284 research outputs found
Remodeling the B-model
We propose a complete, new formalism to compute unambiguously B-model open
and closed amplitudes in local Calabi-Yau geometries, including the mirrors of
toric manifolds. The formalism is based on the recursive solution of matrix
models recently proposed by Eynard and Orantin. The resulting amplitudes are
non-perturbative in both the closed and the open moduli. The formalism can then
be used to study stringy phase transitions in the open/closed moduli space. At
large radius, this formalism may be seen as a mirror formalism to the
topological vertex, but it is also valid in other phases in the moduli space.
We develop the formalism in general and provide an extensive number of checks,
including a test at the orbifold point of A_p fibrations, where the amplitudes
compute the 't Hooft expansion of Wilson loops in lens spaces. We also use our
formalism to predict the disk amplitude for the orbifold C^3/Z_3.Comment: 83 pages, 9 figure
Four-modulus "Swiss Cheese" chiral models
We study the 'Large Volume Scenario' on explicit, new, compact, four-modulus
Calabi-Yau manifolds. We pay special attention to the chirality problem pointed
out by Blumenhagen, Moster and Plauschinn. Namely, we thoroughly analyze the
possibility of generating neutral, non-perturbative superpotentials from
Euclidean D3-branes in the presence of chirally intersecting D7-branes. We find
that taking proper account of the Freed-Witten anomaly on non-spin cycles and
of the Kaehler cone conditions imposes severe constraints on the models.
Nevertheless, we are able to create setups where the constraints are solved,
and up to three moduli are stabilized.Comment: 40 pages, 10 figures, clarifying comments added, minor mistakes
correcte
Twisting K3 x T^2 Orbifolds
We construct a class of geometric twists of Calabi-Yau manifolds of
Voisin-Borcea type (K3 x T^2)/Z_2 and study the superpotential in a type IIA
orientifold based on this geometry. The twists modify the direct product by
fibering the K3 over T^2 while preserving the Z_2 involution. As an important
application, the Voisin-Borcea class contains T^6/(Z_2 x Z_2), the usual
setting for intersecting D6 brane model building. Past work in this context
considered only those twists inherited from T^6, but our work extends these
twists to a subset of the blow-up modes. Our work naturally generalizes to
arbitrary K3 fibered Calabi-Yau manifolds and to nongeometric constructions.Comment: 57 pages, 4 figures; uses harvmac.tex, amssym.tex; v3: minor
corrections, references adde
MOF-based heterogeneous catalysis in continuous flow via incorporation onto polymer-based spherical activated carbon supports
We present an approach to harnessing the tuneable catalytic properties of complex nanomaterials for continuous flow heterogeneous catalysis by combining them with the scalable and industrially implementable properties of carbon pelleted supports. This approach, in turn, will enable these catalytic materials, which largely currently exist in forms unsuitable for this application (e.g. powders), to be fully integrated into large scale, chemical processes. A composite heterogeneous catalyst consisting of a metalâorganic framework-based Lewis acid, MIL-100(Sc), immobilised onto polymer-based spherical activated carbon (PBSAC) support has been developed. The material was characterised by focused ion beam-scanning electron microscopy-energy dispersive X-ray analysis, powder X-ray diffraction, N2 adsorption, thermogravimetric analysis, atomic absorption spectroscopy, light scattering and crush testing with the catalytic activity studied in continuous flow. The mechanically robust spherical geometry makes the composite material ideal for application in packed-bed reactors. The catalyst was observed to operate without any loss in activity at steady state for 9 hours when utilised as a Lewis acid catalyst for the intramolecular cyclisation of (±)-citronellal as a model reaction. This work paves the way for further development into the exploitation of MOF-based continuous flow heterogeneous catalysis
Efficient Hydrogen Evolution from Dimethylamine Borane, Ammonia Borane and Sodium Borohydride Catalyzed by Ruthenium and Platinum Nanoparticles Stabilized by an Amine Modified Polymer Immobilized Ionic Liquid: a Comparative Study
Platinum and ruthenium nanoparticles stabilised by an amine modified polymer immobilised ionic liquid (MNP@NH2-PEGPIILS, Mâ=âPt, Ru) catalyse the hydrolytic liberation of hydrogen from dimethylamine borane (DMAB), ammonia borane (AB) and NaBH4 under mild conditions. While RuNP@NH2-PEGPIILS and PtNP@NH2-PEGPIILS catalyse the hydrolytic evolution of hydrogen from NaBH4 with comparable initial TOFs of 6,250 molesH2.molcatâ1.hâ1 and 5,900 molesH2.molcatâ1.hâ1, respectively, based on the total metal content, RuNP@NH2-PEGPIILS is a markedly more efficient catalyst for the dehydrogenation of DMAB and AB than its platinum counterpart, as RuNP@NH2-PEGPIILS gave initial TOFs of 8,300 molesH2.molcatâ1.hâ1 and 21,200 molesH2.molcatâ1.hâ1, respectively, compared with 3,050 molesH2.molcatâ1.hâ1 and 8,500 molesH2.molcatâ1.hâ1, respectively, for PtNP@NH2-PEGPIILS. Gratifyingly, for