Navegando por Autor "Costa, C. H."
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Artigo Octonacci photonic quasicrystals(Elsevier, 2015-05-09) Anselmo, Dory Hélio Aires de Lima; Brandão, E. R.; Costa, C. H.; Vasconcelos, Manoel Silva de; Mello, V. D.We study theoretically the transmission spectra in one-dimensional photonic quasicrystals, made up of iO2(A) and TiO2(B) materials, organized following the Octonacci sequence, where the nth-stage of the multilayer Sn is given by the rule Sn ¼ Sn1Sn2Sn1, for n P 3 and with S1 ¼ A and S2 ¼ B. The expression for transmittance was obtained by employing a theoretical calculation based on the transfer-matrix method. For normally incident waves, we observe that, for a same generation, the transmission spectra for transverse electric (TE) and transverse magnetic (TM) waves are equal, at least qualitatively, and they present a scaling property where a self-similar behavior is obtained, as an evidence that these spectra are fractals. The spectra show regions where the omnidirectional band gaps emerges for specific generations of Octonacci photonic structure, except to TM waves. For TE waves, we note that all of them have almost the same width, for different generations. We also report the localization of modes as a consequence of the quasiperiodicity of the heterostructureArtigo Static and dynamic properties of [hkl] low-symmetry trilayers(Wiley, 2016-01-28) Celino, K. R.; Costa, C. H.; Bezerra, Claudionor Gomes; Feitosa, Carlos Chesman de AraújoWe present a theoretical study about the influence of magneto‐crystalline anisotropies on the static and dynamic magnetic properties of trilayers coupled via bilinear and biquadratic exchange fields for situations in which the systems are grown in unusual [hkl ] low‐symmetry directions. We apply a realistic phenomenological model, with a total free magnetic energy that includes Zeeman interaction and magneto‐crystalline anisotropies as well as exchange energy terms. We consider parameters from the literature to illustrate our results for Fe/Cr/Fe systems. In particular, a total of six different magnetic scenarios for the [211] and [321] low‐symmetry growth orientations and three sets of exchange fields were analyzed, and the associated magnetization, magnetoresistance, and spin‐wave frequencies were calculated. Our results show that the combination of magneto‐crystalline symmetries and exchange fields leads to various interesting properties, including different values of the saturation field for the magnetization and magnetoresistance curves. Regarding the spin‐wave modes, we observed the presence of Goldstone modes, associated with second‐order phase transitions, resulting from the competition between the Zeeman and biquadratic energies