1,567 research outputs found
Electrostatic interactions between discrete helices of charge
We analytically examine the pair interaction for parallel, discrete helices
of charge. Symmetry arguments allow for the energy to be decomposed into a sum
of terms, each of which has an intuitive geometric interpretation. Truncated
Fourier expansions for these terms allow for accurate modeling of both the
axial and azimuthal terms in the interaction energy and these expressions are
shown to be insensitive to the form of the interaction. The energy is evaluated
numerically through application of an Ewald-like summation technique for the
particular case of unscreened Coulomb interactions between the charges of the
two helices. The mode structures and electrostatic energies of flexible helices
are also studied. Consequences of the resulting energy expressions are
considered for both F-actin and A-DNA aggregates
Disorder induced transition into a one-dimensional Wigner glass
The destruction of quasi-long range crystalline order as a consequence of
strong disorder effects is shown to accompany the strict localization of all
classical plasma modes of one-dimensional Wigner crystals at T=0. We construct
a phase diagram that relates the structural phase properties of Wigner crystals
to a plasmon delocalization transition recently reported. Deep inside the
strictly localized phase of the strong disorder regime, we observe
``glass-like'' behavior. However, well into the critical phase with a plasmon
mobility edge, the system retains its crystalline composition. We predict that
a transition between the two phases occurs at a critical value of the relative
disorder strength. This transition has an experimental signature in the AC
conductivity as a local maximum of the largest spectral amplitude as a function
of the relative disorder strength.Comment: 5 pages, revtex. Typo regarding localization length exponent
corrected. Should read 1 / \delt
Spatial connectedness imposes local‐ and metapopulation‐level selection on life history through feedbacks on demography
Dispersal evolution impacts the fluxes of individuals and hence, connectivity in metapopulations. Connectivity is therefore decoupled from the structural connectedness of the patches within the spatial network. Because of demographic feedbacks, local selection also drives the evolution of other life history traits. We investigated how different levels of connectedness affect trait evolution in experimental metapopulations of the two-spotted spider mite. We separated local- and metapopulation-level selection and linked trait divergence to population dynamics. With lower connectedness, an increased starvation resistance and delayed dispersal evolved. Reproductive performance evolved locally by transgenerational plasticity or epigenetic processes. Costs of dispersal, but also changes in local densities and temporal fluctuations herein are found to be putative drivers. In addition to dispersal, demographic traits are able to evolve in response to metapopulation connectedness at both the local and metapopulation level by genetic and/or non-genetic inheritance. These trait changes impact the persistence of spatially structured populations
Anderson transition of the plasma oscillations of 1D disordered Wigner lattices
We report the existence of a localization-delocalization transition in the
classical plasma modes of a one dimensional Wigner Crystal with a white noise
potential environment at T=0. Finite size scaling analysis reveals a divergence
of the localization length at a critical eigenfrequency. Further scaling
analysis indicates power law behavior of the critical frequency in terms of the
relative interaction strength of the charges. A heuristic argument for this
scaling behavior is consistent with the numerical results. Additionally, we
explore a particular realization of random-bond disorder in a one dimensional
Wigner lattice in which all of the collective modes are observed to be
localized.Comment: 4 pages, 3 figures, Typo for the localization length corrected.
Should read 1 / \n
Extragalactic Radio Sources and the WMAP Cold Spot
We detect a dip of 20-45% in the surface brightness and number counts of NVSS
sources smoothed to a few degrees at the location of the WMAP cold spot. The
dip has structure on scales of approximately 1-10 degrees. Together with
independent all-sky wavelet analyses, our results suggest that the dip in
extragalactic brightness and number counts and the WMAP cold spot are
physically related, i.e., that the coincidence is neither a statistical anomaly
nor a WMAP foreground correction problem. If the cold spot does originate from
structures at modest redshifts, as we suggest, then there is no remaining need
for non-Gaussian processes at the last scattering surface of the CMB to explain
the cold spot. The late integrated Sachs-Wolfe effect, already seen
statistically for NVSS source counts, can now be seen to operate on a single
region. To create the magnitude and angular size of the WMAP cold spot requires
a ~140 Mpc radius completely empty void at z<=1 along this line of sight. This
is far outside the current expectations of the concordance cosmology, and adds
to the anomalies seen in the CMB.Comment: revised version, ApJ, in pres
What drives the translocation of stiff chains?
