164 research outputs found
Coherent resistance of a disordered 1D wire: Expressions for all moments and evidence for non-Gaussian distribution
We study coherent electron transport in a one-dimensional wire with disorder
modeled as a chain of randomly positioned scatterers. We derive analytical
expressions for all statistical moments of the wire resistance . By means
of these expressions we show analytically that the distribution of the
variable is not exactly Gaussian even in the limit of weak
disorder. In a strict mathematical sense, this conclusion is found to hold not
only for the distribution tails but also for the bulk of the distribution
.Comment: Revised version, 8 pages, 4 figures, RevTeX
Making On-Demand Routing Efficient with Route-Request Aggregation
In theory, on-demand routing is very attractive for mobile ad hoc networks
(MANET), because it induces signaling only for those destinations for which
there is data traffic. However, in practice, the signaling overhead of existing
on-demand routing protocols becomes excessive as the rate of topology changes
increases due to mobility or other causes. We introduce the first on-demand
routing approach that eliminates the main limitation of on-demand routing by
aggregating route requests (RREQ) for the same destinations. The approach can
be applied to any existing on-demand routing protocol, and we introduce the
Ad-hoc Demand-Aggregated Routing with Adaptation (ADARA) as an example of how
RREQ aggregation can be used. ADARA is compared to AODV and OLSR using
discrete-event simulations, and the results show that aggregating RREQs can
make on-demand routing more efficient than existing proactive or on-demand
routing protocols
Frictional drag between quantum wells mediated by fluctuating electromagnetic field
We use the theory of the fluctuating electromagnetic field to calculate the
frictional drag between nearby two-and three dimensional electron systems. The
frictional drag results from coupling via a fluctuating electromagnetic field,
and can be considered as the dissipative part of the van der Waals interaction.
In comparison with other similar calculations for semiconductor two-dimensional
system we include retardation effects. We consider the dependence of the
frictional drag force on the temperature , electron density and separation
. We find, that retardation effects become dominating factor for high
electron densities, corresponding thing metallic film, and suggest a new
experiment to test the theory. The relation between friction and heat transfer
is also briefly commented on.Comment: 14 pages, 4 figure
Phonon mediated drag in double layer two dimensional electron systems
Experiments studying phonon mediated drag in the double layer two dimensional
electron gas system are reported. Detailed measurements of the dependence of
drag on temperature, layer spacing, density ratio, and matched density are
discussed. Comparisons are made to theoretical results [M. C. Bonsager et al.,
Phys. Rev. B 57, 7085 (1998)] which propose the existence of a new coupled
electron-phonon collective mode. The layer spacing and density dependence at
matched densities for samples with layer spacings below 2600 A do not support
the existence of this mode, showing behavior expected for independent electron
and phonon systems. The magnitude of the drag, however, suggests the alternate
limit; one in which electrons and phonons are strongly coupled. The results for
still larger layer spacing show significant discrepancies with the behavior
expected for either limit.Comment: 9 pages, 9 figures, Late
Reflecting on loss in Papua New Guinea
This article takes up the conundrum of conducting anthropological fieldwork with people who claim that they have 'lost their culture,' as is the case with Suau people in the Massim region of Papua New Guinea. But rather than claiming culture loss as a process of dispossession, Suau claim it as a consequence of their own attempts to engage with colonial interests. Suau appear to have responded to missionization and their close proximity to the colonial-era capital by jettisoning many of the practices characteristic of Massim societies, now identified as 'kastom.' The rejection of kastom in order to facilitate their relations with Europeans during colonialism, followed by the mourning for kastom after independence, both invite consideration of a kind of reflexivity that requires action based on the presumed perspective of another
Frictional drag between quantum wells mediated by phonon exchange
We use the Kubo formalism to evaluate the contribution of acoustic phonon
exchange to the frictional drag between nearby two-dimensional electron
systems. In the case of free phonons, we find a divergent drag rate
(). However, becomes finite when phonon
scattering from either lattice imperfections or electronic excitations is
accounted for. In the case of GaAs quantum wells, we find that for a phonon
mean free path smaller than a critical value, imperfection
scattering dominates and the drag rate varies as over many
orders of magnitude of the layer separation . When exceeds the
critical value, the drag rate is dominated by coupling through an
electron-phonon collective mode localized in the vicinity of the electron
layers. We argue that the coupled electron-phonon mode may be observable for
realistic parameters. Our theory is in good agreement with experimental results
for the temperature, density, and -dependence of the drag rate.Comment: 45 pages, LaTeX, 8 postscript file figure
Spatio-temporal dynamics of quantum-well excitons
We investigate the lateral transport of excitons in ZnSe quantum wells by
using time-resolved micro-photoluminescence enhanced by the introduction of a
solid immersion lens. The spatial and temporal resolutions are 200 nm and 5 ps,
respectively. Strong deviation from classical diffusion is observed up to 400
ps. This feature is attributed to the hot-exciton effects, consistent with
previous experiments under cw excitation. The coupled transport-relaxation
process of hot excitons is modelled by Monte Carlo simulation. We prove that
two basic assumptions typically accepted in photoluminescence investigations on
excitonic transport, namely (i) the classical diffusion model as well as (ii)
the equivalence between the temporal and spatial evolution of the exciton
population and of the measured photoluminescence, are not valid for
low-temperature experiments.Comment: 8 pages, 6 figure
Hierarchically coupled ultradian oscillators generating robust circadian rhythms
Ensembles of mutually coupled ultradian cellular oscillators have been proposed by a number of authors to explain the generation of circadian rhythms in mammals. Most mathematical models using many coupled oscillators predict that the output period should vary as the square root of the number of participating units, thus being inconsistent with the well-established experimental result that ablation of substantial parts of the suprachiasmatic nuclei (SCN), the main circadian pacemaker in mammals, does not eliminate the overt circadian functions, which show no changes in the phases or periods of the rhythms. From these observations, we have developed a theoretical model that exhibits the robustness of the circadian clock to changes in the number of cells in the SCN, and that is readily adaptable to include the successful features of other known models of circadian regulation, such as the phase response curves and light resetting of the phase
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