Medium/high field magnetoconductance in chaotic quantum dots

The magnetoconductance G in chaotic quantum dots at medium/high magnetic fluxes Phi is calculated by means of a tight binding Hamiltonian on a square lattice. Chaotic dots are simulated by introducing diagonal disorder on surface sites of L x L clusters. It is shown that when the ratio W/L is sufficiently large, W being the leads width, G increases steadily showing a maximum at a flux Phi_max ~ W. Bulk disordered ballistic cavities (with an amount of impurities proportional to L) does not show this effect. On the other hand, for magnetic fluxes larger than that for which the cyclotron radius is of the order of L/2, the average magnetoconductance inceases almost linearly with the flux with a slope proportional to W^2, shows a maximum and then decreases stepwise. These results closely follow a theory proposed by Beenakker and van Houten to explain the magnetoconductance of two point contacts in series.Comment: RevTeX including six postscript figure

Trapping of electrons near chemisorbed hydrogen on graphene

Chemical adsorption of atomic hydrogen on a negatively charged single layer graphene sheet has been analyzed with ab-initio Density Functional Theory calculations. We have simulated both finite clusters and infinite periodic systems to investigate the effect of different ingredients of the theory, e.g. exchange and correlation potentials, basis sets, etc. Hydrogen's electron affinity dominates the energetic balance in the charged systems and the extra electron is predominantly attracted to a region nearby the chemisorbed atom. The main consequences are: (i) the cancellation of the unpaired spin resulting in a singlet ground-state, and (ii) a stronger interaction between hydrogen and the graphene sheet.Comment: 11 pages, 8 figures, to be published in PR

Ab-initio calculation of the effect of stress on the chemical activity of graphene

Graphene layers are stable, hard, and relatively inert. We study how tensile stress affects $\sigma$ and $\pi$ bonds and the resulting change in the chemical activity. Stress affects more strongly $\pi$ bonds that can become chemically active and bind to adsorbed species more strongly. Upon stretch, single C bonds are activated in a geometry mixing $120^{o}$ and $90^{o}$; an intermediate state between $sp^{2}$ and $sp^{3}$ bonding. We use ab-initio density functional theory to study the adsorption of hydrogen on large clusters and 2D periodic models for graphene. The influence of the exchange-correlation functional on the adsorption energy is discussed

Secuencias de propagación del sistema de cabalgamientos de la terminación oriental del manto del Pedraforca y relación con los conglomerados sinorogénicos

El mapa detallado de la terminación oriental del manto inferior del Pedraforca muestra un sistema imbricado de cabalgamientos que constituye una zona de rampas oblicuas. La secuencia de propagación de este sistema imbricado es de bloque superior para los cabalgamientos superiores y mas modernos, tal como lo demuestran las relaciones geométricas entre los cabalgamientos y los sedimentos asociados a éstos (abanicos deltaicos y aluviales). El cabalgamiento inferior del manto inferior del Pedraforca corresponde en superificie, casi en su totalidad, a un anticlinal de contorno oblicuo (anticlinal de Vilada-Lillet) situado en el manto infrayacente del Cadi. Este cabalgamiento inferior es intersectado, al oeste de Vilada, por el cabalgamiento de Vallfogona (cabalgamiento inferior del manto del Cadi). Los conglomerados sinorogenicos asociados al manto inferior del Pedraforca se depositaron durante la etapa de progresiva emersión del manto (emplazamiento) y determinan una edad Luteciense inferior alto-Luteciense medio (51 M.a.) para sus ultimos movimientos (fosilización). La composicibn litológica de los  conglomerados de los abanicos superiores y sus relaciones espaciales con el sinclinal de Ripoll (manto del Cadí) demuestran que el emplazamiento del manto inferior del Pedraforca se produjo en parte sincronicamente con el desplazamiento del manto del Cadi y el apilamiento antiformal del Freser

Classical trajectories in quantum transport at the band center of bipartite lattices with or without vacancies

