Remarks on quiver gauge theories from open topological string theory

We study effective quiver gauge theories arising from a stack of D3-branes on certain Calabi-Yau singularities. Our point of view is a first principle approach via open topological string theory. This means that we construct the natural A-infinity-structure of open string amplitudes in the associated D-brane category. Then we show that it precisely reproduces the results of the method of brane tilings, without having to resort to any effective field theory computations. In particular, we prove a general and simple formula for effective superpotentials

Influence of Ni Catalyst Layer and TiN Diffusion Barrier on Carbon Nanotube Growth Rate

Dense, vertically aligned multiwall carbon nanotubes were synthesized on TiN electrode layers for infrared sensing applications. Microwave plasma-enhanced chemical vapor deposition and Ni catalyst were used for the nanotubes synthesis. The resultant nanotubes were characterized by SEM, AFM, and TEM. Since the length of the nanotubes influences sensor characteristics, we study in details the effects of changing Ni and TiN thickness on the physical properties of the nanotubes. In this paper, we report the observation of a threshold Ni thickness of about 4 nm, when the average CNT growth rate switches from an increasing to a decreasing function of increasing Ni thickness, for a process temperature of 700°C. This behavior is likely related to a transition in the growth mode from a predominantly “base growth” to that of a “tip growth.” For Ni layer greater than 9 nm the growth rate, as well as the CNT diameter, variations become insignificant. We have also observed that a TiN barrier layer appears to favor the growth of thinner CNTs compared to a SiO2 layer

Ty1 integrase overexpression leads to integration of non-Ty1 DNA fragments into the genome of Saccharomyces cerevisiae

The integrase of the Saccharomyces cerevisiae retrotransposon Ty1 integrates Ty1 cDNA into genomic DNA likely via a transesterification reaction. Little is known about the mechanisms ensuring that integrase does not integrate non-Ty DNA fragments. In an effort to elucidate the conditions under which Ty1 integrase accepts non-Ty DNA as substrate, PCR fragments encompassing a selectable marker gene were transformed into yeast strains overexpressing Ty1 integrase. These fragments do not exhibit similarity to Ty1 cDNA except for the presence of the conserved terminal dinucleotide 5′-TG-CA-3′. The frequency of fragment insertion events increased upon integrase overexpression. Characterization of insertion events by genomic sequencing revealed that most insertion events exhibited clear hallmarks of integrase-mediated reactions, such as 5 bp target site duplication and target site preferences. Alteration of the terminal dinucleotide abolished the suitability of the PCR fragments to serve as substrates. We hypothesize that substrate specificity under normal conditions is mainly due to compartmentalization of integrase and Ty cDNA, which meet in virus-like particles. In contrast, recombinant integrase, which is not confined to virus-like particles, is able to accept non-Ty DNA, provided that it terminates in the proper dinucleotide sequence

Scattering of conformal higher spin fields

We develop a formalism for describing the most general notion of tree-level scattering amplitudes in 4d conformal higher spin theory. As conformal higher spin fields obey higher-derivative equations of motion, there are many distinct on-shell external states which may contribute to their scattering, some of which grow polynomially with time, leading to ill-defined amplitudes. We characterize the set of admissible scattering states which produce finite tree amplitudes, noting that there are more such states than just standard massless higher spins obeying two-derivative equations of motion. We use conformal gravity as a prime example, where the set of scattering states includes the usual Einstein graviton and a `ghost' massless spin 1 particle. An extension of the usual spinor helicity formalism allows us to encode these scattering states efficiently in terms of `twistor-spinors'. This leads to compact momentum space expressions for all finite tree-level 3-point amplitudes of conformal higher spin theory. While some of these 3-point amplitudes vanish (including all those with only standard two-derivative higher spin external states), there are many others which are non-vanishing. We also comment on the generalization to scattering of conformal higher spins in AdS$_4$.Comment: 40 pages, no figures. v2: references adde