Ab initio Simulation of Optical Limiting: The Case of Metal-Free Phthalocyanine

We present a fully ab initio, non-perturbative description of the optical limiting properties of a metal-free phthalocyanine, by simulating the effects of a broadband electric field of increasing intensity. The results confirm reverse saturable absorption as leading mechanism for optical limiting phenomena in this system and reveal that a number of dipole-forbidden excitations are populated by excited-state absorption, at more intense external fields. The excellent agreement with the experimental data supports our approach as a powerful tool to predict optical limiting, in view of applications

Surface nano-patterning through styrene adsorption on Si(100)

We present an ab initio study of the structural and electronic properties of styrene molecules adsorbed on the dimerized Si(100) surface at different coverages, ranging from the single-molecule to the full monolayer. The adsorption mechanism primarily involves the vinyl group via a [2+2] cycloaddition process that leads to the formation of covalent Si-C bonds and a local surface derelaxation, while it leaves the phenyl group almost unperturbed. The investigation of the functionalized surface as a function of the coverage (e.g. 0.5 -- 1 ML) and of the substrate reconstruction reveals two major effects. The first results from Si dimer-vinyl interaction and concerns the controlled variation of the energy bandgap of the interface. The second is associated to phenyl-phenyl interactions, which gives rise to a regular pattern of electronic wires at surface, stemming from the pi-pi coupling. These findings suggest a rationale for tailoring the surface nano-patterning of the surface, in a controlled way.Comment: 19 pages (preprint), 4 figures, supplementary materia

A note on Nordhaus-Gaddum-type inequaliies for the automorphic H-chromatic index of graphs

The automorphic H-chromatic index of a graph G is the minimum integer m for which G has a proper edge-coloring with m colors which is preserved by a given automorphism H of G. We consider the sum and the product of the automorphic H-chromatic index of a graph and its complement. We prove upper and lower bounds in terms of the order of the graph when H is chosen to be either a cyclic group of prime order or a group of order four

Wall paintings facies and their possible genetic correlates in the ancient Pompeii: A bio-anthropologic message from the past?

The figurative arts and precisely the ancient Pompeian wall paintings portraits can provide an additional source of information in supplementing bio-anthropological studies. There are several genetic diseases with a wide spectrum of congenital bone stigmata in association to distinctive facial features. Gorlin-Goltz syndrome, also named nevoid basal cell carcinoma syndrome, is an autosomal dominant syndrome characterized by unusual skeletal changes, such as macrocephaly, facial asymmetry, hypertelorism, frontal and parietal bossing caused by germline mutations of the gene PTCH1. The Gorlin syndrome, clinically defined in 1963, existed during Dynastic Egyptian times, as revealed by a spectrum of skeletal findings compatible with the syndrome in mummies dating back to three thousand years ago and, most likely, in the ancient population of Pompeii. In the present research, we discuss the potential relationship between Pompeian wall paintings portrait and the cranio-metric bone changes revealed among the Pompeian skull collections assuming that the ancient portraits can constitute an important tool that should be strictly integrated with osteologic and biomolecular data in order to argue a syndromic diagnosis in ancient population

Manifold Spines and Hyperbolicity Equations

We give a combinatorial representation of compact connected orientable 3-dimensional manifolds with boundary and their special spines by a class of graphs with extrastructure which are strictly related to o-graphs defined and studied in [3] and [4]. Then we describe a simple algorithm for constructing the boundary of these manifolds by using a list of 6-tuples of non-negative integers. Finally we discuss some combinatorial methods for determining the hyperbolicity equations. Examples of hyperbolic 3- manifolds of low complexity illustrate in particular cases the constructions and algorithms presented in the paper

Excitons in carbon nanotubes: an ab initio symmetry-based approach

The optical absorption spectrum of the carbon (4,2) nanotube is computed using an ab-initio many-body approach which takes into account excitonic effects. We develop a new method involving a local basis set which is symmetric with respect to the screw symmetry of the tube. Such a method has the advantages of scaling faster than plane-wave methods and allowing for a precise determination of the symmetry character of the single particle states, two-particle excitations, and selection rules. The binding energy of the lowest, optically active states is approximately 0.8 eV. The corresponding exciton wavefunctions are delocalized along the circumference of the tube and localized in the direction of the tube axis.Comment: 4 pages, 1 LaTex file + 4 eps figure

Electronic properties of polymer crystals: The effect of interchain interactions

We present a theoretical study of the transport parameters in a prototype conjugated-polymer, poly-para-phenylenevinylene, in two different possible crystalline packings. Our analysis is performed through density-functional electronic structure calculations, and allows one to obtain the fundamental parameters describing charge transport. The transfer integrals are found to be a crucial quantity to appreciate the effects of crystalline aggregation on conduction properties: our results indicate that interchain interactions can be viewed as a tunable parameter for the design of efficient electronic devices based on organic materials.The sensitivity of interchain interactions to the specific three-dimensional (3D) structure was demonstrated. It was confirmed that the interactions can be used to tailor the transport properties of conjugated-polymer films

Optical properties of graphene nanoribbons: The role of many-body effects

We investigate from first principles the optoelectronic properties of nanometer-sized armchair graphene nanoribbons (GNRs). We show that many-body effects are essential to correctly describe both energy gaps and optical response. As a signature of the confined geometry, we observe strongly bound excitons dominating the optical spectra, with a clear family-dependent binding energy. Our results demonstrate that GNRs constitute one-dimensional nanostructures whose absorption and luminescence performance can be controlled by changing both family and edge termination.We investigate from first principles the optoelectronic properties of nanometer-sized armchair graphene nanoribbons (GNRs). We show that many-body effects are essential to correctly describe both energy gaps and optical response. As a signature of the confined geometry, we observe strongly bound excitons dominating the optical spectra, with a clear family-dependent binding energy. Our results demonstrate that GNRs constitute one-dimensional nanostructures whose absorption and luminescence performance can be controlled by changing both family and edge termination. © 2008 The American Physical Society

Kite systems of order 8;Embedding of kite systems into bowtie systems

This article consist of two parts. In the first part, we enumerate the kite systems of order 8; in the second part, we consider embedding kite systems into bowtie systems