11,918 research outputs found
Many-body mobility edge due to symmetry-constrained dynamics and strong interactions
We provide numerical evidence combined with an analytical understanding of
the many-body mobility edge for the strongly anisotropic spin-1/2 XXZ model in
a random magnetic field. The system dynamics can be understood in terms of
symmetry-constrained excitations about parent states with ferromagnetic and
anti-ferromagnetic short range order. These two regimes yield vastly different
dynamics producing an observable, tunable many-body mobility edge. We compute a
set of diagnostic quantities that verify the presence of the mobility edge and
discuss how weakly correlated disorder can tune the mobility edge further.Comment: 10 pages, 5 figure
A semi-analytical approach to perturbations in mutated hilltop inflation
We study cosmological perturbations and observational aspects for mutated
hilltop model of inflation. Employing mostly analytical treatment, we evaluate
observable parameters during inflation as well as post-inflationary
perturbations. This further leads to exploring observational aspects related to
Cosmic Microwave Background (CMB) radiation. This semi-analytical treatment
reduces complications related to numerical computation to some extent for
studying the different phenomena related to CMB angular power spectrum for
mutated hilltop inflation.Comment: 7 pages, 2 figures. Improved version to appear in IJMP
Designing fuzzy rule based classifier using self-organizing feature map for analysis of multispectral satellite images
We propose a novel scheme for designing fuzzy rule based classifier. An SOFM
based method is used for generating a set of prototypes which is used to
generate a set of fuzzy rules. Each rule represents a region in the feature
space that we call the context of the rule. The rules are tuned with respect to
their context. We justified that the reasoning scheme may be different in
different context leading to context sensitive inferencing. To realize context
sensitive inferencing we used a softmin operator with a tunable parameter. The
proposed scheme is tested on several multispectral satellite image data sets
and the performance is found to be much better than the results reported in the
literature.Comment: 23 pages, 7 figure
Transient stability analysis and enhancement of renewable energy conversion system during LVRT
Grid-connected renewable energy conversion systems (RECSs) are usually required by grid codes to possess the low voltage ride through (LVRT) and reactive power support capabilities so as to cope with grid voltage sags. During LVRT, RECS's terminal voltage becomes sensitive and changeable with its output current, which brings a great challenge for the RECS to resynchronize with the grid by means of phase-locked loops (PLLs). This paper indicates that loss of synchronism (LOS) of PLLs is responsible for the transient instability of grid-connected RECSs during LVRT, and the LOS is essentially due to the transient interaction between the PLL and the weak terminal voltage. For achieving a quantitative analysis, an equivalent swing equation model is developed to describe the transient interaction. Based on the model, the transient instability mechanism of RECSs during LVRT is clarified. Furthermore, a transient stability enhancement method is proposed to avoid the possibility of transient instability. Simulations performed on the New England 39-bus test system verify the effectiveness of the method
A momentum-conserving, consistent, Volume-of-Fluid method for incompressible flow on staggered grids
The computation of flows with large density contrasts is notoriously
difficult. To alleviate the difficulty we consider a consistent mass and
momentum-conserving discretization of the Navier-Stokes equation.
Incompressible flow with capillary forces is modelled and the discretization is
performed on a staggered grid of Marker and Cell type. The Volume-of-Fluid
method is used to track the interface and a Height-Function method is used to
compute surface tension. The advection of the volume fraction is performed
using either the Lagrangian-Explicit / CIAM (Calcul d'Interface Affine par
Morceaux) method or the Weymouth and Yue (WY) Eulerian-Implicit method. The WY
method conserves fluid mass to machine accuracy provided incompressiblity is
satisfied which leads to a method that is both momentum and mass-conserving. To
improve the stability of these methods momentum fluxes are advected in a manner
"consistent" with the volume-fraction fluxes, that is a discontinuity of the
momentum is advected at the same speed as a discontinuity of the density. To
find the density on the staggered cells on which the velocity is centered, an
auxiliary reconstruction of the density is performed. The method is tested for
a droplet without surface tension in uniform flow, for a droplet suddenly
accelerated in a carrying gas at rest at very large density ratio without
viscosity or surface tension, for the Kelvin-Helmholtz instability, for a
falling raindrop and for an atomizing flow in air-water conditions
A Multi-Phase Transport Model for Relativistic Heavy Ion Collisions
We describe in detail how the different components of a multi-phase transport
(AMPT) model, that uses the Heavy Ion Jet Interaction Generator (HIJING) for
generating the initial conditions, Zhang's Parton Cascade (ZPC) for modeling
partonic scatterings, the Lund string fragmentation model or a quark
coalescence model for hadronization, and A Relativistic Transport (ART) model
for treating hadronic scatterings, are improved and combined to give a coherent
description of the dynamics of relativistic heavy ion collisions. We also
explain the way parameters in the model are determined, and discuss the
sensitivity of predicted results to physical input in the model. Comparisons of
these results to experimental data, mainly from heavy ion collisions at the
Relativistic Heavy Ion Collider (RHIC), are then made in order to extract
information on the properties of the hot dense matter formed in these
collisions.Comment: 33 pages, 38 figures, revtex. Added 9 figures, version published in
Phys. Rev. C. The full source code of the AMPT model in the Fortran 77
language and instructions for users are available from the EPAPS ftp site
(ftp://ftp.aip.org/epaps/phys_rev_c/E-PRVCAN-72-781512/) and the OSCAR
website (http://www-cunuke.phys.columbia.edu/OSCAR/
Magnetic field induced orientation of superconducting MgB crystallites determined by X-ray diffraction
X-ray diffraction studies of fine polycrystalline samples of MgB in the
superconducting state reveal that crystals orient with their \emph{c}-axis in a
plane normal to the direction of the applied magnetic field. The MgB
samples were thoroughly ground to obtain average grain size 5 - 10 m in
order to increase the population of free single crystal grains in the powder.
By monitoring Bragg reflections in a plane normal to an applied magnetic field
we find that the powder is textured with significantly stronger (\emph{0,0,l})
reflections in comparison to (\emph{h,k,0}), which remain essentially
unchanged. The orientation of the crystals with the \emph{ab}-plane parallel to
the magnetic field at all temperatures below demonstrates that the sign
of the torque under magnetic field does not alter, in disagreement with current
theoretical predictions
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