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.

Toward an understanding of intermediate- and short-range defects in ZnO single crystals. A combined experimental and theoretical study

A joint use of experimental and theoretical techniques allows us to understand the key role of intermediate- and short-range defects in the structural and electronic properties of ZnO single crystals obtained by means of both conventional hydrothermal and microwave-hydrothermal synthesis methods. X-ray diffraction, Raman spectra, photoluminescence, scanning electronic and transmission electron microscopies were used to characterize the thermal properties, crystalline and optical features of the obtained nano and microwires ZnO structures. In addition, these properties were further investigated by means of two periodic models, crystalline and disordered ZnO wurtzite structure, and first principles calculations based on density functional theory at the B3LYP level. The theoretical results indicate that the key factor controlling the electronic behavior can be associated with a symmetry breaking process, creating localized electronic levels above the valence band.

Different origins of green-light photoluminescence emission in structurally ordered and disordered powders of calcium molybdate

A strong greenish-light photoluminescence (PL) emission was measured at room temperature for disordered and ordered powders of CaMoO4 prepared by the polymeric precursor method. The structural evolution from disordered to ordered powders was accompanied by XRD. Raman spectroscopy, and TEM imagery. High-level quantum mechanical calculations in the density functional framework were used to interpret the formation of the structural defects of disorder powders in terms of band diagram and density of states. Complex cluster vacancies [MoO3 center dot V-O(z)] and [CaO7 center dot V-O(z)] (where V-O(z) = V-O(X), V-O(center dot), V-O(center dot center dot)) were suggested to be responsible to the appearance of new states shallow and deeply inserted in the band gap. These defects give rise to the PL in disordered powders. The natural PL emission of ordered CaMoO4 was attributed to an intrinsic slight distortion of the [MoO4] tetrahedral in the short range.

RHEOLOGICAL CHARACTERIZATION of SUSPENSIONS of SUCROSE CRYSTALS IN SATURATED SUCROSE SOLUTIONS

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

A Joint Experimental and Theoretical Study on the Nanomorphology of CaWO4 Crystals

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Electronic structure, growth mechanism and photoluminescence of CaWO4 crystals

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Effect of variations in annealing temperature and metallic cations on nanostructured molybdate thin films

Crystalline molybdate thin films were prepared by the complex polymerization method. The AMoO(4) (A = Ca, Sr, Ba) films were deposited onto Si wafers by the spinning technique. The Mo-O bond in the AMoO(4) structure was confirmed by FTIR spectra. X-ray diffraction revealed the presence of crystalline scheelite-type phase. The mass, size, and basicity of A(2+) cations was found to be dependent on the intrinsic characteristics of the materials. The grain size increased in the following order: CaMoO4 < SrMoO4 < BaMoO4. The emission band wavelength was detected at around 576 nm. Our findings suggest that the material's morphology and photoluminescence were both affected by the variations in cations (Ca, Sr, or Ba) and in the thermal treatment.

Crystallization study of molybdate phosphate glasses by thermal analysis

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

A simple spectrophotometric method for the determination of captopril in pharmaceutical preparations using ammonium molybdate

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Role of intermediary cells in Peltodon radicans (Lamiaceae) in the transfer of calcium and formation of calcium oxalate crystals

Com o objetivo de estudar a relação entre cristais de oxalato de cálcio e floema, fragmentos de folhas de Peltodon radicans foram fixados e processados, segundo métodos usuais, para estudos ao microscópio de luz e eletrônico de transmissão. Observou-se que os cristais ocorrem nas células da bainha do feixe, lateralmente em relação ao floema. Células intermediárias estabelecem conexão entre elemento crivado e células da bainha, portadoras de cristais, com crescimento intrusivo entre estas. Íons cálcio são abundantes no citoplasma das células da bainha que contém cristais de oxalato de cálcio. Nas células intermediárias a detecção ultra-citoquímica de cálcio também apresentou resultados positivos, enquanto nos elementos crivados a presença deste íon não foi constatada. Há, portanto, um gradiente crescente de concentração de cálcio dos elementos crivados para as células da bainha. Assim, formulamos a hipótese de que a formação de cristais de oxalato de cálcio tem, em P. radicans, o objetivo de controlar os níveis de cálcio citossólico nos elementos crivados.

Vibrational spectra of pilocarpine hydrochloride crystals

Pilocarpine is a natural substance with potential application in the treatment of several diseases. In this work Fourier Transform (FT)-Raman spectrum and the Fourier Transform infra red (FT-IR) spectrum of pilocarpine hydrochloride C11H17N2O2+.Cl- were investigated at 300 K. Vibrational wavenumber and wave vector have been predicted using density functional theory (B3LYP) calculations with the 6-31 G(d,p) basis set. A comparison with experiment allowed us to assign most of the normal modes of the crystal.

Comparative vibrational spectra of pilosine and epiisopilosine crystals

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Existence and stability of new nanoreactors: Highly swollen hexagonal liquid crystals

We report the preparation of direct hexagonal liquid crystals, constituted of oil-swollen cylinders arranged on a triangular lattice in water. The volume ratio of oil over water, rho, can be as large as 3.8. From the lattice parameter measured by small-angle X-ray scattering, we show that all the oil is indeed incorporated into the cylinders, thus allowing the diameter of the cylinders to be controlled over one decade range, provided that the ionic strength of the aqueous medium and rho are varied concomitantly. These hexagonal swollen liquid crystals (SLCs) have been first reported with sodium dodecyl sulfate as anionic surfactant, cyclohexane as solvent, 1-pentanol as co-surfactant, and sodium chloride as salt (Ramos, L.; Fabre, P. Langmuir 1997, 13, 13). The stability of these liquid crystals is investigated when the pH of the aqueous medium or the chemical nature of the components (salt and surfactant) is changed. We demonstrate that the range of stability is quite extended, rendering swollen hexagonal phases potentially useful for the fabrication of nanomaterials. As illustrations, we finally show that gelation of inorganic particles in the continuous aqueous medium of a SLC and polymerization within the oil-swollen cylinders of a SLC can be conducted without disrupting the hexagonal order of the system.

Fabrication and analysis of self-assembled photonic crystals structures

This paper presents the fabrication and analysis of a three-dimensional FCC photonic crystal (PhC) based on a self-assembly synthesis of monodispersive latex spheres. Experimental optical characterization, achieved by measurements of the specular reflectance under variable angles, indicated the clear presence of a Bragg diffraction pattern. Results are further explored by theoretical calculations based on the Finite Difference Time Domain (FDTD) method to determine the full PhC band structure.

Holographic imaging with multimode, large free spectral range lasers in photorefractive sillenite crystals

The holographic imaging of rigid objects with diode lasers emitting in many wavelengths in a sillenite Bi12TiO20 photorefractive crystal is both theoretically an experimentally investigated. It is shown that, due to the multi-wavelength emission and the typically large free spectral range of this light source, contour fringes appear on the holographic image corresponding to the surface relief, even in single-exposure recordings. The influence of the number of emitted modes on the fringe width is analysed, and the possible applications of the contour fringes in the field of optical metrology are pointed out.