Wannier-function approach to electron states in superlattices under an electric field

Electron quasi-stationary states in a periodic semiconductor superlattice are calculated, as linear combinations of Wannier-Kohn functions, for different values of an electric field applied along the heterostructure. A comparison with an alternative approach, which is based on the localization of quasi-stationary states, is performed. (C) 2004 Elsevier Ltd. All rights reserved.

Competition between local potentials and attractive particle-particle interactions in superlattices

Naturally occuring or man-made systems displaying periodic spatial modulations of their properties on a nanoscale constitute superlattices. Such modulated structures are important both as prototypes of simple nanotechnological devices and as particular examples of emerging spatial inhomogeneity in interacting many-electron systems. Here we investigate the effect different types of modulation of the system parameters have on the ground-state energy and the charge-density distribution of the system. The superlattices are described by the inhomogeneous attractive Hubbard model, and the calculations are performed by density-functional and density-matrix renormalization group techniques. We find that modulations in local electric potentials are much more effective in shaping the system's properties than modulations in the attractive on-site interaction. This is the same conclusion we previously [M.F. Silva, N.A. Lima, A.L. Malvezzi, K. Capelle, Phys. Rev. B 71 (2005) 125130.] obtained for repulsive interactions, suggesting that it is not an artifact of a specific state, but a general property of modulated structures. (c) 2007 Elsevier Ltd. All rights reserved.

Band-edge states of the zero(th)-order gap in quasi-periodic photonic superlattices

The photonic modes of Thue-Morse and Fibonacci lattices with generating layers A and B, of positive and negative indices of refraction, are calculated by the transfer-matrix technique. For Thue-Morse lattices, as well for periodic lattices with AB unit cell, the constructive interference of reflected waves, corresponding to the zero(th)-order gap, takes place when the optical paths in single layers A and B are commensurate. In contrast, for Fibonacci lattices of high order, the same phenomenon occurs when the ratio of those optical paths is close to the golden ratio. In the long wavelength limit, analytical expressions defining the edge frequencies of the zero(th) order gap are obtained for both quasi-periodic lattices. Furthermore, analytical expressions that define the gap edges around the zero(th) order gap are shown to correspond to the = 0 and = 0 conditions.

Suppression of Anderson localization of light in one-dimensional disordered photonic superlattices

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Multiperiodic magnetic structures in Hubbard superlattices

We consider fermions in one-dimensional superlattices (SL's), modeled by site-dependent Hubbard-U couplings arranged in a repeated pattern of repulsive (i.e., U>0) and free (U=0) sites. Density matrix renormalization group diagonalization of finite systems is used to calculate the local moment and the magnetic structure factor in the ground state. We have found four regimes for magnetic behavior: uniform local moments forming a spin-density wave (SDW), floppy local moments with short-ranged correlations, local moments on repulsive sites forming long-period SDW's superimposed with short-ranged correlations, and local moments on repulsive sites solely with long-period SDW's; the boundaries between these regimes depend on the range of electronic densities ρ and on the SL aspect ratio. Above a critical electronic density, ρ↑↓, the SDW period oscillates both with ρ and with the spacer thickness. The former oscillation allows one to reproduce all SDW wave vectors within a small range of electronic densities, unlike the homogeneous system. The latter oscillation is related to the exchange oscillation observed in magnetic multilayers. A crossover between regimes of thin to thick layers has also been observed.

On the photonic dispersion of periodic superlattices made of left-handed materials

Studies of the band gap properties of one-dimensional superlattices with alternate layers of air and left-handed materials are carried out within the framework of Maxwell's equations. By left-handed material, we mean a material with dispersive negative electric and magnetic responses. Modeling them by Drude-type responses or by fabricated ones, we characterize the n(ω) = 0 gap, i.e., the zeroth order gap, which has been predicted and detected. The band structure and analytic equations for the band edges have been obtained in the long wavelength limit in case of periodic, Fibonacci, and Thue-Morse superlattices. Our studies reveal the nature of the width of the zeroth order band gap, whose edge equations are defined by null averages of the response functions. Oblique incidence is also investigated, yielding remarkable results. © 2010 Springer Science+Business Media B.V.

Avaliação da reabsorção radicular apical em incisivos submetidos à retração anterior com alças T e Bull

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)

Bulk plasmon polariton-gap soliton-induced transparency in one-dimensional Kerr-metamaterial superlattices

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Magnetic properties of YBCO/LCMO superlattices with and without STO interlayers

Superlattices formed of two antagonic characteristics have been broadly studied in literature mainly in order to clarify the effects of proximity and interface interactions. Here, we present a study of superlattice introducing an insulator between each superconducting and ferromagnetic layer. The electrical insulator STO, YBCO and LCMO layer are deposited by PLD method. The samples with STO show more intergrowth surface morphology, which favors the application providing better contacts between the grains. The magnetic measurements indicated higher Tc values and high anisotropy for SLs with STO, which is dependent on the relative thickness of LCMO and YBCO.

