2,306 research outputs found
Long-Tailed Trapping Times and Levy Flights in a Self-Organized Critical Granular System
We present a continuous time random walk model for the scale-invariant
transport found in a self-organized critical rice pile [Christensen et al.,
Phys. Rev. Lett. 77, 107 (1996)]. From our analytical results it is shown that
the dynamics of the experiment can be explained in terms of L\'evy flights for
the grains and a long-tailed distribution of trapping times. Scaling relations
for the exponents of these distributions are obtained. The predicted
microscopic behavior is confirmed by means of a cellular automaton model.Comment: 4 pages, RevTex, includes 3 PostScript figures, submitted to Phys.
Rev. Let
Transport Properties of Highly Aligned Polymer Light-Emitting-Diodes
We investigate hole transport in polymer light-emitting-diodes in which the
emissive layer is made of liquid-crystalline polymer chains aligned
perpendicular to the direction of transport. Calculations of the current as a
function of time via a random-walk model show excellent qualitative agreement
with experiments conducted on electroluminescent polyfluorene demonstrating
non-dispersive hole transport. The current exhibits a constant plateau as the
charge carriers move with a time-independent drift velocity, followed by a long
tail when they reach the collecting electrode. Variation of the parameters
within the model allows the investigation of the transition from non-dispersive
to dispersive transport in highly aligned polymers. It turns out that large
inter-chain hopping is required for non-dispersive hole transport and that
structural disorder obstructs the propagation of holes through the polymer
film.Comment: 4 pages, 5 figure
Onset and Role of the Antarctic Circumpolar Current.
The major role for the ACC, as inferred in the past, is to have caused or stabilised full
Antarctic glaciation. This role has since been questioned, and other hypothesised roles are
relatively minor. Using a “smoking gun” assumption, determination of the time of onset of an
ACC will resolve uncertainties in its role, and constrain the importance of ocean circulation to
climate. To this end, we summarise all published estimates of ACC onset. The time of onset, of
shallow circulation or deep, is extremely uncertain, whether based on tectonic studies or the
interpretation of changes in the sediment record. Two potential final barriers to circumpolar
flow have been identified; south of Tasmania and south of South America. The former is wellconstrained
by tectonics and marine geology to before 32 Ma for a deep gap, with a shallow
gap in place by 35.5 Ma at the latest. These ages fit nicely with the onset of full Antarctic
glaciation at 33-34 Ma, although some workers deny the causality. Estimates of the time of
opening of the latter range very widely, whether based on tectonics or sedimentary geology,
from as recently as 6 Ma to as early as 41 Ma, with the gap depth uncertain also. Resolution of
the tectonics-based uncertainties by additional survey being most probably both timeconsuming
and inconclusive, and the geological estimates being open to alternative
interpretations, we define an optimal strategy for additional sampling and measurement,
designed to resolve the time of onset much more certainly, possibly also resolving between
deep and shallow opening, and thereby constraining the ACC role. Sample sites would have to
be close to likely final barriers, to avoid extraneous influence, and within modern zones of
ACC influence, ideally would form a depth transect, and would have continuous, mixed
terrigenous and biogenic sections. A wide range of carefully-selected parameters would be
measured at each
Multiscale Analysis of the Stress State in a Granular Slope in Transition to Failure
By means of contact dynamics simulations, we analyze the stress state in a
granular bed slowly tilted towards its angle of repose. An increasingly large
number of grains are overloaded in the sense that they are found to carry a
stress ratio above the Coulomb yield threshold of the whole packing. Using this
property, we introduce a coarse-graining length scale at which all stress
ratios are below the packing yield threshold. We show that this length
increases with the slope angle and jumps to a length comparable to the depth of
the granular bed at an angle below the angle of repose. This transition
coincides with the onset of dilatation in the packing. We map this transition
into a percolation transition of the overloaded grains, and we argue that in
the presence of long-range correlations above the transition angle, the
granular slope is metastable.Comment: 11 pages, 14 Fig, submitted to PR
Boundary Effects on the Determination of Metamaterial Parameters from Normal Incidence Reflection and Transmission Measurements
A method is described for the determination of the effective electromagnetic
parameters of a metamaterial based only on external measurements or
simulations, taking boundary effects at the interfaces between a conventional
