8,308 research outputs found
Treating some solid state problems with the Dirac equation
The ambiguity involved in the definition of effective-mass Hamiltonians for
nonrelativistic models is resolved using the Dirac equation. The multistep
approximation is extended for relativistic cases allowing the treatment of
arbitrary potential and effective-mass profiles without ordering problems. On
the other hand, if the Schrodinger equation is supposed to be used, our
relativistic approach demonstrate that both results are coincidents if the
BenDaniel and Duke prescription for the kinetic-energy operator is implemented.
Applications for semiconductor heterostructures are discussed.Comment: 06 pages, 5 figure
Co-evolution of strategy and structure in complex networks with dynamical linking
Here we introduce a model in which individuals differ in the rate at which
they seek new interactions with others, making rational decisions modeled as
general symmetric two-player games. Once a link between two individuals has
formed, the productivity of this link is evaluated. Links can be broken off at
different rates. We provide analytic results for the limiting cases where
linking dynamics is much faster than evolutionary dynamics and vice-versa, and
show how the individual capacity of forming new links or severing inconvenient
ones maps into the problem of strategy evolution in a well-mixed population
under a different game. For intermediate ranges, we investigate numerically the
detailed interplay determined by these two time-scales and show that the scope
of validity of the analytical results extends to a much wider ratio of time
scales than expected
Quantifying the Drivers of Star Formation on Galactic Scales. I. The Small Magellanic Cloud
We use the star formation history of the Small Magellanic Cloud (SMC) to
place quantitative limits on the effect of tidal interactions and gas infall on
the star formation and chemical enrichment history of the SMC. The coincident
timing of two recent (< 4 Gyr) increases in the star formation rate and
SMC/Milky Way(MW) pericenter passages suggests that global star formation in
the SMC is driven at least in part by tidal forces due to the MW. The Large
Magellanic Cloud (LMC) is the other potential driver of star formation, but is
only near the SMC during the most recent burst. The poorly constrained LMC-SMC
orbit is our principal uncertainty. To explore the correspondence between
bursts and MW pericenter passages further, we model star formation in the SMC
using a combination of continuous and tidally-triggered star formation. The
behavior of the tidally-triggered mode is a strong inverse function of the
SMC-MW separation (preferred behavior ~ r^-5, resulting in a factor of ~100
difference in the rate of tidally-triggered star formation at pericenter and
apocenter). Despite the success of these closed-box evolutionary models in
reproducing the recent SMC star formation history and current chemical
abundance, they have some systematic shortcomings that are remedied by
postulating that a sizable infall event (~ 50% of the total gas mass) occured
about 4 Gyr ago. Regardless of whether this infall event is included, the
fraction of stars in the SMC that formed via a tidally triggered mode is > 10%
and could be as large as 70%.Comment: Accepted for publication in Ap
Between old broadcast media and new networked media: materiality and media consumption practices
Past tensions between content and materiality have prevented communication researchers from forming a deeper conceptualization of the role played by the material character of communication technologies in shaping social arrangements and cultural forms of expression. Drawing on the findings of a comparative research project on European audiences, we examine the interrelation between material facets of media technologies and the practices they afford on the audience side. Relations between "old" mass media, such as television, and "new" networked and individualized media, such as the Internet, are discussed, focusing on the transitions between different media technologies and the processes of substitution or supplementation occurring in these transitions.info:eu-repo/semantics/publishedVersio
Magnetic-field effects on transport in carbon nanotube junctions
Here we address a theoretical study on the behaviour of electronic states of
heterojunctions and quantum dots based on carbon nanotubes under magnetic
fields. Emphasis is put on the analysis of the local density of states, the
conductance, and on the characteristic curves of current versus voltage. The
heterostructures are modeled by joining zigzag tubes through single
pentagon-heptagon pair defects, and described within a simple tight binding
calculation. The conductance is calculated using the Landauer formula in the
Green functions formalism. The used theoretical approach incorporates the
atomic details of the topological defects by performing an energy relaxation
via Monte Carlo calculation. The effect of a magnetic field on the conductance
gap of the system is investigated and compared to those of isolated constituent
tubes. It is found that the conductance gap of the studied CNHs exhibits
oscillations as a function of the magnetic flux. However, unlike the pristine
tubes case, they are not Aharonov-Bohm periodic oscillations
Avaliação da fermentação de caldo de sorgo sacarino empregando leveduras comerciais.
Visando explorar o potencial deste material, empregando o caldo de sorgo sacarino BRS 511, foi avaliado o comportamento de três leveduras comerciais (CAT-1, PE-2 e SA-1) em quatro ciclos fermentativos
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