Ariel A. Valladares

Cooper pair dispersion relation for weak to strong coupling

Cooper pairing in two dimensions is analyzed with a set of renormalized equations to determine its binding energy for any fermion number density and all coupling assuming a generic pairwise residual interfermion interaction. Also considered are Cooper pairs (CPs) with nonzero center-of-mass momentum (CMM) and their binding energy is expanded analytically in powers of the CMM up to quadratic terms. A Fermi-sea-dependent linear term in the CMM dominates the pair excitation energy in weak coupling (also called the BCS regime) while the more familiar quadratic term prevails in strong coupling (the Bose regime). The crossover, though strictly unrelated to BCS theory per se, is studied numerically as it is expected to play a central role in a model of superconductivity as a Bose-Einstein condensation of CPs where the transition temperature vanishes for all dimensionality

Optical gaps of ab initio generated random networks for a-SiNx alloys

dl~2

Exploring the Surface Reactivity of 3d Metal Endofullerenes: A Density-Functional Theory Study

for quadratic dispersion, but is nonzero for all

Linear to quadratic crossover of Cooper-pair dispersion relation

dg~1

Low-dimensional BEC

for linear dispersion.

Ab initio cluster simulation of N-doped tetrahedral amorphous carbon

We report optical gaps for ab initio generated random networks of silicon–nitrogen alloys, a-SiNx, for thirteen values of x from 0 to x=1.29, a nearly stoichiometric composition. The random networks were constructed by amorphizing 64-atom periodically-continued diamond-like cells containing silicon and nitrogen, with a new thermal process and a Harris-functional based molecular dynamics code in the local density approximation. The electron energy levels were then calculated and the optical gaps obtained using a Tauc-like procedure that is not sensitive to gap states and band tails. Our results agree with experiment.

A New Approach to the Computer Modeling of Amorphous Nanoporous Structures of Semiconducting and Metallic Materials: A Review

Changes in the preferential sites of electrophilic, nucleophilic, and radical attacks on the pristine C60 surface with endohedral doping using 3d transition metal atoms were studied via two useful reactivity indices, namely the Fukui functions and the molecular electrostatic potential. Both of these were calculated at the density functional BPW91 level of theory with the DNP basis set. Our results clearly show changes in the preferential reactivity sites on the fullerene surface when it is doped with Mn, Fe, Co, or Ni atoms, whereas there are no significant changes in the preferential reactivity sites on the C60 surface upon endohedral doping with Cu and Zn atoms. Electron affinities (EA), ionization potentials (IP), and HOMO-LUMO gaps (Eg) were also calculated to complete the study of the endofullerenes surface reactivity. These findings provide insight into endofullerene functionalization, an important issue in their application.

Ab initio generation of amorphous carbon structures

Abstract Cooper pairing is studied in three dimensions to determine its binding energy for all coupling using a general separable interfermion interaction. Also considered are Cooper pairs (CPs) with nonzero center-of-mass momentum (CMM). A coupling-independent linear term in the CMM dominates the pair excitation energy in weak coupling and/or high fermion density, while the more familiar quadratic term prevails only in the extreme low-density (i.e., vacuum) limit for any nonzero coupling. The linear-to-quadratic crossover of the CP dispersion relation is analyzed numerically, and is expected to play a central role in a model of superconductivity (and superfluidity) simultaneously accommodating a Bardeen–Cooper–Schrieffer condensate as well as a Bose–Einstein condensate of CP bosons.

New Approaches to the Computer Simulation of Amorphous Alloys: A Review

The Bose-Einstein condensation (BEC) temperature Tc of Cooper pairs (CPs) created from a general interfermion interaction is determined for a linear, as well as the usually assumed quadratic, energy vs center-of-mass momentum dispersion relation. This explicit Tc is then compared with a widely applied implicit one of Wen & Kan (1988) in d=2+∈ dimensions, for small ∈, for a geometry of an infinite stack of parallel (e.g., copperoxygen) planes as in, say, a cuprate superconductor, and with a new result for linear-dispersion CPs. The implicit formula gives Tc values only slightly lower than those of the explicit formula for typical cuprate parameters.

Atomic topology and radial distribution functions of a-SiNx alloys: ab initio simulations

Abstract The electronic structure of nitrogen-doped tetrahedral carbon clusters, both amorphous and crystalline, with 21, 57 and 59 carbon atoms and various ring topologies, were studied using the self-consistent ab initio density functional theory-local density approximation (DFT-LDA). All clusters were hydrogen saturated. Clusters with n =0,1 and 4 nitrogen atoms were analyzed for each structure using an initial interatomic distance of 0.154 nm as in the bulk. All clusters were energy optimized maintaining tetrahedral symmetry and the position of the outermost atoms in order to simulate the inertia of the bulk. For all the clusters the energy gap increases with one N. For the 21-atom cluster which contains only 6-atom boat-rings the gap remains practically constant as n increases from 1, 6.60 to 4, 6.51 eV, unlike the other clusters. The Fermi energy varies from the top of the valence band to the bottom of the conduction band as the nitrogen concentration increases. In the forced fourfold coordinated environment N is a shallow donor as is P in Si; however, when N becomes essentially threefold coordinated, states appear in the middle of the gap.