CCET - DFTE - Artigos publicados em periódicos
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Navegando CCET - DFTE - Artigos publicados em periódicos por Autor "Albuquerque, Eudenilson Lins de"
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Artigo Crossover-behavior of the saturation magnetic fields of (100) and (110) Fe/Cr/Fe trilayers(Physics Letters A, 2006-01-27) Feitosa, Carlos Chesman de Araújo; Bezerra, Claudionor Gomes; Albuquerque, Eudenilson Lins de; Mariz, Ananias MonteiroWe use a simple and realistic theoretical model to investigate the saturation magnetic fields of ultrathin (100) and (110) Fe/Cr/Fe trilayer system. We consider the Fe films presenting magneto-crystalline anisotropies (cubic and uniaxial) and interacting through bilinear and biquadratic exchange couplings. From the equilibrium configurations, which are determined by the absolute minimum of the magnetic energy, we were able to set up field-dependent phase diagrams for the situation where the external static magnetic field H is applied in the plane of the films at an arbitrary angle θH . We have obtained the critical frontier between the saturated and non-saturated phases. Two main results should be pointed out: (i) the change of the symmetry axis according to the magneto-crystalline fields and (ii) a sudden change (on a multicritical point) of the character of the magnetic phase transitions, typical of a crossover behaviorArtigo 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 Effects of the magneto-crystalline anisotropy on the magnetic properties of Fe/Cr/Fe (110) trilayer(Springer Verlag, 2004-07-23) Bezerra, Claudionor Gomes; Feitosa, Carlos Chesman de Araújo; Albuquerque, Eudenilson Lins de; Azevedo, A.In this paper we present a theoretical study about the influence of the magneto-crystalline anisotropy on the magnetic properties of magnetic metallic trilayers Fe/Cr/Fe (110). The theory is based on a realistic phenomenological model which includes the following contributions to the free magnetic energy: Zeeman, cubic and uniaxial anisotropy, as well as bilinear and biquadratic exchange energies. The experimental parameters used here are based on experimental data known from the literature. We present numerical results of magnetization versus external applied field to illustrate the behavior of the system. Our numerical results show that in some situations the saturation field can not be correctly determined by magnetoresistance measuresArtigo Magnetic polaritons in metamagnet layered structures: spectra and localization properties(Elsevier, 2007-09-11) Anselmo, Dory Hélio Aires de Lima; Araujo, C. A. A.; Albuquerque, Eudenilson Lins de; Vasconcelos, M. S.The magnetic polariton propagation in metamagnet layered structures is theoretically studied by using a transfer matrix approach. The layered structures considered here are made up by the stacking of two different layers (also known as building blocks, named A and B), where one of them is a metamagnetic thin film (A), while the other is a non-magnetic insulator thin layer (B). We take into account both the antiferromagnetic (AFM) and ferromagnetic (FM) phases of the metamagnetic material. For the periodic arrangement, the bulk modes are characterized by two large symmetric bands, with non-reciprocal surface modes between them. The quasiperiodic metamagnetic structure is then built up by following the Fibonacci sequence, whose long-range order effect is then investigated, giving rise to an interesting self-similar spectra and a power-law profileArtigo Magnetoresistance profiles of quasiperiodic Fe/Cr structures(Elsevier, 2003-06-10) Albuquerque, Eudenilson Lins de; Bezerra, Claudionor Gomes; Feitosa, Carlos Chesman de AraújoWe study theoretically the magnetoresistance properties of magnetic films (Fe/Cr) grown following the Fibonacci sequence. We use a theoretical Hamiltonian which includes Zeeman, cubic and uniaxial anisotropy, bilinear and biquadratic exchange energies. In particular, the presence of the uniaxial anisotropy in the Hamiltonian gives new symmetries to the system which were not discussed in previous works. Our physical parameters are based on experimental data recently reported, which contain biquadratic exchange coupling with magnitude comparable to the bilinear exchange coupling. We show the influence of the biquadratic exchange and anisotropies on the self-similar properties of the magnetoresistance profilesArtigo A multifractal analysis of optical phonon excitations in quasicrystals(Elsevier, 2006-04-01) Anselmo, Dory Hélio Aires de Lima; Dantas, Ana Lúcia; Albuquerque, Eudenilson Lins deIt is shown that the allowed frequencies of optical phonon modes propagating in quasiperiodic AlN–GaN structures display a multifractal behavior. This feature is confirmed through a scaling-exponent analysis of a parametric spectrum of singularities f ðaÞ. This is done by using an alternative counting method, avoiding the use of a Legendre transform, which could lead to distortions. The Cantor-like spectrum is also investigated, and scaling properties are shown for Fibonacci, Thue–Morse and double-period quasiperiodic