17,153 research outputs found
Holographic Operator Mixing and Quasinormal Modes on the Brane
We provide a framework for calculating holographic Green's functions from
general bilinear actions and fields obeying coupled differential equations in
the bulk. The matrix-valued spectral function is shown to be independent of the
radial bulk coordinate. Applying this framework we improve the analysis of
fluctuations in the D3/D7 system at finite baryon density, where the
longitudinal perturbations of the world-volume gauge field couple to the scalar
fluctuations of the brane embedding. We compute the spectral function and show
how its properties are related to the quasinormal mode spectrum. We study the
crossover from the hydrodynamic diffusive to the reactive regime and the
movement of quasinormal modes as functions of temperature and density. We also
compute their dispersion relations and find that they asymptote to the
lightcone for large momenta.Comment: 42 pages, 12 figure
Gender differences and the timing of first marriages
In this article we provide a simple model of the marriage market where singles search for
spouses. In our model economy men and women live for many years and they differ in their
survival probabilities, in their fecundity, and in their earnings. We show that modelling the
marriage decision in a very simple model economy is sufficient to account for much of the
observed marriage behavior in the United States in the year 2000. We conclude that gender
differences in fecundity are all important in accounting for marriage behavior, and that
differences in earnings matter little. We also conclude that, even though they are in short supply,
the market power of fecund women is not enough for them to demand compensation in all
cases. And that studying the marriage decision without modelling explicitly the roles played by
age and by fecundity, as has been typically done by the previous literature, makes little sense
Environmental conditions during early life accelerate the rate of senescence in a short-lived passerine bird
Environmental conditions experienced in early life may shape subsequent phenotypic traits including life history. We investigated how predation risk caused by domestic cats (Felis silvestris catus) and local breeding density affected patterns of reproductive and survival senescence in Barn Swallows (Hirundo rustica) breeding semicolonially in Denmark. We recorded the abundance of cats and the number of breeding pairs at 39 breeding sites during 24 years and related these to age-specific survival rate and reproductive senescence to test predictions of the life history theory of senescence. We found evidence for actuarial senescence for the first time in this species. Survival rate increased until reaching a plateau in midlife and then decreased later. We also found that survival rate was higher for males than females. Local breeding density or predation risk did not affect survival as predicted by theory. Barn Swallows with short lives did not invest more in reproduction in early life, inconsistent with expectations for trade-offs between reproduction and survival as theory suggests. However, we found that the rate of reproductive decline during senescence was steeper for individuals exposed to intense competition, and predation pressure accelerated the rate of reproductive senescence, but only in sites with many breeding pairs. These latter results are in accordance with one of the predictions suggested by the life history theory of aging. These results emphasize the importance of considering intraspecific competition and interspecific interactions such as predation when analyzing reproductive and actuarial senescence
Some results on rational surfaces and Fano varieties
The goal of this article is to study the equations and syzygies of embeddings
of rational surfaces and certain Fano varieties. Given a rational surface X and
an ample and base-point-free line bundle L on X, we give an optimal numerical
criterion for L to satisfy property Np. This criterion turns out to be a
characterization of property Np if X is anticanonical. We also prove syzygy
results for adjunction bundles and a Reider type theorem for higher syzygies.
For certain Fano varieties we also prove results on very ampleness and higher
syzygies.Comment: 26 pages, AMSTe
Modeling Space-Charge Limited Currents in Organic Semiconductors: Extracting Trap Density and Mobility
We have developed and applied a mobility edge model that takes into account
drift and diffusion currents to characterize the space charge limited current
in organic semiconductors. The numerical solution of the drift-diffusion
equation allows the utilization of asymmetric contacts to describe the built-in
potential within the device. The model has been applied to extract information
of the distribution of traps from experimental current-voltage measurements of
a rubrene single crystal from Krellner et al. [Phys. Rev. B, 75(24), 245115]
showing excellent agreement across several orders of magnitude of current.
Although the two contacts are made of the same metal, an energy offset of 580
meV between them, ascribed to differences in the deposition techniques
(lamination vs. evaporation) was essential to correctly interpret the shape of
the current-voltage characteristics at low voltage. A band mobility 0.13 cm2/Vs
for holes was estimated, which is consistent with transport along the long axis
of the orthorhombic unit cell. The total density of traps deeper than 0.1 eV
was 2.2\times1016 cm-3. The sensitivity analysis and error estimation in the
obtained parameters shows that it is not possible to accurately resolve the
shape of the trap distribution for energies deeper than 0.3 eV or shallower
than 0.1 eV above the valence band edge. The total number of traps deeper than
0.3 eV however can be estimated. Contact asymmetry and the diffusion component
of the current play an important role in the description of the device at low
bias, and are required to obtain reliable information about the distribution of
deep traps
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