Here we report on several anomalies in quantum transport at the band center of a bipartite lattice with vacancies that are surely due to its chiral symmetry, namely: no weak localization effect shows up, and, when leads have a single channel the transmission is either one or zero. We propose that these are a consequence of both the chiral symmetry and the large number of states at the band center. The probability amplitude associated to the eigenstate that gives unit transmission ressembles a classical trajectory both with or without vacancies. The large number of states allows to build up trajectories that elude the blocking vacancies explaining the absence of weak localization.Comment: 5 pages, 5 figure

Magnetic molecules created by hydrogenation of Polycyclic Aromatic Hydrocarbons

Present routes to produce magnetic organic-based materials adopt a common strategy: the use of magnetic species (atoms, polyradicals, etc.) as building blocks. We explore an alternative approach which consists of selective hydrogenation of Polycyclic Aromatic Hydrocarbons. Self-Consistent-Field (SCF) (Hartree-Fock and DFT) and multi-configurational (CISD and MCSCF) calculations on coronene and corannulene, both hexa-hydrogenated, show that the formation of stable high spin species is possible. The spin of the ground states is discussed in terms of the Hund rule and Lieb's theorem for bipartite lattices (alternant hydrocarbons in this case). This proposal opens a new door to magnetism in the organic world.Comment: 6 pages, 4 figures and 2 table

Secuencias de propagación del sistema de cabalgamientos de la terminación oriental del manto del Pedraforca y relación con los conglomerados sinorogénicos

El mapa detallado de la terminación oriental del manto inferior del Pedraforca muestra un sistema imbricado de cabalgamientos que constituye una zona de rampas oblicuas. La secuencia de propagación de este sistema imbricado es de bloque superior para los cabalgamientos superiores y mas modernos, tal como lo demuestran las relaciones geométricas entre los cabalgamientos y los sedimentos asociados a éstos (abanicos deltaicos y aluviales). El cabalgamiento inferior del manto inferior del Pedraforca corresponde en superificie, casi en su totalidad, a un anticlinal de contorno oblicuo (anticlinal de Vilada-Lillet) situado en el manto infrayacente del Cadi. Este cabalgamiento inferior es intersectado, al oeste de Vilada, por el cabalgamiento de Vallfogona (cabalgamiento inferior del manto del Cadi). Los conglomerados sinorogenicos asociados al manto inferior del Pedraforca se depositaron durante la etapa de progresiva emersión del manto (emplazamiento) y determinan una edad Luteciense inferior alto-Luteciense medio (51 M.a.) para sus ultimos movimientos (fosilización). La composicibn litológica de los  conglomerados de los abanicos superiores y sus relaciones espaciales con el sinclinal de Ripoll (manto del Cadí) demuestran que el emplazamiento del manto inferior del Pedraforca se produjo en parte sincronicamente con el desplazamiento del manto del Cadi y el apilamiento antiformal del Freser

Conductance scaling at the band center of wide wires with pure non--diagonal disorder

Kubo formula is used to get the scaling behavior of the static conductance distribution of wide wires showing pure non-diagonal disorder. Following recent works that point to unusual phenomena in some circumstances, scaling at the band center of wires of odd widths has been numerically investigated. While the conductance mean shows a decrease that is only proportional to the inverse square root of the wire length, the median of the distribution exponentially decreases as a function of the square root of the length. Actually, the whole distribution decays as the inverse square root of the length except close to G=0 where the distribution accumulates the weight lost at larger conductances. It accurately follows the theoretical prediction once the free parameter is correctly fitted. Moreover, when the number of channels equals the wire length but contacts are kept finite, the conductance distribution is still described by the previous model. It is shown that the common origin of this behavior is a simple Gaussian statistics followed by the logarithm of the E=0 wavefunction weight ratio of a system showing chiral symmetry. A finite value of the two-dimensional conductance mean is obtained in the infinite size limit. Both conductance and the wavefunction statistics distributions are given in this limit. This results are consistent with the 'critical' character of the E=0 wavefunction predicted in the literature.Comment: 10 pages, 9 figures, RevTeX macr