A. Calles

A simple algorithm for the group theoretical classification of quantum states

Here we present a simple and easily handled algorithm for the group theoretical classification of quantum states that although, it is implicit in a general group theory formulation, it is not presented the way it is discussed here, and which facilitates its application by non-specialists. It consists in finding the minimal set of symmetry operations needed to identify unequivocally the symmetry for each state and a direct calculation of the character of the representation expanded by the vector state. The algorithm allows simple and fast calculations of the symmetry properties, even for large symmetry groups and systems with many degrees of freedom. In particular the occurrence of accidental degeneracy is distinguished clearly using this method. In order to illustrate the application of the method, we discuss the classification of the vibrational states for two different molecular systems.

Influence of 5-atom rings on the optical properties of amorphous carbon nitride

Abstract The optical properties of three amorphous tetrahedral carbon-based clusters of the type α- C 17 - i N i H 24 with 5-atom rings have been calculated. The hydrogens are used to passivate the outermost dangling bonds and t is 0, 1 or 4. The theoretical DFT-LDA based method used minimizes the energy of the clusters and provides the energy spectrum and the optical transitions for the minimum energy structures. The optical absorption spectrum of each cluster is analyzed and a comparison is made with previously reported spectra for clusters with 6-atom boat-type rings in order to see the influence of the present atomic topology.

A THEORETICAL-ANALYSIS OF PHONON-SPECTRA IN HIGH-TC SUPERCONDUCTORS

Abstract The normal modes of vibration of the superconductors Y 1 Ba 2 Cu 3 O 7 and related compounds are calculated using the harmonic aproximation. The conditions for the Jahn-Teller effect and its influence on the phonon density of states are analysed.

Ab initio Computationally Generated Nanoporous Carbon and its Comparison to Experiment

Nanoporous carbon is a widely studied material due to its potential applications in hydrogen storage or for filtering undesirable products. Most of the developments have been experimental although some simulation work has been carried out based on the use of graphene sheets and/or carbon chains and classical molecular dynamics. Here we present an application of our recently developed ab initio method [1] for the generation of group IV porous materials. The method consists in constructing a crystalline diamond supercell with 216 atoms of carbon and a density of 3.546 g/cm 3 , then lengthening the supercell edge to obtain a density of 1.38 g/cm 3 , yielding a porosity of 61.1 % in order to be able to compare with experimental results reported in the literature [2]. We then subject the resulting supercell to an ab initio molecular dynamics process at 1000 K during 295 steps. The radial distribution functions obtained are compared to experiment to discern coincidences and discrepancies.

A theoretical phonon calculation in the Bi2Sr2CaCu2O8 superconductor

Abstract The normal modes of vibration of the Bi 2 Sr 2 CaCu 2 O 8 superconducting material (with Tc near 85 K) are calculated within the harmonic approximation. The theoretical group analysis of the normal modes and the phonon density of states are reported.

The Energetics of Hydrogen Adsorbed in Nanoporous Silicon. An ab initio Simulational Study

Porous silicon may be an interesting alternative to store hydrogen. Unlike carbon, its bonding multiplicity is limited, and because of this, the probability of having more dangling bonds on the pore surface is larger than in carbon. Using nanoporous silicon periodic supercells with 216 atoms and 50 % porosity, constructed with a novel ab initio approach devised by us, the dangling bonds of the silicon atoms were first saturated with hydrogen, then relaxed and its total energy calculated. Next the same number of hydrogen atoms was placed within the pore in the pure silicon supercell, then the sample relaxed, and finally its total energy calculated, with and without hydrogens. From these results the average energy per hydrogen atom is obtained. We compare our results to SiH bond energies and to previous results for hydrogenated carbon; conclusions are drawn concerning the possibility of using porous silicon as a fuel tank for hydrogen.

Jahn-Teller effect in the LaSrCuO superconductor

The CuO 6 octahedron in the T structure of the La-Sr-Cu-O superconductor is considered within the quasi-molecular approximation in order to calculate the electronic structure and the vibrational modes that could participate in the Jahn-Teller effect. The electronic calculations are made within the extended Huckel model and the vibrations are taken from previously reported results. Once the electronic and vibrational participants are determined using group theory analysis, the intensity of the electron-phonon interaction is calculated to establish the Jahn-Teller deformation

Jahn-Teller calculations for CuO4 and FeO4 clusters in the Nd1.85Ce0.15Cu0.99Fe0.01O4−δ

The Jahn-Teller distortion mechanism in the Nd2−xCexCuO4 is analyzed. This analysis is based on a quasi-molecular model for the CuO4 cluster havingD4h symmetry. In order to compare with experimental Mössbauer measurements in the doped Nd1.85Ce0.15Cu0.99Fe0.01O4−δ superconductor, the FeO4 cluster is also analyzed for the case of Fe2+ in the high spin state. The results show not evidence of a Jahn-Teller effect in these superconductors.

Experimental and computational conductivity study of multilayer graphene in polypropylene nanocomposites

We study the electric conductivity of compounds formed by multilayer graphene in polypropylene. Our study makes a comparative analysis between the experimental and computational results. To obtain an experimental measurement of the electronic properties, we deposited multilayer graphene (MLG) nanoparticles over a polypropylene matrix. The deposition was made over several stages, in which we added to the polymer matrix different percentages of MLG nanoparticles using the melt compounding technique, and we studied the conductivities of the nanocomposites by means of electrochemical impedance spectroscopy (EIS). The second part consists of computational calculations, in which we studied the electronic properties of a graphene sheet under a polypropylene molecule with different slabs in the monomer. In both analyses, there is a strong percolation phenomenon with a percolation threshold of around 18% of the MLG nanoparticles. Before the percolation threshold, the charge carriers are constrained in the polypropylene molecule, making the system an insulating material and creating p-type doping. After the percolation threshold, the charge carriers are constrained in the graphene, making the system a conductor material and creating n-type doping with conductivity values of around 20 S m−1. This phenomenon is a consequence of a change in the mechanism of charge transfer in the interface between the polypropylene molecule and graphene sheet. To describe the charge transfer mechanism, it is necessary to consider the quantum effect. The incorporation of the quantum effects and the percolation phenomenon make it possible for the theoretical conductivity to be close to the conductivity measured experimentally.

Magnetic fields produced by rotating symmetrical bodies with homogeneous surface charge density

We present a numerical calculation for the stationary magnetic field produced by different rotating bodies with homogeneous and constant surface charge density. The calculation is done by superposing the magnetic field produced by a set of loops of current which mimic the magnetic field produced by belts of current defined by slices of fixed width. We consider the cases of a sphere, ellipsoids, open and closed cylinders and a combination of these in a dumbbell-like shell. We also plot their magnetic field lines using a technique that make use of the Runge–Kutta fourth-order method. Up to our knowledge, the case of closed cylinders was not calculated before. In contrast to previous results, we find that the magnetic field inside finite hollow bodies is homogeneous only in the case of a sphere. This is consequence of the fact that, for the sphere, the surface of any slice taken perpendicularly to the rotation axis, depends only on its thickness, like in the case of an infinite cylinder.