We study the dynamics of the passage of a stiff chain through a pore into a
cell containing particles that bind reversibly to it. Using Brownian Molecular
Dynamics simulations we investigate the mean-first-passage time as a function
of the length of the chain inside, for different concentrations of binding
particles. As a consequence of the interactions with these particles, the chain
experiences a net force along its length whose calculated value from the
simulations accounts for the velocity at which it enters the cell. This force
can in turn be obtained from the solution of a generalized diffusion equation
incorporating an effective Langmuir adsorption free energy for the chain plus
binding particles. These results suggest a role of binding particles in the
translocation process which is in general quite different from that of a
Brownian ratchet. Furthermore, non-equilibrium effects contribute significantly
to the dynamics, \emph{e.g.}, the chain often enters the cell faster than
particle binding can be saturated, resulting in a force several times smaller
than the equilibrium value.Comment: 7 pages, 4 figure
Exact longitudinal plasmon dispersion relations for one and two dimensional Wigner crystals
We derive the exact longitudinal plasmon dispersion relations, of
classical one and two dimensional Wigner crystals at T=0 from the real space
equations of motion, of which properly accounts for the full unscreened Coulomb
interactions. We make use of the polylogarithm function in order to evaluate
the infinite lattice sums of the electrostatic force constants. From our exact
results we recover the correct long-wavelength behavior of previous approximate
methods. In 1D, , validating the known
RPA and bosonization form. In 2D , agreeing remarkably
with the celebrated Ewald summation result. Additionally, we extend this
analysis to calculate the band structure of tight-binding models of
non-interacting electrons with arbitrary power law hopping.Comment: 4 pages, 1 figure. Important typos and errors fixed, 2D dispersion
adde
A Comparison of X-ray and Radio Emission from the Supernova Remnant Cassiopeia A
We compare the radio and soft X-ray brightness as a function of position
within the young supernova remnant Cassiopeia A. A moderately strong
correlation (r = 0.7) was found between the X-ray emission (corrected for
interstellar absorption) and radio emission, showing that the thermal and
relativistic plasmas occupy the same volumes and are regulated by common
underlying parameters. The logarithmic slope of the relationship, ln(Sx-ray) =
1.2 x Sradio + ln(k) implies that the variations in brightness are primarily
due to path length differences. The X-ray and radio emissivities are both high
in the same general locations, but their more detailed relationship is poorly
constrained and probably shows significant scatter. The strongest radio and
X-ray absorption is found at the western boundary of Cas A. Based on the
properties of Cas A and the absorbing molecular cloud, we argue that they are
physically interacting. We also compare ASCA derived column densities with 21
cm H I and 18 cm OH optical depths in the direction of Cas A, in order to
provide an independent estimate of ISM properties. We derive an average value
for the H I spin temperature of about 40 K and measure the ratio of OH to
molecular hydrogen to be nominally larger than previous estimates.
Keywords: Cas A, Cassiopeia A, interstellar medium, molecular clouds, radio
astronomy, supernova remnants, X-ray astronomyComment: To appear in Vol. 446 of The Astrophysical Journal on Aug. 1, 1996;
10 pages with 5 embedded figures; replaced because of updated reference
Potential vorticity structure in the North Atlantic western boundary current from underwater glider observations
Author Posting. © American Meteorological Society, 2016. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 46 (2016): 327–348, doi:10.1175/JPO-D-15-0112.1.Potential vorticity structure in two segments of the North Atlantic’s western boundary current is examined using concurrent, high-resolution measurements of hydrography and velocity from gliders. Spray gliders occupied 40 transects across the Loop Current in the Gulf of Mexico and 11 transects across the Gulf Stream downstream of Cape Hatteras. Cross-stream distributions of the Ertel potential vorticity and its components are calculated for each transect under the assumptions that all flow is in the direction of measured vertically averaged currents and that the flow is geostrophic. Mean cross-stream distributions of hydrographic properties, potential vorticity, and alongstream velocity are calculated for both the Loop Current and the detached Gulf Stream in both depth and density coordinates. Differences between these mean transects highlight the downstream changes in western boundary current structure. As the current increases its transport downstream, upper-layer potential vorticity is generally reduced because of the combined effects of increased anticyclonic relative vorticity, reduced stratification, and increased cross-stream density gradients. The only exception is within the 20-km-wide cyclonic flank of the Gulf Stream, where intense cyclonic relative vorticity results in more positive potential vorticity than in the Loop Current. Cross-stream gradients of mean potential vorticity satisfy necessary conditions for both barotropic and baroclinic instability within the western boundary current. Instances of very low or negative potential vorticity, which predispose the flow to various overturning instabilities, are observed in individual transects across both the Loop Current and the Gulf Stream.Glider operations in the Gulf Stream were supported by the National Science Foundation under Grant OCE-0220769. Glider operations in the Gulf of Mexico were supported by BP. R.E.T. was supported by the Penzance Endowed Fund in Support of Assistant Scientists and the Independent Research and Development Program at WHOI.2016-07-0
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