817 research outputs found
Thermal noise and dephasing due to electron interactions in non-trivial geometries
We study Johnson-Nyquist noise in macroscopically inhomogeneous disordered
metals and give a microscopic derivation of the correlation function of the
scalar electric potentials in real space. Starting from the interacting
Hamiltonian for electrons in a metal and the random phase approximation, we
find a relation between the correlation function of the electric potentials and
the density fluctuations which is valid for arbitrary geometry and
dimensionality. We show that the potential fluctuations are proportional to the
solution of the diffusion equation, taken at zero frequency. As an example, we
consider networks of quasi-1D disordered wires and give an explicit expression
for the correlation function in a ring attached via arms to absorbing leads. We
use this result in order to develop a theory of dephasing by electronic noise
in multiply-connected systems.Comment: 9 pages, 6 figures (version submitted to PRB
Quantum dynamics of the avian compass
The ability of migratory birds to orient relative to the Earth's magnetic
field is believed to involve a coherent superposition of two spin states of a
radical electron pair. However, the mechanism by which this coherence can be
maintained in the face of strong interactions with the cellular environment has
remained unclear. This Letter addresses the problem of decoherence between two
electron spins due to hyperfine interaction with a bath of spin 1/2 nuclei.
Dynamics of the radical pair density matrix are derived and shown to yield a
simple mechanism for sensing magnetic field orientation. Rates of dephasing and
decoherence are calculated ab initio and found to yield millisecond coherence
times, consistent with behavioral experiments
Mechanical unfolding of RNA hairpins
Mechanical unfolding trajectories, generated by applying constant force in
optical tweezer experiments, show that RNA hairpins and the P5abc subdomain of
the group I intron unfold reversibly. We use coarse-grained Go-like models for
RNA hairpins to explore forced-unfolding over a broad range of temperatures. A
number of predictions that are amenable to experimental tests are made. At the
critical force the hairpin jumps between folded and unfolded conformations
without populating any discernible intermediates. The phase diagram in the
force-temperature (f,T) plane shows that the hairpin unfolds by an all-or-none
process. The cooperativity of the unfolding transition increases dramatically
at low temperatures. Free energy of stability, obtained from time averages of
mechanical unfolding trajectories, coincide with ensemble averages which
establishes ergodicity. The hopping time between the the native basin of
attraction (NBA) and the unfolded basin increases dramatically along the phase
boundary. Thermal unfolding is stochastic whereas mechanical unfolding occurs
in "quantized steps" with great variations in the step lengths. Refolding
times, upon force quench, from stretched states to the NBA is "at least an
order of magnitude" greater than folding times by temperature quench. Upon
force quench from stretched states the NBA is reached in at least three stages.
In the initial stages the mean end-to-end distance decreases nearly
continuously and only in the last stage there is a sudden transition to the
NBA. Because of the generality of the results we propose that similar behavior
should be observed in force quench refolding of proteins.Comment: 23 pages, 6 Figures. in press (Proc. Natl. Acad. Sci.
Exploring the influence of micro-structure on the mechanical properties and crack bridging mechanisms of fibrous tufts
AbstractA constitutive model for tufts bridging a mode I delamination is presented. The tuft is modelled as a rod, laterally supported by an elastic medium and clamped at both ends. A fracture mechanics approach is introduced to describe the progressive debonding of the tuft from the embedding laminate. The debonding model requires the identification of stiffness, strength and toughness properties, which depend both on the laminate/tuft architecture and the constituent materials. Such identification is carried out via experimental data obtained from tensile tests on single tufts inserted in a pre-delaminated non-crimp fabric composite. The experimental results are complemented by micro-scale finite element analysis. The mode I bridging law obtained from the constitutive model is implemented into a meso-scale cohesive zone formulation. This formulation is applied to predict the response to delamination of tufted Double Cantilever Beam (DCB) coupons. The cohesive zone approach is validated by means of experimental data from DCB tests. It is shown that the proposed micro- to meso-scale modelling approach yields results in good agreement with the experiments
DNA Methylation Mediates the Discriminatory Power of Associative Long-Term Memory in Honeybees
Memory is created by several interlinked processes in the brain, some of which require long-term gene regulation. Epigenetic mechanisms are likely candidates for regulating memory-related genes. Among these, DNA methylation is known to be a long lasting genomic mark and may be involved in the establishment of long-term memory. Here we demonstrate that DNA methyltransferases, which induce and maintain DNA methylation, are involved in a particular aspect of associative long-term memory formation in honeybees, but are not required for short-term memory formation. While long-term memory strength itself was not affected by blocking DNA methyltransferases, odor specificity of the memory (memory discriminatory power) was. Conversely, perceptual discriminatory power was normal. These results suggest that different genetic pathways are involved in mediating the strength and discriminatory power of associative odor memories and provide, to our knowledge, the first indication that DNA methyltransferases are involved in stimulus-specific associative long-term memory formation