each substrate tested RuNP@NH2-PEGPIILS and PtNP@NH2-PEGPIILS were markedly more active than commercial 5wt % Ru/C and 5wt% Pt/C, respectively. The apparent activation energies of 55.7 kJ molâ1 and 27.9 kJ molâ1 for the catalytic hydrolysis of DMAB and AB, respectively, with RuNP@NH2-PEGPIILS are significantly lower than the respective activation energies of 74.6 kJ molâ1 and 35.7 kJ molâ1 for its platinum counterpart, commensurate with the markedly higher initial rates obtained with the RuNPs. In comparison, the apparent activation energies of 44.1 kJ molâ1 and 46.5 kJ molâ1, for the hydrolysis NaBH4 reflect the similar initial TOFs obtained for both catalysts. The difference in apparent activation energies for the hydrolysis of DMAB compared with AB also reflect the higher rates of hydrolysis for the latter. Stability and reuse studies revealed that RuNP@NH2-PEGPIILS recycled efficiently as high conversions for the hydrolysis of DMAB were maintained across five runs with the catalyst retaining 97% of its activity
Topological Strings on Grassmannian Calabi-Yau manifolds
We present solutions for the higher genus topological string amplitudes on
Calabi-Yau-manifolds, which are realized as complete intersections in
Grassmannians. We solve the B-model by direct integration of the holomorphic
anomaly equations using a finite basis of modular invariant generators, the gap
condition at the conifold and other local boundary conditions for the
amplitudes. Regularity of the latter at certain points in the moduli space
suggests a CFT description. The A-model amplitudes are evaluated using a mirror
conjecture for Grassmannian Calabi-Yau by Batyrev, Ciocan-Fontanine, Kim and
Van Straten. The integrality of the BPS states gives strong evidence for the
conjecture.Comment: 36 pages, 1 eps figur
Amine-modified polyionic liquid supports enhance the efficacy of PdNPs for the catalytic hydrogenation of COâ to formate
Palladium nanoparticles stabilised by aniline modified polymer immobilised ionic liquid is a remarkably active catalyst for the hydrogenation of COâ to formate; the initial TOF of 500 hâ»Âč is markedly higher than either unmodified catalyst or its benzylamine and N,N-dimethylaniline modified counterparts and is among the highest to be reported for a PdNP-based catalyst
Selective Partial Reduction of Nitroarenes to the Hydrazoarene Catalyzed by AmineâModified Ordered Mesoporous Silica Immobilized Ionic Liquid (OMSIIL) Stabilised RuNPs
Ruthenium nanoparticles stabilised by an amine-modified Ordered Mesoporous Silica Immobilized Ionic Liquid (OMSIIL) are efficient catalysts for the partial reduction of nitrobenzene to hydrazobenzene with 100â% selectivity as well as the complete reduction to aniline. High selectivity for the partial reduction of nitrobenzene to hydrazobenzene was obtained when the reaction was conducted in ethanol with 0.5â
mol% catalyst and NaBHâ as the hydrogen donor whereas aniline was obtained as the sole product in water when dimethylamine borane (DMAB) was used as the hydrogen donor. Interestingly, while a range of electron poor nitroarenes were reduced to the corresponding hydrazoarene with high selectivities and good conversions, nitroarenes substituted with electron donating groups resulted in complete reduction to the aniline. Composition-time profiles suggest that reductions conducted in ethanol with sodium borohydride occur via the condensation pathway while those conducted in water using dimethylamine borane as the hydrogen source may well go via the direct pathway. This is the first example of the selective reduction of nitrobenzene to hydrazobenzene using a ruthenium nanoparticle-based catalyst and the initial TOF of 320â
mol nitrobenzene converted mol Ruâ»Âč hâ»Âč for the partial reduction of nitrobenzene to hydrazobenzene is markedly higher than previous literature reports. A study of the catalyst performance as a function of the surface modification revealed that each component has a direct and dramatic effect on the efficacy as RuNPs stabilised by COK-12 modified with imidazolium-based ionic liquid and a primary amine gave the highest conversion while selective removal of either component or replacement of the primary amine with a tertiary amine resulted in a marked reduction in efficiency
The Spherically Symmetric Standard Model with Gravity
Spherical reduction of generic four-dimensional theories is revisited. Three
different notions of "spherical symmetry" are defined. The following sectors
are investigated: Einstein-Cartan theory, spinors, (non-)abelian gauge fields
and scalar fields. In each sector a different formalism seems to be most
convenient: the Cartan formulation of gravity works best in the purely
gravitational sector, the Einstein formulation is convenient for the Yang-Mills
sector and for reducing scalar fields, and the Newman-Penrose formalism seems
to be the most transparent one in the fermionic sector. Combining them the
spherically reduced Standard Model of particle physics together with the
usually omitted gravity part can be presented as a two-dimensional (dilaton
gravity) theory.Comment: 58 pages, 2 eps figure
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