1,222 research outputs found
Modified conjugated gradient method for diagonalising large matrices
We present an iterative method to diagonalise large matrices. The basic idea
is the same as the conjugated gradient (CG) method, i.e, minimizing the
Rayleigh quotient via its gradient and avoiding reintroduce errors to the
directions of previous gradients. Each iteration step is to find lowest
eigenvector of the matrix in a subspace spanned by the current trial vector and
the corresponding gradient of the Rayleigh quotient, as well as some previous
trial vectors. The gradient, together with the previous trail vectors, play a
similar role of the conjugated gradient of the original CG algorithm. Our
numeric tests indicate that this method converges significantly faster than the
original CG method. And the computational cost of one iteration step is about
the same as the original CG method. It is suitably for first principle
calculations.Comment: 6 Pages, 2EPS figures. (To appear in Phys. Rev. E
Analysis by x-ray microtomography of a granular packing undergoing compaction
Several acquisitions of X-ray microtomography have been performed on a beads
packing while it compacts under vertical vibrations. An image analysis allows
to study the evolution of the packing structure during its progressive
densification. In particular, the volume distribution of the pores reveals a
large tail, compatible to an exponential law, which slowly reduces as the
system gets more compact. This is quite consistent, for large pores, with the
free volume theory. These results are also in very good agreement with those
obtained by a previous numerical model of granular compaction.Comment: 4 pages, 4 figures. Latex (revtex4). to be published in Phys. Rev.
Reactive dynamics of inertial particles in nonhyperbolic chaotic flows
Anomalous kinetics of infective (e.g., autocatalytic) reactions in open,
nonhyperbolic chaotic flows are important for many applications in biological,
chemical, and environmental sciences. We present a scaling theory for the
singular enhancement of the production caused by the universal, underlying
fractal patterns. The key dynamical invariant quantities are the effective
fractal dimension and effective escape rate, which are primarily determined by
the hyperbolic components of the underlying dynamical invariant sets. The
theory is general as it includes all previously studied hyperbolic reactive
dynamics as a special case. We introduce a class of dissipative embedding maps
for numerical verification.Comment: Revtex, 5 pages, 2 gif figure
Glassy dynamics in granular compaction: sand on random graphs
We discuss the use of a ferromagnetic spin model on a random graph to model
granular compaction. A multi-spin interaction is used to capture the
competition between local and global satisfaction of constraints characteristic
for geometric frustration. We define an athermal dynamics designed to model
repeated taps of a given strength. Amplitude cycling and the effect of
permanently constraining a subset of the spins at a given amplitude is
discussed. Finally we check the validity of Edwards' hypothesis for the
athermal tapping dynamics.Comment: 13 pages Revtex, minor changes, to appear in PR
On the study of jamming percolation
We investigate kinetically constrained models of glassy transitions, and
determine which model characteristics are crucial in allowing a rigorous proof
that such models have discontinuous transitions with faster than power law
diverging length and time scales. The models we investigate have constraints
similar to that of the knights model, introduced by Toninelli, Biroli, and
Fisher (TBF), but differing neighbor relations. We find that such knights-like
models, otherwise known as models of jamming percolation, need a ``No Parallel
Crossing'' rule for the TBF proof of a glassy transition to be valid.