superlatticesArtigo Oscillatory behavior of the specific heat at low temperature in quasiperiodic structures(Elsevier, 2004-12-15) Bezerra, Claudionor Gomes; Albuquerque, Eudenilson Lins de; Mauriz, P. W.; Vasconcelos, Manoel Silva deWe investigate the thermodynamical properties of plasmon–polaritons that propagate in multiple semiconductor layers arranged in a quasi-periodical fashion of Fibonacci and Thue–Morse types. We have chosen these two types of quasi-periodic structures for the following reasons: (i) both were realized experimentally, so they are not mere academic examples of a quasi-crystal; (ii) it is believed that the Thue–Morse chain is more disordered than the Fibonacci one, i.e., it has a degree of aperiodicity intermediate between the Fibonacci chain and the random systems. More precisely, the Fourier amplitude spectrum of the Thue–Morse sequence is singular continuous, while for the Fibonacci case we found δ-function peaks not arranged periodically. Both analytical and numerical studies on the temperature dependence of the excitation's specific heat associated with the generation number n=1,2,3,… for their multiscale fractal energy spectra are presented. We show that when T tends to zero, the specific heat displays oscillations and when T tends to infinity, the specific heat goes to zero as T−2, because the energy spectrum considered is boundedArtigo Specific heat spectra for quasiperiodic ladder sequences(Springer Verlag, 2007-01-05) Moreira, Darlan Araújo; Albuquerque, Eudenilson Lins de; Bezerra, Claudionor GomesWe performed a theoretical study of the specific heat C(T) as a function of the temperature for double-strand quasiperiodic sequences. To mimic DNA molecules, the sequences are made up from the nucleotides guanine G, adenine A, cytosine C and thymine T, arranged according to the Fibonacci and Rudin-Shapiro quasiperiodic sequences. The energy spectra are calculated using the two-dimensional Schrödinger equation, in a tight-binding approximation, with the on-site energy exhibiting long-range disorder and non-random hopping amplitudes. We compare the specific heat features of these quasiperiodic artificial sequences to the spectra considering a segment of the first sequenced human chromosome 22 (Ch22), a real genomic DNA sequenceArtigo Specific heat spectra of non-interacting fermions in a quasiperiodic ladder sequence(Elsevier BV, 2008-06-11) Anselmo, Dory Hélio Aires de Lima; Moreira, Darlan Araújo; Albuquerque, Eudenilson Lins deWe compute the specific heat spectra of non-interacting fermions whose energy spectrum was obtained from a quasiperiodic ladder sequence (Fibonacci and Rudin–Shapiro type), mimicking a DNA molecule model. The specific heat is calculated from their underlying multi-fractal energy spectrum, considering several values of energy densities. Comparisons are made with a real DNA sequence, namely the human chromosome 22 (Ch22)Artigo Spin waves in quasi-periodic Fe/Cr(100) thin films(IOP Publishing, 2002-02-15) Bezerra, Claudionor Gomes; Mauriz, Paulo Wilson; Albuquerque, Eudenilson Lins deIn this work we have calculated the ferromagnetic (FM) resonance curves of Fe/Cr(100) thin films, which follow the Fibonacci sequence. Our approach is based on the equations of motion for the small-signal magnetization deviation from the equilibrium directions. The equilibrium positions of the magnetizations are calculated from the total energy, which includes the following contributions: Zeeman, bilinear exchange, biquadratic exchange, dipolar, cubic anisotropy and surface anisotropy terms. We also consider the presence of an external magnetic field applied in the plane of the films and parallel to the easy axes. The experimental parameters used in our calculations were recently reported and lie in three regions of interest, namely (i) near to the first antiferromagnetic (AF) peak (strong bilinear exchange coupling), (ii) near to the first FM-AF transition (moderate bilinear exchange coupling) and (iii) near to the second AF peak (weak bilinear exchange coupling comparable to the biquadratic exchange coupling). Our results show the effect of the quasi-periodic arrangement in the spin wave dispersion relation of these artificial structuresArtigo Transmission fingerprints in quasiperiodic magnetic structures(Elsevier B.V., 2003-11) Bezerra, Claudionor Gomes; Vasconcelos, Manoel Silva de; Albuquerque, Eudenilson Lins de; Mariz, Ananias MonteiroIn this paper we study the spin wave transmission spectra of quasiperiodic magnetic multilayers following Fibonacci, double period and Thue–Morse sequences. We consider multilayers that are composed of two simple cubic ferromagnetic materials surrounded by two semi-infinite slabs of a third ferromagnetic material. A transfer matrix treatment is employed with the calculations carried out for the exchange-dominated regime within the framework of the Heisenberg model and taking into account the random phase approximation. The numerical results illustrate the self-similar aspects of the spectra as well as their magnetic transmission fingerprints, which can be used to identify the sequence applied to construct the system