spectra related to electronic transitions were obtained, as well as the infrared absorption spectra after computing the vibrational modes of the crystals at q = 0. Dielectric optical permittivities and polarizabilities at ω = 0 and ∞ were also calculated. The results show that GGA-optimized geometries are more accurate than LDA ones, and the Kohn–Sham band structures obtained for the CdSnO3 polymorphs confirm that ilmenite has an indirect band gap, while perovskite has a direct band gap, both being semiconductors. Effective masses for both crystals are obtained for the first time, being highly isotropic for electrons and anisotropic for holes. The optical properties reveal a very small degree of anisotropy of both crystals with respect to different polarization planes of incident light. The phonon calculation at q = 0 for perovskite CdSnO3 does not show any imaginary frequencies, in contrast to a previous report suggesting the existence of a more stable crystal of perovskite CdSnO3 with ferroelectric properties