On the perturbative corrections around D-string instantons

We study ${\cal F}^4$-threshold corrections in an eight dimensional S-dual pair of string theories, as a prototype of dual string vacua with sixteen supercharges. We show that the orbifold CFT description of D-string instantons gives rise to a perturbative expansion similar to the one appearing on the fundamental string side. By an explicit calculation, using the Nambu-Goto action in the static gauge, we show that the first subleading term agrees precisely on the two sides. We then give a general argument to show that the agreement extends to all orders.Comment: 12 page

AdS/CFT correspondence and D1/D5 systems in theories with 16 supercharges

We discuss spectra of $AdS_3$ supergravities, arising in the near horizon geometry of D1/D5 systems in orbifolds/orientifolds of type IIB theory with 16 supercharges. These include models studied in a recent paper (hep-th/0012118), where the group action involves also a shift along a transversal circle, as well as IIB/$\Omega I_4$, which is dual to IIB on $K3$. After appropriate assignements of the orbifold group eigenvalues and degrees to the supergravity single particle spectrum, we compute the supergravity elliptic genus and find agreement, in the expected regime of validity, with the elliptic genus obtained using U-duality map from (4,4) CFTs of U-dual backgrounds. Since this U-duality involves the exchange of KK momentum $P$ and D1 charge $N$, it allows us to test the (4,4) CFTs in the $P < N/4$ and $N < P/4$ regimes by two different supergravity duals.Comment: 28 pages, no figure

Modeling of amorphous carbon structures with arbitrary structural constraints

In this paper we describe a method to generate amorphous structures with arbitrary structural constraints. This method employs the Simulated Annealing algorithm to minimize a simple yet carefully tailored Cost Function (CF). The Cost Function is composed of two parts: a simple harmonic approximation for the energy-related terms and a cost that penalizes configurations that do not have atoms in the desired coordinations. Using this approach, we generated a set of amorphous carbon structures spawning nearly all the possible combinations of $sp$, $sp^2$ and $sp^3$ hybridizations. The bulk moduli of this set of amorphous carbons structures was calculated using Brenner's potential. The bulk modulus strongly depends on the mean coordination, following a power law behavior with an exponent $\nu=1.51 \pm 0.17$. A modified Cost Function that segregates carbon with different hybridizations is also presented, and another set of structures was generated. With this new set of amorphous materials, the correlation between the bulk modulus and the mean coordination weakens. This method proposed can be easily modified to explore the effects on physical properties of the presence hydrogen, dangling bonds, and structural features such as carbon rings.Comment: 20 pages, 5 figures; v2: minor text modifications, included analysis of angular width; v3: grammar revision, recalculation of the bulk modulu

Closed String Tachyons on C/Z_N

We analyse the condensation of closed string tachyons on the $C/Z_N$ orbifold. We construct the potential for the tachyons upto the quartic interaction term in the large $N$ limit. In this limit there are near marginal tachyons. The quartic coupling for these tachyons is calculated by subtracting from the string theory amplitude for the tachyons, the contributions from the massless exchanges, computed from the effective field theory. We argue that higher point interaction terms are are also of the same order in 1/N as the quartic term and are necessary for existence of the minimum of the tachyon potential that is consistent with earlier analysis.Comment: 19 pages, 1 figure; comments added about $N$ dependence of contact ter

Superspace WZW Models and Black Holes

We show how to write an off-shell action for the $SU(2)\times U(1)$ supersymmetric WZW model in terms of $N=2$ chiral and twisted chiral multiplets. We discuss the $N=4$ supersymmetry of this model and exhibit the $N=4$ superconformal current algebra. Finally, we show that the off-shell formulation makes it possible to perform a duality transformation, which leads to a supersymmetric sigma model on a manifold with a black hole type singularity.Comment: 12 page

Produção de mudas de palma-forrageira utilizando fragmentos de cladódios.

A palma (Opuntia ficus-indica L.) é uma das principais alternativas para alimentação de caprinos, ovinos e bovinos no Semiárido brasileiro. Contudo, o ataque da cochonilhado-carmim (Dactylopius opuntiae Cockerel) erradicou grande parte da área plantada, já que a maioria das variedades cultivadas são suscetíveis à praga. Três novas variedades resistentes à praga foram identificadas: Orelha-de-elefante mexicana, Miúda ou Doce e a variedade Mão-de-moça (clones IPA-Sertânea e PALMEPA-PB1). Contudo, a disponibilidade de material propagativo (cladódios ou raquetes) ainda é baixa, aumentando o preço da palma-semente. Assim, desenvolveu-se uma metodologia utilizando-se fragmentos (pedaços) de raquetes para a obtenção de novas mudas

Applications of the Ashtekar gravity to four dimensional hyperk\"ahler geometry and Yang-Mills Instantons

The Ashtekar-Mason-Newman equations are used to construct the hyperk\"ahler metrics on four dimensional manifolds. These equations are closely related to anti self-dual Yang-Mills equations of the infinite dimensional gauge Lie algebras of all volume preserving vector fields. Several examples of hyperk\"ahler metrics are presented through the reductions of anti self-dual connections. For any gauge group anti self-dual connections on hyperk\"ahler manifolds are constructed using the solutions of both Nahm and Laplace equations.Comment: 9pages, Figures are not include

Produção de toxinas inseticidas por isolados de Fusarlum spp. patogênicos a Dactylopius opuntiae Cockerel.

Neste trabalho buscou-se avaliar a produção de toxinas por seis isolados de Fusarium spp. obtidos em diferentes regiões do Agreste e Sertão pernambucano.Resumo 1733-2

Influência da compactação do solo na produtividade da rebrota de eucalipto.

Normalmente, as áreas de rebrota sofrem a influência das operações de colheita, aplicação de herbicidas, adubos e resíduos, que concorrem para a compactação do solo. Em solos de texturas diferentes em áreas da Cia. Suzano de Papel e Celulose, nos municípios de Itatinga, SP, e São Miguel Arcanjo, SP, foram selecionadas 30 linhas de plantio para determinação da produtividade de eucalipto (Eucalyptus grandis Hill ex Maiden) aos sete anos de idade, bem como atributos físico-hídricos do solo a 50 cm da linha de plantio. Foram obtidas amostras indeformadas de solo em três profundidades: 0 a 10, 10 a 20 e 20 a 30 cm; a resistência do solo foi obtida com penetrômetro – SC 60 – até 60 cm de profundidade. As linhas de eucaliptos foram separadas, segundo a intensidade de movimentação das máquinas de colheita, em corte, galhada e tráfego. A redução da produtividade do eucalipto chegou até dois terços, comparando-se linhas de tráfego intenso das máquinas e sem tráfego. A camada superficial do solo (0 a 10 cm) recuperou parcialmente a estrutura durante os sete anos. A compactação foi máxima na camada de 10 a 20 cm, no solo argiloso e na camada de 20 a 30 cm, no solo arenoso. Houve correlação negativa (r² = 0,86) entre a densidade global, na profundidade de 10 a 20 cm, e o volume de madeira das linhas de eucalipto no solo arenoso. No solo argiloso, essa correlação (r² = 0,77) negativa ocorreu na profundidade de 20 a 30 cm