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

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

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

Geometría del margen S.E. de la Cuenca del Ebro: discordancias progresivas en el Grupo Scala Dei. Serra de La Llena. (Tarragona)

The Scala Dei Group, located at the southern margin of the Ebro foreland basin, represented the sedimentary response to the Catalan Coastal Ranges thrusting. At la Serra de la Llena the 1600 m of continental deposits can be divided in 4 main megasequences (M,- M,) separated by angular unconformities. New time-control data based on micromammal fauna indicates an influx of coarse material from Upper Lutetian-Bartonian to Middle-Upper Chattian (although the conglomeratic sedimentation related with la Serra del Montsant continued ti11 the Uppermost Chattian). Sedimentation rates at the la Serra de la Llena were 13 cm/Ky during the first megasequence (M,), 7,s cm/Ky during M,-M, and 6 cm/Ky during the last and finer grained M,. The general configuration of the syntectonic Scala Dei deposits with a double monoclinal flexure dipping to the northwest indicates tectonic control by two different structures at depth, interpreted as a fault-propagation folds. Theprogressiveunconformities developed in both forelimbs of these folds show continuous motion and a general steady state between rates of uplift and rates of sediment accumulation. The progressive unconformities are developed mainly by the vertical component of the tectonic motion and by a continuous rotation of the forelimb of the anticlines

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

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

Joan Roura-Parella: pensament i pedagogia

El objetivo de este artículo es clarificar la noción de «vida auténtica» tal y como es descrita por Joan Roura-Parella en su última obra Tema y variaciones de la personalidad (1950). Por «vida auténtica» se entiende una vida autogobernada, que permite al individuo realizar su propia singularidad y, por tanto, preservar su autonomía y dignidad como persona. Una vida dominada por factores ajenos al propio individuo, sean cuales sean éstos, es una vida alienada, que no permite el desarrollo del individuo en tanto la persona que es. Se señala la relevancia filosófica de la propuesta de Roura-Parella, que es la de ofrecer una justificación no-evidencial para una forma de vida cristiana, agapística. De acuerdo con Roura-Parella, una forma de vida basada en la entrega amorosa para con el prójimo está justificada en tanto que ésta es el resultado último de nuestra propia condición natural, constituyendo así un acto de afirmación personal y siendo, por tanto, condición necesaria para gozar de una vida auténtica — «la vida alcanza su hondo sentido al salirse de sí misma para darse a los demás» (Roura-Parella, Tema y variaciones de la personalidad, 1950, p. 229). Se señalan las diferencias y semejanzas de la propuesta de Roura-Parella con respecto a las de Miguel de Unamuno y Joaquim Xirau, autores que fueron cercanos a Roura-Parella y quienes, aunque partiendo de razonamientos distintos a los de Roura-Parella, también concibieron la entrega amorosa para con el prójimo como un acto de afirmación personal

Effects of Fermi energy, dot size and leads width on weak localization in chaotic quantum dots

Magnetotransport in chaotic quantum dots at low magnetic fields is investigated by means of a tight binding Hamiltonian on L x L clusters of the square lattice. Chaoticity is induced by introducing L bulk vacancies. The dependence of weak localization on the Fermi energy, dot size and leads width is investigated in detail and the results compared with those of previous analyses, in particular with random matrix theory predictions. Our results indicate that the dependence of the critical flux Phi_c on the square root of the number of open modes, as predicted by random matrix theory, is obscured by the strong energy dependence of the proportionality constant. Instead, the size dependence of the critical flux predicted by Efetov and random matrix theory, namely, Phi_c ~ sqrt{1/L}, is clearly illustrated by the present results. Our numerical results do also show that the weak localization term significantly decreases as the leads width W approaches L. However, calculations for W=L indicate that the weak localization effect does not disappear as L increases.Comment: RevTeX, 8 postscript figures include