Morfo-anatomia de plântulas e número cromossômico de Cybistax antisyphilitica (Mart.) Mart. (Bignoniaceae)

Plântulas de Cybistax antisyphilitica (ipê-verde), espécie arbórea que ocorre no Estado de São Paulo, Brasil, foram estudadas morfoanatomicamente e citogeneticamente. As plântulas são eudicotiledôneas, fanerocotiledonares e epigéias. Suas sementes apresentam alas hialinas, assimétricas e de textura papirácea. A raiz é axial com tecido epidérmico irregular. O hipocótilo é verde, glabro e apresenta os elementos do xilema em diferenciação. Os cotilédones são verdes, foliáceos, reniformes, com mesofilo heterogêneo, epiderme pilosa e feixes vasculares colaterais. Os eófilos são glabros, peciolados, de filotaxia oposta, dorsiventrais, hipoestomáticos e possuem mesofilo heterogêneo e assimétrico. Não há diferenças anatômicas significativas entre os eófilos e os metáfilos. O sistema vascular do pecíolo dos metáfilos dispõe-se em forma de ferradura. A espécie apresenta número cromossômico mitótico 2n = 40 com comprimento cromossômico geral médio de 1,042 µm ± 0,140 e amplitude variando de 0,58 µm até 1,60 µm.

NEW PHYTOSEIIDAE (ACARI: MESOSTIGMATA) FROM COSTA RICA, WITH ADDITIONAL INFORMATION on OTHER SPECIES

This paper reports on a study of phytoseiids collected from the La Selva Biological Field Station, the ALAS (Arthropods of La Selva) Project, made available to us by the Canadian National Collection via Dr. E. E. Lindquist. This material includes two new species, Proprioseiopsis versus n. sp. and Transeius costarricensis n. sp., three new records for Central America, and additional information for nine species previously reported from La Selva.

Bloch-Kohn and Wannier-Kohn functions in one dimension

Bloch and Wannier functions of the Kohn type for a quite general one-dimensional Hamiltonian with inversion symmetry are studied. Important clarifications on null minigaps and the symmetry of those functions are given, with emphasis on the Kronig-Penney model. The lack of a general selection rule on the miniband index for optical transitions between edge states in semiconductor superlattices is discussed. A direct method for the calculation of Wannier-Kohn functions is presented.

Wannier functions of isolated bands in one-dimensional crystals

We present a simple procedure to obtain the maximally localized Wannier function of isolated bands in one-dimensional crystals with or without inversion symmetry. First, we discuss the generality of dealing with real Wannier functions. Next, we use a transfer-matrix technique to obtain nonoptimal Bloch functions which are analytic in the wave number. This produces two classes of real Wannier functions. Then, the minimization of the variance of the Wannier functions is performed, by using the antiderivative of the Berry connection. In the case of centrosymmetric crystals, this procedure leads to the Wannier-Kohn functions. The asymptotic behavior of the Wannier functions is also analyzed. The maximally localized Wannier functions show the expected exponential and power-law decays. Instead, nonoptimal Wannier functions may show reduced exponential and anisotropic power-law decays. The theory is illustrated with numerical calculations of Wannier functions for conduction electrons in semiconductor superlattices.

Sistema de Apoio Ósseo para Mecânica Ortodôntica (SAO®): miniplacas para ancoragem ortodôntica. Parte I: tratamento da mordida aberta

Este trabalho descreve uma nova forma de ancoragem por meio de miniplacas denominada SAO®, Sistema de Apoio Ósseo para Mecânica Ortodôntica. Após a descrição do sistema, protocolos de tratamento para mordidas abertas esqueléticas são apresentados. A aplicação de cantiléveres e alças apoiadas diretamente nos tubos do sistema de ancoragem permite que associações de problemas verticais e sagitais (Classe II e III) sejam tratadas de formas distintas. A aplicação de forças leves e constantes e o controle tridimensional das forças aplicadas são o grande diferencial desse novo sistema.

Estudo morfo-anatômico de folhas de Jacaratia spinosa (Aubl.) A.DC. (Caricaceae)

Aspectos morfológicos e anatômicos da folha de Jacaratia spinosa (Aubl.) A.DC. (Caricaceae) foram estudados com detalhe. O limbo foliar possui organização dorsiventral, estômago anomocíticos, glândulas, laticíferos do tipo articulado anastomosado e epiderme papilosa na face abaxial. A venação é bronquidódroma terminando em alças no bordo foliar.