Navegando por Autor "Freire, V.N."
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Artigo Acoustic phonon transmission spectra in piezoelectric AlN/GaN Fibonacci phononic crystals(Springer Verlag, 2007-09-22) Sesion Júnior, P.D.; Albuquerque, E.L.; Feitosa, Carlos Chesman de Araújo; Freire, V.N.We study the acoustic-phonon transmission spectra in periodic and quasiperiodic (Fibonacci type) superlattices made up from the III-V nitride materials AlN and GaN. The phonon dynamics is described by a coupled elastic and electromagnetic equations within the static field approximation model, stressing the importance of the piezoelectric polarization field in a strained condition. We use a transfer-matrix treatment to simplify the algebra, which would be otherwise quite complicated, allowing a neat analytical expressions for the phonon transmission coefficients. Numerical results, for the normal incidence case, show a strike self-similar pattern for both hexagonal (class 6 mm) and cubic symmetries crystalizations of the nitridesArtigo Dielectric function spectra from a nondegenerate polaron gas(Elsevier, 2007-02-07) Barboza, Carlos A.; Albuquerque, Eudenilson Lins de; Feitosa, Carlos Chesman de Araújo; Freire, V.N.A Green function formalism is employed to perform an analytical calculation of the dielectric function due to an electron–phonon interaction within a Hartree–Fock approximation. Our theoretical approach is based on the polar or Fröhlich type of electron–phonon interaction, with the electronic hopping between the first-neighbor, to characterize the polaron model. Our numerical results for the dielectric function profile show resonant peaks, all of them at lower frequencies than the main one, attributed to the polaron crossover regime between the quasi free-electrons and the small polaronsArtigo The DNA electronic specific heat at low temperature: the role of aperiodicity(Elsevier, 2012) Sarmento, R.G.; Mendes, G.A.; Albuquerque, Eudenilson Lins de; Fulco, Umberto Laico; Vasconcelos, Manoel Silva de; Ujsághy, O.; Freire, V.N.; Caetano, E.W.S.The electronic specific heat spectra at constant volume (CV ) of a long-range correlated extended ladder model, mimicking a DNA molecule, is theoretically analyzed for a stacked array of a double-stranded structure made up from the nucleotides guanine G, adenine A, cytosine C and thymine T . The role of the aperiodicity on CV is discussed, considering two different nucleotide arrangements with increasing disorder, namely the Fibonacci and the Rudin–Shapiro quasiperiodic structures. Comparisons are made for different values of the band fillings, considering also a finite segment of natural DNA, as part of the human chromosome Ch22.