Pomerons and BCFW recursion relations for strings on D-branes

We derive pomeron vertex operators for bosonic strings and superstrings in the presence of D-branes. We demonstrate how they can be used in order to compute the Regge behavior of string amplitudes on D-branes and the amplitude of ultrarelativistic D-brane scattering. After a lightning review of the BCFW method, we proceed in a classification of the various BCFW shifts possible in a field/string theory in the presence of defects/D-branes. The BCFW shifts present several novel features, such as the possibility of performing single particle momentum shifts, due to the breaking of momentum conservation in the directions normal to the defect. Using the pomeron vertices we show that superstring amplitudes on the disc involving both open and closed strings should obey BCFW recursion relations. As a particular example, we analyze explicitly the case of 1 -> 1 scattering of level one closed string states off a D-brane. Finally, we investigate whether the eikonal Regge regime conjecture holds in the presence of D-branes.Comment: 49 pages; v2 corrected references and minor typos; v3 minor typos corrected, version to appear in NP

Supersymmetric moduli of the SU(2) x R linear dilaton background and NS5-branes

We study several classes of marginal deformations of the conformal field theory SU(2) x R. This theory describes the near-horizon region of a stack of parallel and coincident NS5-branes and is related holographically to little string theory. We investigate the supersymmetry properties of these deformations and we elucidate their role in the context of holography. The conformal field theory moduli space contains "non-holographic" operators that do not seem to have a simple interpretation in little string theory. Subsequently, we analyze several NS5-brane configurations in terms of SU(2) x R deformations. We discuss in detail interesting phenomena, like the excision of the strong coupling region associated with the linear dilaton and the manifestation of the symmetries of an NS5-brane setup in the deforming operators. Finally, we present a class of conformally hyperkaehler geometries that arise as "non-holographic" deformations of SU(2) x R.Comment: 38 pages, 1 figure, 1 table; version to appear in JHE

Discrete Torsion, Covering Groups and Quiver Diagrams

Without recourse to the sophisticated machinery of twisted group algebras, projective character tables and explicit values of 2-cocycles, we here present a simple algorithm to study the gauge theory data of D-brane probes on a generic orbifold G with discrete torsion turned on. We show in particular that the gauge theory can be obtained with the knowledge of no more than the ordinary character tables of G and its covering group G*. Subsequently we present the quiver diagrams of certain illustrative examples of SU(3)-orbifolds which have non-trivial Schur Multipliers. The paper serves as a companion to our earlier work (arXiv:hep-th/0010023) and aims to initiate a systematic and computationally convenient study of discrete torsion.Comment: 26 pages, 8 figures, some errors correcte

Supersymmetric deformations of F1-NS5-branes and their exact CFT description

We consider certain classes of operators in the exact conformal field theory SL(2,R) x SU(2) x U(1)^4 describing strings in an AdS(3) x S(3) x T4 geometry supported by Neveu--Schwarz 3-form fluxes. This background arises in the near-horizon limit of a system of NS5-branes wrapped on a 4-torus and F1-branes smeared on the 4-torus when both types of branes are located at the same point in their common transverse space. We find a class of operators that lead to spacetime supersymmetric deformations. It is remarkable that most of these operators are not chiral primary with respect to the N=2 superconformal algebra on the wordsheet. A subset of these worldsheet conformal field theory deformations admits an interpretation either as a geometric deformation of the brane system or as a deformation of the distribution of the F1-branes, viewed as smooth instantons, inside the wrapped NS5-brane worldvolume. The 2-dimensional conformal field theory, however, seems to lack operators corresponding to arbitrary NS5-brane deformations, in contrast to pure NS5-brane systems where all geometric deformations can be accounted for by chiral primary operators.Comment: 30+1 pages, 1 table; v2 minor changes, version to appear in JHE

Worldsheet theories for non-geometric string backgrounds

We show that twisted doubled tori can be used to construct a general class of worldsheet models describing non-geometric string backgrounds. By employing a first order formulation of interacting chiral bosons, we first refine the analysis on the general conditions of worldsheet Lorentz invariance and then prove that twisted doubled tori provide good duality symmetric backgrounds. Subsequently we apply our general analysis to several examples which enable us to gain new insight on the difference between geometric, locally geometric and genuine non-geometric backgrounds.Comment: Latex, 25 pages; v2 Fixed typos; v3 Sections 2 and 3 expanded; v4 Published versio

