2,242 research outputs found

    Mixtures of Bose gases confined in concentrically coupled annular traps

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    A two-component Bose-Einstein condensate confined in an axially-symmetric potential with two local minima, resembling two concentric annular traps, is investigated. The system shows a number of quantum phase transitions that result from the competition between phase coexistence, and radial/azimuthal phase separation. The ground-state phase diagram, as well as the rotational properties, including the (meta)stability of currents in this system, are analysed.Comment: 6 pages, 5 figures, minor revision

    Randomly Diluted e_g Orbital-Ordered Systems

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    Dilution effects on the long-range ordered state of the doubly degenerate ege_g orbital are investigated. Quenched impurities without the orbital degree of freedom are introduced in the orbital model where the long-range order is realized by the order-from-disorder mechanism. It is shown by the Monte-Carlo simulation and the cluster-expansion method that a decrease in the orbital ordering temperature by dilution is remarkable in comparison with that in the randomly diluted spin models. Tiltings of orbitals around impurity cause this unique dilution effects on the orbital systems. The present theory provides a new view point for the recent experiments in KCu1−x_{1-x}Znx_xF3_3.Comment: 4 pages, 4 figure

    Charged black holes: Wave equations for gravitational and electromagnetic perturbations

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    A pair of wave equations for the electromagnetic and gravitational perturbations of the charged Kerr black hole are derived. The perturbed Einstein-Maxwell equations in a new gauge are employed in the derivation. The wave equations refer to the perturbed Maxwell spinor Ί0\Phi_0 and to the shear σ\sigma of a principal null direction of the Weyl curvature. The whole construction rests on the tripod of three distinct derivatives of the first curvature Îș\kappa of a principal null direction.Comment: 12 pages, to appear in Ap.

    Dark-bright mixing of interband transitions in symmetric semiconductor quantum dots

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    In photoluminescence spectra of symmetric [111] grown GaAs/AlGaAs quantum dots in longitudinal magnetic fields applied along the growth axis we observe in addition to the expected bright states also nominally dark transitions for both charged and neutral excitons. We uncover a strongly non-monotonous, sign changing field dependence of the bright neutral exciton splitting resulting from the interplay between exchange and Zeeman effects. Our theory shows quantitatively that these surprising experimental results are due to magnetic-field-induced \pm 3/2 heavy-hole mixing, an inherent property of systems with C_3v point-group symmetry.Comment: 5 pages, 3 figure

    Impact of heavy hole-light hole coupling on optical selection rules in GaAs quantum dots

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    We report strong heavy hole-light mixing in GaAs quantum dots grown by droplet epitaxy. Using the neutral and charged exciton emission as a monitor we observe the direct consequence of quantum dot symmetry reduction in this strain free system. By fitting the polar diagram of the emission with simple analytical expressions obtained from k⋅\cdotp theory we are able to extract the mixing that arises from the heavy-light hole coupling due to the geometrical asymmetry of the quantum dot.Comment: 4 pages, 2 figure

    Characterization of chitosan and polycaprolactone membranes designed for wound repair application

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    Polycaprolactone (PCL) and chitosan (Ch) are nontoxic, biocompatible, and biodegradable polymers of vast interest for wound repair. The aim of this work was to prepare Ch/PCL membranes in different proportions (90:10 and 80:20 w/w) in the presence and absence of the surfactant Pluronic F68 (PF68). The membranes were evaluated regarding morphology, thermal behavior, and viscoelastic properties. Sample swelling and degradation in phosphate-buffered saline (PBS), simulated body fluid (SBF), and fetal bovine serum (FBS) were determined by differential scanning calorimetry (DSC) and dynamical mechanical analysis (DMA), while cell toxicity to L929 and Vero fibroblasts was evaluated using the MTT reduction assay and cell proliferation, by DNA quantification and confocal laser microscopy. After 60 days in SBF, marked Ch matrix loss and advanced degradation of PCL particles were noticed by scanning electron microscopy (SEM). No significant differences in melting temperature (Tm) and enthalpy (DHm) were detected by DSC. However, the surfactant increased the DHm. After 30 days, the membranes obtained in the presence of PF68 had absorbed more blood serum and were more degraded after exposure to simulated blood fluid for 30 days. All membranes had low cytotoxicity, and higher cell proliferation was noticed for samples obtained in the presence of the surfactant. In conclusion, the Ch/PCL membranes showed satisfactory degradability and biocompatibility, which enhances their potential for application in wound repair.The authors thank the PhD student Sofia Caridade (3B's Research Group-Universidade do Minho, Portugal) for her assistance in the DMA analyses. The financial support provided by the Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq-150984/2009-0) in Brazil is gratefully acknowledged for this work