Furthermore, most knight-like models fail a ``No Perpendicular Crossing''
requirement, and thus need modification to be made rigorous. We also show how
the ``No Parallel Crossing'' requirement can be used to evaluate the provable
glassiness of other correlated percolation models, by looking at models with
more stable directions than the knights model. Finally, we show that the TBF
proof does not generalize in any straightforward fashion for three-dimensional
versions of the knights-like models.Comment: 13 pages, 18 figures; Spiral model does satisfy property
The Grad-Shafranov Reconstruction of Toroidal Magnetic Flux Ropes: Method Development and Benchmark Studies
We develop an approach of Grad-Shafranov (GS) reconstruction for toroidal
structures in space plasmas, based on in-situ spacecraft measurements. The
underlying theory is the GS equation that describes two-dimensional
magnetohydrostatic equilibrium as widely applied in fusion plasmas. The
geometry is such that the arbitrary cross section of the torus has rotational
symmetry about the rotation axis , with a major radius . The magnetic
field configuration is thus determined by a scalar flux function and a
functional that is a single-variable function of . The algorithm is
implemented through a two-step approach: i) a trial-and-error process by
minimizing the residue of the functional to determine an optimal
axis orientation, and ii) for the chosen , a minimization process
resulting in the range of . Benchmark studies of known analytic solutions
to the toroidal GS equation with noise additions are presented to illustrate
the two-step procedures and to demonstrate the performance of the numerical GS
solver, separately. For the cases presented, the errors in and are
9 and 22\%, respectively, and the relative percent error in the
numerical GS solutions is less than 10\%. We also make public the computer
codes for these implementations and benchmark studies.Comment: submitted to Sol. Phys. late Dec 2016; under review; code will be
made public once review is ove
Deterministically Driven Avalanche Models of Solar Flares
We develop and discuss the properties of a new class of lattice-based
avalanche models of solar flares. These models are readily amenable to a
relatively unambiguous physical interpretation in terms of slow twisting of a
coronal loop. They share similarities with other avalanche models, such as the
classical stick--slip self-organized critical model of earthquakes, in that
they are driven globally by a fully deterministic energy loading process. The
model design leads to a systematic deficit of small scale avalanches. In some
portions of model space, mid-size and large avalanching behavior is scale-free,
being characterized by event size distributions that have the form of
power-laws with index values, which, in some parameter regimes, compare
favorably to those inferred from solar EUV and X-ray flare data. For models
using conservative or near-conservative redistribution rules, a population of
large, quasiperiodic avalanches can also appear. Although without direct
counterparts in the observational global statistics of flare energy release,
this latter behavior may be relevant to recurrent flaring in individual coronal
loops. This class of models could provide a basis for the prediction of large
solar flares.Comment: 24 pages, 11 figures, 2 tables, accepted for publication in Solar
Physic
A new photon recoil experiment: towards a determination of the fine structure constant
We report on progress towards a measurement of the fine structure constant to
an accuracy of or better by measuring the ratio of the
Planck constant to the mass of the cesium atom. Compared to similar
experiments, ours is improved in three significant ways: (i) simultaneous
conjugate interferometers, (ii) multi-photon Bragg diffraction between same
internal states, and (iii) an about 1000 fold reduction of laser phase noise to
-138 dBc/Hz. Combining that with a new method to simultaneously stabilize the
phases of four frequencies, we achieve 0.2 mrad effective phase noise at the
location of the atoms. In addition, we use active stabilization to suppress
systematic effects due to beam misalignment.Comment: 12 pages, 9 figure
Axially and radially expandable modular helical soft actuator for robotic implantables
Soft robotics has advanced the field of biomedical engineering by creating safer technologies for interfacing with the human body. One of the challenges in this field is the realization of modular soft basic constituents and accessible assembly methods to increase the versatility of soft robots. We present a soft pneumatic actuator composed of two elastomeric strands that provide interdependent axial and radial expansion due to the modularity of the components and their helical arrangement. The actuator reaches 35% of elongation with respect to its initial height and both chambers achieve forces of 1N at about 19kPa. We describe the design, fabrication, modeling and benchtop testing of the soft actuator towards realizing 3D functional structures with potential medical applications. An example of application for soft medical robots is tissue regenerative for the long-gap esophageal atresia condition
and Oxygen Stoichiometry: Structure, Resistivity, Fermi Surface Topology and Normal State Properties
(2212) single crystal samples
were studied using transmission electron microscopy (TEM), plane
() and axis () resistivity, and high resolution
angle-resolved ultraviolet photoemission spectroscopy (ARUPS). TEM reveals that
the modulation in the axis for doped 2212 is dominantly
of type that is not sensitive to the oxygen content of the system, and the
system clearly shows a structure of orthorhombic symmetry. Oxygen annealed
samples exhibit a much lower axis resistivity and a resistivity minimum at
K. He-annealed samples exhibit a much higher axis resistivity and
behavior below 300K. The Fermi surface (FS) of oxygen annealed
2212 mapped out by ARUPS has a pocket in the FS around the
point and exhibits orthorhombic symmetry. There are flat, parallel sections of
the FS, about 60\% of the maximum possible along , and about 30\%
along . The wavevectors connecting the flat sections are about
along , and about along , rather than . The symmetry of the near-Fermi-energy dispersing
states in the normal state changes between oxygen-annealed and He-annealed
samples.Comment: APS_REVTEX 3.0, 49 pages, including 11 figures, available upon
request. Submitted to Phys. Rev. B
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