Preparação e caracterização de vidros de TeO2 dopados com terras raras

Mestrado em Engenharia FísicaForam preparados pelo método de fusão vidros tendo como constituinte maioritário o TeO2, com composição molar (70-x-y)TeO2-20WO3-10Y2O3- xEr2O3-yYb2O3, com x=0 e 0,5 e y=0, 0,5, 1, 2 e 4. Este sistema vítreo relevou ser promissor para potenciais aplicações em amplificação ótica, em particular em amplificadores óticos de fibra dopada com érbio (EDFA – “Erbium-doped fiber amplifier”) e de Raman, e também em conversão espectral para aplicações na tecnologia fotovoltaica. Através de espectroscopia de Raman e com a linha de excitação de 532 nm, verificou-se que este sistema vítreo apresenta um espectro com bandas que caracterizam as ligações Te-O e W-O. Os componentes minoritários Y2O3, Er2O3 e Yb2O3 são nesse contexto considerados como pequenas perturbações. A largura de banda de Raman torna este sistema vítreo promissor no desenvolvimento de amplificadores de Raman. A utilização da linha de excitação UV (325 nm) induz alterações na superfície destas amostras. A análise ótica permitiu verificar um aumento da intensidade das emissões de upconversion com o aumento da concentração de iões sensitizadores Yb3+. A emissão do Er3+ na região espectral do vermelho (transição 4F9/2 4I15/2) é a mais favorecida com o aumento de concentração dos sensitizadores. A emissão do Er3+ em torno dos 1540 nm apresenta uma largura a meia altura de cerca de 100 nm para temperaturas entre os 70 e 300 K, tratando-se de um valor bastante promissor quando comparado com os valores reportados para outros sistemas vítreos, considerando a amplificação ótica como uma potencial aplicação. A análise elétrica permitiu verificar um aumento da condutividade dc com o aumento da energia de ativação na região de mais altas temperaturas, indicando que deverá ocorrer ai um aumento considerável do número de portadores de carga disponíveis. No caso ac, na gama de altas temperaturas, a condutividade tende a diminuir com o aumento da concentração de Yb3+, enquanto que a energia de ativação tende a aumentar, indicando que o parâmetro mais importante para este mecanismo de condução é a mobilidade dos portadores de carga. Para todas as amostras, a constante dielétrica não varia significativamente com a temperatura e frequência. Esta grandeza tende a diminuir com o aumento da concentração de Yb3+.TeO2 based glasses were prepared by the melt-quenching technique, with the molar compostion (70-x-y)TeO2-20WO3-10Y2O3-xEr2O3-yYb2O3, with x=0 e 0,5 e y=0, 0,5, 1, 2 e 4. This vitreous system was found to be promising for potential applications in optical amplification, in particular in erbium doped fiber optical amplifiers (EDFA) and Raman amplifiers, and also in spectral conversion for applications in photovoltaic technology. Through Raman spectroscopy and using the laser excitation of 532 nm it was verified that this vitreous system has a spectrum with bands associated to Te-O and W-O vibrations. The minority components Y2O3, Er2O3 and Yb2O3 are regarded as small pertubations. The observed bandwidth makes this system promising for the development of Raman amplifiers. The use of the UV laser excitation of 325 nm induced surface modifications on the samples. The optical analysis showed an increase of the intensity of the upconversion emissions with the increase of the content of the sensitizer ions Yb3+. The Er3+ emission arround 1540 nm has a full width at half maximum of about 100 nm in the temperature range of 70-300 K, which is a very interesting value when compared to those reported for other glass systems, considering optical amplification as a potencial application. The electrical analysis allowed to verify an increase of the dc conductivity with the increase of the activation energy in the region of high temperatures, indicating an increase of the number of avaible charge carriers. In the ac regime, and in the high temperature region, the conductivity tends to decrease with the increase of the concentration of Yb3+ ions, while the activation energy tends to increase, suggesting that the most important parameter for this conduction mechanism is the mobility of the charge carriers. For all the samples, the dielectric constant does not change significantly with the temperature and frequency. This quantity tends to decrease with the increase of the concentration of Yb3+ ions

Polarização elétrica de biomateriais baseados em hidroxiapatite, como filmes em substratos metálicos, para aumento da bioatividade