Interpreting the Wide Scattering of Synchronized Traffic Data by Time Gap Statistics
Based on the statistical evaluation of experimental single-vehicle data, we
propose a quantitative interpretation of the erratic scattering of flow-density
data in synchronized traffic flows. A correlation analysis suggests that the
dynamical flow-density data are well compatible with the so-called jam line
characterizing fully developed traffic jams, if one takes into account the
variation of their propagation speed due to the large variation of the netto
time gaps (the inhomogeneity of traffic flow). The form of the time gap
distribution depends not only on the density, but also on the measurement cross
section: The most probable netto time gap in congested traffic flow upstream of
a bottleneck is significantly increased compared to uncongested freeway
sections. Moreover, we identify different power-law scaling laws for the
relative variance of netto time gaps as a function of the sampling size. While
the exponent is -1 in free traffic corresponding to statistically independent
time gaps, the exponent is about -2/3 in congested traffic flow because of
correlations between queued vehicles.Comment: For related publications see http://www.helbing.or
Estimating Acceleration and Lane-Changing Dynamics Based on NGSIM Trajectory Data
The NGSIM trajectory data sets provide longitudinal and lateral positional
information for all vehicles in certain spatiotemporal regions. Velocity and
acceleration information cannot be extracted directly since the noise in the
NGSIM positional information is greatly increased by the necessary numerical
differentiations. We propose a smoothing algorithm for positions, velocities
and accelerations that can also be applied near the boundaries. The smoothing
time interval is estimated based on velocity time series and the variance of
the processed acceleration time series. The velocity information obtained in
this way is then applied to calculate the density function of the
two-dimensional distribution of velocity and inverse distance, and the density
of the distribution corresponding to the ``microscopic'' fundamental diagram.
Furthermore, it is used to calculate the distributions of time gaps and
times-to-collision, conditioned to several ranges of velocities and velocity
differences. By simulating virtual stationary detectors we show that the
probability for critical values of the times-to-collision is greatly
underestimated when estimated from single-vehicle data of stationary detectors.
Finally, we investigate the lane-changing process and formulate a quantitative
criterion for the duration of lane changes that is based on the trajectory
density in normalized coordinates. Remarkably, there is a very noisy but
significant velocity advantage in favor of the targeted lane that decreases
immediately before the change due to anticipatory accelerations
Regulated dicing of pre-mir-144 via reshaping of its terminal loop.
Although the route to generate microRNAs (miRNAs) is often depicted as a linear series of sequential and constitutive cleavages, we now appreciate multiple alternative pathways as well as diverse strategies to modulate their processing and function. Here, we identify an unusually profound regulatory role of conserved loop sequences in vertebrate pre-mir-144, which are essential for its cleavage by the Dicer RNase III enzyme in human and zebrafish models. Our data indicate that pre-mir-144 dicing is positively regulated via its terminal loop, and involves the ILF3 complex (NF90 and its partner NF45/ILF2). We provide further evidence that this regulatory switch involves reshaping of the pre-mir-144 apical loop into a structure that is appropriate for Dicer cleavage. In light of our recent findings that mir-144 promotes the nuclear biogenesis of its neighbor mir-451, these data extend the complex hierarchy of nuclear and cytoplasmic regulatory events that can control the maturation of clustered miRNAs
Towards a Macroscopic Modelling of the Complexity in Traffic Flow
We present a macroscopic traffic flow model that extends existing fluid-like
models by an additional term containing the second derivative of the safe
velocity. Two qualitatively different shapes of the safe velocity are explored:
a conventional Fermi-type function and a function exhibiting a plateau at
intermediate densities. The suggested model shows an extremely rich dynamical
behaviour and shows many features found in real-world traffic data.Comment: submitted to Phys. Rev.
Direct observation of twist mode in electroconvection in I52
I report on the direct observation of a uniform twist mode of the director
field in electroconvection in I52. Recent theoretical work suggests that such a
uniform twist mode of the director field is responsible for a number of
secondary bifurcations in both electroconvection and thermal convection in
nematics. I show here evidence that the proposed mechanisms are consistent with
being the source of the previously reported SO2 state of electroconvection in
I52. The same mechanisms also contribute to a tertiary Hopf bifurcation that I
observe in electroconvection in I52. There are quantitative differences between
the experiment and calculations that only include the twist mode. These
differences suggest that a complete description must include effects described
by the weak-electrolyte model of electroconvection
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