    Revealing the potential of squid chitosan-based structures for biomedical applications

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    In recent years, much attention has been given to different marine organisms, namely as potential sources of valuable materials with a vast range of properties and characteristics. In this work, ÎČ-chitin was isolated from the endoskeleton of the giant squid Dosidicus gigas and further deacetylated to produce chitosan. Then, the squid chitosan was processed into membranes and scaffolds using solvent casting and freeze-drying, respectively, to assess their potential biomedical application. The developed membranes have shown to be stiffer and less hydrophobic than those obtained with commercial chitosan. On the other hand, the morphological characterization of the developed scaffolds, by SEM and micro-computed tomography, revealed that the matrices were formed with a lamellar structure. The findings also indicated that the treatment with ethanol prior to neutralization with sodium hydroxide caused the formation of larger pores and loss of some lamellar features. The in vitro cell culture study has shown that all chitosan scaffolds exhibited a non-cytotoxic effect over the mouse fibroblast-like cell line, L929 cells. Thus, chitosan produced from the endoskeletons of the giant squid Dosidicus gigas has proven to be a valuable alternative to existing commercial materials when considering its use as biomaterial.This work was partially funded by FEDER through INTERREG III A Project Proteus and POCTEP Project IBEROMARE. The Portuguese Foundation for Science and Technology is gratefully acknowledged for post-doctoral grants of THS, JMO and SSS. The authors would also like to acknowledge to Dr Julio Maroto from the Fundacion CETMAR and Roi Vilela from PESCANOVA S.A, Spain, for the kind offer of squid pens and to Dr Ramon Novoa, Professor Ricardo Riguera and Professor Mariana Landin from the University of Santiago of Compostela for the SEC-MALLS measurements

    Biomineralization in chitosan/BioglassÂź composite membranes under different dynamic mechanical conditions

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    Fundamental aspects of biomineralization may be important in order to understand and improve calcification onto the surface of biomaterials. The biomineralization process is mainly followed in vitro by assessing the evolution of the apatite layer that is formed upon immersion of the material in Simulated Body Fluid (SBF). In this work we propose an innovative methodology to monitor apatite deposition by looking at the evolution of the mechanical/viscoelastic properties of the sample while immersed in SBF, using non-conventional dynamic mechanical analysis (DMA) performed under distinct displacement amplitudes (d). The biomimetic biomineralization process in composite membranes of chitosan (CTS) with BioglassÂź (BG) was followed by measuring the change of the storage modulus, Eâ€Č, and the loss factor, tan ÎŽ, at 37 °C and in SBF, both online (d = 10 ÎŒm and d = 30 ÎŒm) and offline (d = 0 ÎŒm). The online experiments revealed that the Eâ€Č decreased continuously up in the first hours of immersion in SBF that should be related to the dissolution of BG particles. After that, an increase of the stiffness was verified due to the apatite deposition. SEM/EDS observations upon 24 h of immersion in SBF showed higher development of apatite deposition with increasing displacement amplitude.This work was financially supported by Foundation for Science and Technology (FCT) by the projects PTDC/QUI/69263/2006, PTDC/CTM-BPC/112774/2009 and, through the scholarship SFRH/BD/64601/2009 granted to Sofia G. Caridade
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