Foi desenvolvido de raiz um sistema experimental que permite carregar amostras através de uma descarga controlável num tríodo de corona. O sistema desenvolvido permite produzir uma descarga de polaridade positiva ou negativa, aplicar o método de carregamento com corrente de carga constante, seguir em tempo real o aumento do potencial de superfície da amostra, controlar a temperatura de descarga até 200 ºC e possui uma atmosfera reprodutível de baixa humidade. O sistema foi desenvolvido com o intuito de carregar com uma descarga de corona negativa revestimentos bioactivos de hidroxiapatite depositados por dois processos: spray de plasma e CoBlast, este último um processo relativamente recente. Foram estudados os seguintes parâmetros de processo do CoBlast: razão mássica entre abrasivo e dopante, distância de ejeção e pressão de ejeção. Mostrou-se que razões mássicas de 50/50 e distâncias inferiores a 30 mm são vantajosas. O método de carregamento com corrente de carga constante não é possível ser aplicado nos revestimentos produzidos por CoBlast, pois estes são caracterizados como tendo regiões onde substrato metálico está directamente exposto à descarga. Os revestimentos produzidos por spray de plasma, com uma espessura média de 70 m, foram carregados negativamente a 200 ºC com sucesso, atingindo potenciais de superfície na gama dos - 1400-1800 V, traduzindo-se em campos eléctricos nas amostras praticamente impossíveis de serem atingidos por polarização convencional de contacto. O controlo de corrente de carga é melhor para mais baixas correntes de carga. Ensaios e medidas complementares feitas em pastilhas de hidroxiapatite revelaram densidades de carga armazenada na gama dos 10-5 - 10-4 C/cm2, bem como uma estabilidade temporal da carga armazenada muito promissora. Os testes biológicos in vitro revelaram uma maior proliferação osteoblástica nos revestimentos carregados comparativamente com os revestimentos de controlo não carregados, indicando também um estágio mais avançado de formação de nova hidroxiapatite em solução simuladora de fluido corporal.A corona triode experimental system was developed “from scratch”. The developed system is able to produce a negative or positive discharge, to apply the constant charging current method, to follow in real-time the surface potential buildup of the sample, to control the discharge temperature up to 200 ºC and a low humidity, reproducible atmosphere is maintained in all the charging experiments. The system was developed in order to charge with a negative corona discharge hydroxyapatite bioactive coatings produced by two processes: plasma spray and CoBlast, the former being a relatively recent process. The following CoBlast process parameters were studied: the weight ratio between the abrasive and dopant, the blasting distance and the blasting process. It was shown that a weight ratio of 50/50 and distances lower than 30 mm are preferable. The constant charging current method cannot be applied in the coatings produced through the CoBlast process, because they are characterized by having regions where the metallic substrate is directly exposed to the discharge. The coatings produced through the plasma spray process, with an average thickness of 70 m, were successively charged at 200 ºC, reaching surface potentials in the - 1400-1800 V range, translating in electric fields across the samples which are practically impossible of being reached through conventional contact polarization. The charging current controllability is better for lower charging current values. Complementary experiments performed in hydroxyapatite coatings revealed stored charge densities in the 10-5 – 10-4 C/cm2 range, as well as a very promising temporal stability of the stored charge. The in vitro biological tests revealed an increased osteoblastic proliferation in the charged coatings compared to non-charged control coatings, also indicating a more advanced stage of new hydroxyapatite development in a simulated body fluid solution.Programa Doutoral em Físic

NS5-branes, holography and CFT deformations

A few supergravity solutions representing configurations of NS5-branes admit exact conformal field theory (CFT) description. Deformations of these solutions should be described by exactly marginal operators of the corresponding theories. We briefly review the essentials of these constructions and present, as a new case, the operators responsible for turning on angular momentum.Comment: 6 pages, 2 figures, Proceedings contribution to the 9th Hellenic School on Elementary Particle Physics and Gravity}, Corfu, Greece, September 200

Structural Characterization of Lithium Niobate Nanoparticles Prepared by the Sol-Gel Process, Using X-Ray and Raman Spectroscopy and Scanning Electron Microscopy

The widespread use of lithium niobate (LN) in several technological applications, notably in optical and electrooptical systems, is a consequence of its remarkable piezoelectric, electrooptical, photoelastic, acousto-optic, and nonlinear optical coefficients. In this chapter, the structural and electrical characterization of LN nanosized particles synthesized by the Pechini route is discussed. Compared to solid-state reaction processes, wet chemistry processes can be advantageous alternatives for the synthesis of polycrystalline LN, because they require lower processing temperatures, and thus the loss of stoichiometry and formation of secondary phases can be minimized. The powders obtained by drying the gel (base powder) were heat-treated for 4 h at temperatures between 400 and 1000°C, according to the differential thermal analysis (DTA) results. It was found that the powders sintered at 450°C contain only the LN phase, while those heat-treated at 500°C already contain the secondary LiNb3O8 phase. The structural and electrical characterization of the samples sintered at 450°C, for different times, was performed using X-ray diffraction (XRD) in conjunction with Rietveld refinement, Raman spectroscopy, scanning electron microscopy (SEM), and impedance spectroscopy in the temperature range between 200 and 360 K and in the frequency range between 100 Hz and 1 MHz and by measuring the ac and dc conductivities