426 research outputs found
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 and bonds and the resulting change in the
chemical activity. Stress affects more strongly bonds that can become
chemically active and bind to adsorbed species more strongly. Upon stretch,
single C bonds are activated in a geometry mixing and ; an
intermediate state between and 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
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