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São Luís, Brazil

Paraguassu W.,Federal University of Para | MacZka M.,Institute of Low Temperature And Structure Research | Guerini S.,University Federal Do Maranho | Freire P.T.C.,Federal University of Ceara | And 3 more authors.
Journal of Physics Condensed Matter

RbNd(WO4)2 was investigated by high pressure Raman spectroscopy in the 0.112.3GPa pressure interval. The assignment of modes was made based on lattice dynamics calculations and the results of these calculations helped us to also discuss the high pressure behavior of phonon spectra in this material. Our results show that a double oxygen bridge plays a fundamental role in the vibrational properties of this system. A density functional theory(DFT) calculation of hydrostatic pressure effects on RbNd(WO4)2 was performed in order to understand the effect of internal bond changes on the vibrational properties of RbNd(WO 4)2. No pressure induced structural phase transition was observed in the Raman study at room temperature, and the DFT calculation (T=0K) is consistent with this result. The anomalous softening of the bridge stretching mode at 770cm1 was attributed to the decrease of WO1W bond angle with increasing pressure. © 2011 IOP Publishing Ltd. Source

Maia Jr. F.F.,Rural University | Maia Jr. F.F.,Federal University of Ceara | Freire V.N.,Federal University of Ceara | Caetano E.W.S.,Federal University of Ceara | And 5 more authors.
Journal of Chemical Physics

We present the structural, electronic, and optical properties of anhydrous crystals of DNA nucleobases (guanine, adenine, cytosine, and thymine) found after DFT (Density Functional Theory) calculations within the local density approximation, as well as experimental measurements of optical absorption for powders of these crystals. Guanine and cytosine (adenine and thymine) anhydrous crystals are predicted from the DFT simulations to be direct (indirect) band gap semiconductors, with values 2.68 eV and 3.30 eV (2.83 eV and 3.22 eV), respectively, while the experimentally estimated band gaps we have measured are 3.83 eV and 3.84 eV (3.89 eV and 4.07 eV), in the same order. The electronic effective masses we have obtained at band extremes show that, at low temperatures, these crystals behave like wide gap semiconductors for electrons moving along the nucleobases stacking direction, while the hole transport are somewhat limited. Lastly, the calculated electronic dielectric functions of DNA nucleobases crystals in the parallel and perpendicular directions to the stacking planes exhibit a high degree of anisotropy (except cytosine), in agreement with published experimental results. © 2011 American Institute of Physics. Source

Vasconcelos M.S.,Federal University of Rio Grande do Norte | Azevedo D.L.,University Federal Do Maranho | Hadad A.,University Federal Do Maranho | Galvao D.S.,University of Campinas
Journal of Physics Condensed Matter

In this paper we address a theoretical calculation of the electronic spectra of an SiGe atomic chain that is arranged in a Fibonacci quasi-periodic sequence, by using a semi-empirical quantum method based on the Hückel extended model. We apply the Fibonacci substitutional sequences in the atomic building blocks A(Si) and B(Ge) through the inflation rule or a recursion relation. In our ab initio calculations we use only a single point, which is sufficient for considering all the orbitals and charge distribution across the entire system. Although the calculations presented here are more complete than the models adopted in the literature which take into account the electronic interaction only up to the second and third neighbors, an interesting property remains in their electronic spectra: the fractality (which is the main signature of this kind of system). We discuss this fractality of the spectra and we compare them with the random arrangement of the SiGe atomic chain, and with previous results based on the tight-binding approximation of the Schrödinger equation considering up to the nearest neighbor. © 2011 IOP Publishing Ltd. Source

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