790 research outputs found
Resonances in a two-dimensional electron waveguide with a single delta-function scatterer
We study the conductance properties of a straight two-dimensional electron
waveguide with an s-like scatterer modeled by a single delta-function potential
with a finite number of modes. Even such a simple system exhibits interesting
resonance phenomena. These resonances are explained in terms of quasi-bound
states both by using a direct solution of the Schroedinger equation and by
studying the Green's function of the system. Using the Green's function we
calculate the survival probability as well as the power absorption and show the
influence of the quasi-bound states on these two quantities.Comment: 5 pages, 6 figures, to be published in Physical Review
Conductance and localization in disordered wires: role of evanescent states
This paper extends an earlier analytical scattering matrix treatment of
conductance and localization in coupled two- and three Anderson chain systems
for weak disorder when evanescent states are present at the Fermi level. Such
states exist typically when the interchain coupling exceeds the width of
propagating energy bands associated with the various transverse eigenvalues of
the coupled tight-binding systems. We calculate reflection- and transmission
coefficients in cases where, besides propagating states, one or two evanescent
states are available at the Fermi level for elastic scattering of electrons by
the disordered systems. We observe important qualitative changes in these
coefficients and in the related localization lengths due to ineffectiveness of
the evanescent modes for transmission and reflection in the various scattering
channels. In particular, the localization lengths are generally significantly
larger than the values obtained when evanescent modes are absent. Effects
associated with disorder mediated coupling between propagating and evanescent
modes are shown to be suppressed by quantum interference effects, in lowest
order for weak disorder
Childrenās friendships in middle childhood: how number of friends, reciprocity and friendship quality relate to peer and school identification, and general self-worth
Childrenās friendships are important for well-being and school adjustment, but few studies have examined multiple indices of friendships together in middle childhood. The current study surveyed 7 to 11-year olds (n = 314) about their friendships, best friendships, friendship quality and indices of self-worth, identification with peers and identification with school. Peer relationships were positively related to self-worth, but not identification with peers or school. Best friendship quality moderated the relationship between number of reciprocated friendship nominations and self-worth. Children with a reciprocated best friend had higher friendship quality and peer identification than others. Where best friendship was reciprocated, the relationship with identification with peers was mediated via positive friendship quality. The results suggest that friendship reciprocity is particularly relevant for childrenās self-worth and identification with peers. The findings are discussed in relation to the importance of fostering the development of reciprocated friendships
Effect of deconfinement on resonant transport in quantum wires
The effect of deconfinement due to finite band offsets on transport through
quantum wires with two constrictions is investigated. It is shown that the
increase in resonance linewidth becomes increasingly important as the size is
reduced and ultimately places an upper limit on the energy (temperature) scale
for which resonances may be observed.Comment: 6 pages, 6 postscript files with figures; uses REVTe
Fluctuations in the transmission properties of a quantum dot with interface roughness and impurities
Transmission Through Carbon Nanotubes With Polyhedral Caps
We study electron transport between capped carbon nanotubes and a substrate,
and relate the transmission probability to the local density of states in the
cap. Our results show that the transmission probability mimics the behavior of
the density of states at all energies except those that correspond to localized
states in the cap. Close proximity of a substrate causes hybridization of the
localized state. As a result, new transmission paths open from the substrate to
nanotube continuum states via the localized states in the cap. Interference
between various transmission paths gives rise to antiresonances in the
transmission probability, with the minimum transmission equal to zero at
energies of the localized states. Defects in the nanotube that are placed close
to the cap cause resonances in the transmission probability, instead of
antiresonances, near the localized energy levels. Depending on the spatial
position of defects, these resonant states are capable of carrying a large
current. These results are relevant to carbon nanotube based studies of
molecular electronics and probe tip applications
Coulomb Blockade Resonances in Quantum Wires
The conductance through a quantum wire of cylindrical cross section and a
weak bulge is solved exactly for two electrons within the Landauer-Buettiker
formalism. We show that this 'open' quantum dot exhibits spin-dependent Coulomb
blockade resonances resulting in two anomalous structure on the rising edge to
the first conductance plateau, one near 0.25(2e^2/h), related to a singlet
resonance, and one near 0.7(2e^2/h), related to a triplet resonance. These
resonances are generic and robust, occurring for other types of quantum wire
and surviving to temperatures of a few degrees.Comment: 5 pages, 3 postscript files with figures; uses REVTe
Aharonov-Bohm effect and resonances in the circular quantum billiard with two leads
We calculate the conductance through a circular quantum billiard with two
leads and a point magnetic flux at the center. The boundary element method is
used to solve the Schrodinger equation of the scattering problem, and the
Landauer formula is used to calculate the conductance from the transmission
coefficients. We use two different shapes of leads, straight and conic, and
find that the conductance is affected by lead geometry, the relative positions
of the leads and the magnetic flux. The Aharonov-Bohm effect can be seen from
shifts and splittings of fluctuations. When the flux is equal to (h/2e) and the
angle between leads is 180 degree, the conductance tends to be suppressed to
zero in the low energy range due to the Aharonov-Bohm effect.Comment: LaTeX2e, 8 pages, 6 figures, submitted to Phys. Rev. B (Two
references added. A discussion on discrete symmetries removed.
Increasing Dominance - the Role of Advertising, Pricing and Product Design
Despite the empirical relevance of advertising strategies in concentrated markets, the economics literature is largely silent on the effect of persuasive advertising
strategies on pricing, market structure and increasing (or decreasing) dominance. In a simple model of persuasive advertising and pricing with differentiated goods,
we analyze the interdependencies between ex-ante asymmetries in consumer appeal, advertising and prices. Products with larger initial appeal to consumers will
be advertised more heavily but priced at a higher level - that is, advertising and price discounts are strategic substitutes for products with asymmetric initial appeal.
We find that the escalating effect of advertising dominates the moderating effect of pricing so that post-competition market shares are more asymmetric than pre-competition differences in consumer appeal. We further find that collusive advertising (but competitive pricing) generates the same market outcomes, and that network effects lead to even more extreme market outcomes, both directly and via
the effect on advertising
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