material and metamaterial into account. Plane-wave reflection and transmission
coefficients at the interfaces are regarded as additional unknowns to be
determined, rather than explicitly dependent on the material parameters. Our
technique is thus analogous to the line-reflect-line (LRL) calibration method
in microwave measurements. The refractive index can be determined from
S-parameters for two samples of different thickness. The effective wave
impedance requires the additional assumption that generalized sheet transition
conditions (GSTCs) account for the boundary effects. Expressions for the bulk
permittivity and permeability then follow easily. Our method is validated by
comparison with the results using the Nicolson-Ross-Weir (NRW) for determining
properties of an ordinary material measured in a coaxial line. Utilizing
S-parameters obtained from 3-D full wave simulations, we test the method on
magnetodielectric metamaterials. We compare the results from our method and the
conventional one that does not consider boundary effects. Moreover, it is shown
that results from our method are consistent under changes in reference plane
location, whereas the results from other methods are not.Comment: 16 pages, 16 figures. Submitted to IEEE Transactions on Antennas and
Propagatio
Implications of reflectance measurements on the mechanism for superconductivity in MgB
Recent optical studies in c-axis oriented superconducting MgB films
indicate that the electron-phonon coupling is weak [tu01]. We reinforce this
conclusion by examining the raw reflectance data; its frequency dependence is
incompatible with strong electron-phonon scattering. This is further
strengthened by analysis of the real part of the conductivity, and by the
temperature dependence of the effective Drude scattering rate. Using a
realistic electron-phonon spectral shape [kong01], we find , in agreement with Tu et al. [tu01]. To the extent that
, this disagrees sharply with model
calculations [kong01,kortus01,an01], and is far too low to provide the means
for K. A simple model is constructed with coupling to a high
frequency excitation, which is consistent with both the low frequency optical
data and the high .Comment: 4 pages, 4 figure
Intra- and interspecific polymorphisms ofLeishmania donovani andL. tropica minicircle DNA
A pair of degenerate polymerase chain reaction (PCR) primers (LEI-1, TCG GAT CC[C,T] [G,C]TG GGT AGG GGC GT; LEI-2, ACG GAT CC[G,C] [G,C][A,C]C TAT [A,T]TT ACA CC) defining a 0.15-kb segment ofLeishmania minicircle DNA was constructed. These primers amplified not only inter- but also intraspecifically polymorphic sequences. Individual sequences revealed a higher intraspecific than interspecific divergence. It is concluded that individual sequences are of limited relevance for species determination. In contrast, when a data base of 19 different sequences was analyzed in a dendrographic plot, an accurate species differentiation was feasible
Electrical transport and percolation in magnetoresistive manganite / insulating oxide composites: case of La0.7Ca0.3MnO3 / Mn3O4
We report the results of electrical resistivity measurements carried out on
well-sintered La0.7Ca0.3MnO3 / Mn3O4 composite samples with almost constant
composition of the magnetoresistive manganite phase (La0.7Ca0.3MnO3). A
percolation threshold (fc) occurs when the La0.7Ca0.3MnO3 volume fraction is ~
0.19. The dependence of the electrical resistivity as a function of
La0.7Ca0.3MnO3 volume fraction (fLCMO) can be described by percolation-like
phenomenological equations. Fitting the conducting regime (fLCMO > fc) by the
percolation power law returns a critical exponent t value of 2.0 +/- 0.2 at
room temperature and 2.6 +/-0.2 at 5 K. The increase of t is ascribed to the
influence of the grain boundaries on the electrical conduction process at low
temperature.Comment: 7 pages, 3 figures, accepted for publication in Phys. Rev.
Temperature and Field Dependence of the Mobility in Liquid-Crystalline Conjugated Polymer Films
The transport properties of organic light-emitting diodes in which the
emissive layer is composed of conjugated polymers in the liquid-crystalline
phase have been investigated. We have performed simulations of the current
transient response to an illumination pulse via the Monte Carlo approach, and
from the transit times we have extracted the mobility of the charge carriers as
a function of both the electric field and the temperature. The transport
properties of such films are different from their disordered counterparts, with
charge carrier mobilities exhibiting only a weak dependence on both the
electric field and temperature. We show that for spatially ordered polymer
films, this weak dependence arises for thermal energy being comparable to the
energetic disorder, due to the combined effect of the electrostatic and thermal
energies. The inclusion of spatial disorder, on the other hand, does not alter
the qualitative behaviour of the mobility, but results in decreasing its
absolute value.Comment: 9 pages, 8 figures, submitted to Phys. Rev.
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