5,579 research outputs found
Nuclear G-Matrix Elements from Nonlocal Potentials
We study effects of nonlocality in the nuclear force on the G-matrix elements
for finite nuclei. Nuclear G-matrix elements for \O16 are calculated in the
harmonic oscillator basis from a nonlocal potential which models quark exchange
effects between two nucleons. We employ a simple form of potential that gives
the same phase shifts as a realistic local nucleon potential. The G-matrix
elements calculated from the nonlocal potential show moderate increase in
repulsion from those derived from the local potential.Comment: 11 page, LaTeX, 2 PS figures, uses epsf.st
A unified approach to linking experimental, statistical and computational analysis of spike train data
A fundamental issue in neuroscience is how to identify the multiple biophysical mechanisms through which neurons generate observed patterns of spiking activity. In previous work, we proposed a method for linking observed patterns of spiking activity to specific biophysical mechanisms based on a state space modeling framework and a sequential Monte Carlo, or particle filter, estimation algorithm. We have shown, in simulation, that this approach is able to identify a space of simple biophysical models that were consistent with observed spiking data (and included the model that generated the data), but have yet to demonstrate the application of the method to identify realistic currents from real spike train data. Here, we apply the particle filter to spiking data recorded from rat layer V cortical neurons, and correctly identify the dynamics of an slow, intrinsic current. The underlying intrinsic current is successfully identified in four distinct neurons, even though the cells exhibit two distinct classes of spiking activity: regular spiking and bursting. This approach – linking statistical, computational, and experimental neuroscience – provides an effective technique to constrain detailed biophysical models to specific mechanisms consistent with observed spike train data.Published versio
New Recursion Relations and a Flat Space Limit for AdS/CFT Correlators
We consider correlation functions of the stress-tensor or a conserved current
in AdS_{d+1}/CFT_d computed using the Hilbert or the Yang-Mills action in the
bulk. We introduce new recursion relations to compute these correlators at tree
level. These relations have an advantage over the BCFW-like relations described
in arXiv:1102.4724 and arXiv:1011.0780 because they can be used in all
dimensions including d=3. We also introduce a new method of extracting
flat-space S-matrix elements from AdS/CFT correlators in momentum space. We
show that the (d+1)-dimensional flat-space amplitude of gravitons or gluons can
be obtained as the coefficient of a particular singularity of the d-dimensional
correlator of the stress-tensor or a conserved current; this technique is valid
even at loop-level in the bulk. Finally, we show that our recursion relations
automatically generate correlators that are consistent with this observation:
they have the expected singularity and the flat-space gluon or graviton
amplitude appears as its coefficient.Comment: 22+6 pages (v2) typos fixe
Nonlinear field theories during homogeneous spatial dilation
The effect of a uniform dilation of space on stochastically driven nonlinear
field theories is examined. This theoretical question serves as a model problem
for examining the properties of nonlinear field theories embedded in expanding
Euclidean Friedmann-Lema\^{\i}tre-Robertson-Walker metrics in the context of
cosmology, as well as different systems in the disciplines of statistical
mechanics and condensed matter physics. Field theories are characterized by the
speed at which they propagate correlations within themselves. We show that for
linear field theories correlations stop propagating if and only if the speed at
which the space dilates is higher than the speed at which correlations
propagate. The situation is in general different for nonlinear field theories.
In this case correlations might stop propagating even if the velocity at which
space dilates is lower than the velocity at which correlations propagate. In
particular, these results imply that it is not possible to characterize the
dynamics of a nonlinear field theory during homogeneous spatial dilation {\it a
priori}. We illustrate our findings with the nonlinear Kardar-Parisi-Zhang
equation
Experiments to investigate particulate materials in reduced gravity fields
Study investigates agglomeration and macroscopic behavior in reduced gravity fields of particles of known properties by measuring and correlating thermal and acoustical properties of particulate materials. Experiment evaluations provide a basis for a particle behavior theory and measure bulk properties of particulate materials in reduced gravity
Wrapping interactions at strong coupling -- the giant magnon
We derive generalized Luscher formulas for finite size corrections in a
theory with a general dispersion relation. For the AdS_5xS^5 superstring these
formulas encode leading wrapping interaction effects. We apply the generalized
mu-term formula to calculate finite size corrections to the dispersion relation
of the giant magnon at strong coupling. The result exactly agrees with the
classical string computation of Arutyunov, Frolov and Zamaklar. The agreement
involved a Borel resummation of all even loop-orders of the BES/BHL dressing
factor thus providing a strong consistency check for the choice of the dressing
factor.Comment: 35 pages, 2 figures; v2: comments and references adde
Growth instability due to lattice-induced topological currents in limited mobility epitaxial growth models
The energetically driven Ehrlich-Schwoebel (ES) barrier had been generally
accepted as the primary cause of the growth instability in the form of
quasi-regular mound-like structures observed on the surface of thin film grown
via molecular beam epitaxy (MBE) technique. Recently the second mechanism of
mound formation was proposed in terms of a topologically induced flux of
particles originating from the line tension of the step edges which form the
contour lines around a mound. Through large-scale simulations of MBE growth on
a variety of crystalline lattice planes using limited mobility, solid-on-solid
models introduced by Wolf-Villain and Das Sarma-Tamborenea in 2+1 dimensions,
we propose yet another type of topological uphill particle current which is
unique to some lattice, and has hitherto been overlooked in the literature.
Without ES barrier, our simulations produce spectacular mounds very similar, in
some cases, to what have been observed in many recent MBE experiments. On a
lattice where these currents cease to exist, the surface appears to be
scale-invariant, statistically rough as predicted by the conventional continuum
growth equation.Comment: 10 pages, 12 figure
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