8,619 research outputs found
The Rockstar Phase-Space Temporal Halo Finder and the Velocity Offsets of Cluster Cores
We present a new algorithm for identifying dark matter halos, substructure,
and tidal features. The approach is based on adaptive hierarchical refinement
of friends-of-friends groups in six phase-space dimensions and one time
dimension, which allows for robust (grid-independent, shape-independent, and
noise-resilient) tracking of substructure; as such, it is named Rockstar
(Robust Overdensity Calculation using K-Space Topologically Adaptive
Refinement). Our method is massively parallel (up to 10^5 CPUs) and runs on the
largest current simulations (>10^10 particles) with high efficiency (10 CPU
hours and 60 gigabytes of memory required per billion particles analyzed). A
previous paper (Knebe et al 2011) has shown Rockstar to have class-leading
recovery of halo properties; we expand on these comparisons with more tests and
higher-resolution simulations. We show a significant improvement in
substructure recovery as compared to several other halo finders and discuss the
theoretical and practical limits of simulations in this regard. Finally, we
present results which demonstrate conclusively that dark matter halo cores are
not at rest relative to the halo bulk or satellite average velocities and have
coherent velocity offsets across a wide range of halo masses and redshifts. For
massive clusters, these offsets can be up to 350 km/s at z=0 and even higher at
high redshifts. Our implementation is publicly available at
http://code.google.com/p/rockstar .Comment: 20 pages, 14 figures. Minor revisions to match accepted versio
Large Zero Point Density Fluctuations in Fluids
Zero point density fluctuations in a liquid and their potential observation
by light scattering are discussed. It is suggested that there are two distinct
effects of interest. One gives an average number of scattered photons, and
depends upon an inverse power of the photon wavelength. The second effect
arises in the scattering of finite size photon wave packets and depends upon an
inverse power of the spatial size of the wave packet. This effect appears as
large fluctuations in the number of scattered photons, and is analogous to the
vacuum fluctuations of spacetime averages of the energy density in quantum
field theory.Comment: 9 pages, 2 figure
Motion Control of Robotic Arm with Command Shaping Method
In the manufacturing industry, lots of rapid point-to-point motion is required while the residual vibration is unfavorable. Residual vibrations caused by flexible elements are limiting the performance of mechanical system, especially when the system needs to make rapid point-to-point motion. As proved in earlier studies, avoiding natural frequencies of the mechanical system reduces the residual vibrations. This work is based on a non-linear, two-link flexible jointed robot with configuration dependent resonance. Command shaping method consisting different combinations of base functions and weighting factors are compared in this work. The compatibility of command shaping with classical feedback control structure allows a computationally effective method for real time implementation. By measuring the acceleration after the input stops, the residual vibration is analyzed. The best case would be the one with the least peak-to-peak residual acceleration and reasonable peak acceleration during the motion. Experimental results show that the residual vibrations can be reduced considerably after the implementation of command shaping method. As verified in this work, command shaping method is a practical way to control motion with flexible elements without exciting the system’s natural frequencies and demanding significantly more computation capability
Controlling crystal symmetries in phase-field crystal models
We investigate the possibility to control the symmetry of ordered states in
phase-field crystal models by tuning nonlinear resonances. In two dimensions,
we find that a state of square symmetry as well as coexistence between squares
and hexagons can be easily obtained. In contrast, it is delicate to obtain
coexistence of squares and liquid. We develop a general method for constructing
free energy functionals that exhibit solid-liquid coexistence with desired
crystal symmetries. As an example, we develop a free energy functional for
square-liquid coexistence in two dimensions. A systematic analysis for
determining the parameters of the necessary nonlinear terms is provided. The
implications of our findings for simulations of materials with simple cubic
symmetry are discussed.Comment: 19 pages, 6 figure
Space and time averaged quantum stress tensor fluctuations
We extend previous work on the numerical diagonalization of quantum stress tensor operators in the Minkowski vacuum state, which considered operators averaged in a finite time interval, to operators averaged in a finite spacetime region. Since real experiments occur over finite volumes and durations, physically meaningful fluctuations may be obtained from stress tensor operators averaged by compactly supported sampling functions in space and time. The direct diagonalization, via a Bogoliubov transformation, gives the eigenvalues and the probabilities of measuring those eigenvalues in the vacuum state, from which the underlying probability distribution can be constructed. For the normal-ordered square of the time derivative of a massless scalar field in a spherical cavity with finite degrees of freedom, analysis of the tails of these distributions confirms previous results based on the analytical treatment of the high moments. We find that the probability of large vacuum fluctuations is reduced when spatial averaging is included, but the tail still decreases more slowly than exponentially as the magnitude of the measured eigenvalues increases, suggesting vacuum fluctuations may not always be subdominant to thermal fluctuations and opening up the possibility of experimental observation under the right conditions.We extend previous work on the numerical diagonalization of quantum stress tensor operators in the Minkowski vacuum state, which considered operators averaged in a finite time interval, to operators averaged in a finite spacetime region. Since real experiments occur over finite volumes and durations, physically meaningful fluctuations may be obtained from stress tensor operators averaged by compactly supported sampling functions in space and time. The direct diagonalization, via a Bogoliubov transformation, gives the eigenvalues and the probabilities of measuring those eigenvalues in the vacuum state, from which the underlying probability distribution can be constructed. For the normal-ordered square of the time derivative of a massless scalar field in a spherical cavity with finite degrees of freedom, analysis of the tails of these distributions confirms previous results based on the analytical treatment of the high moments. We find that the probability of large vacuum fluctuations is reduced when spatial averaging is included, but the tail still decreases more slowly than exponentially as the magnitude of the measured eigenvalues increases, suggesting vacuum fluctuations may not always be subdominant to thermal fluctuations and opening up the possibility of experimental observation under the right conditions.Peer reviewe
Frequency Spectra Analysis of Space and Time Averaged Quantum Stress Tensor Fluctuations
Observing physical effects of large quantum stress tensor fluctuations
requires knowledge of the interaction between the probe and the particles of
the underlying quantum fields. The quantum stress tensor operators must first
be averaged in time alone or space and time to confer meaningful results, the
details of which may correspond to the physical measurement process. We build
on prior results to characterize the particle frequencies associated with
quantum fluctuations of different magnitudes. For the square of time
derivatives of the massless scalar field in a spherical cavity, we find that
these frequencies are bounded above in a power law behavior. Our findings
provide a way identify the largest quantum fluctuation that may be probed in
experiments relying on frequency-dependent interactions.Comment: 23 pages, 4 figures, 1 tabl
Maser-beam instability of Bernstein waves
The present study constitutes a continuation and improvement of the preceding work by Yoon et al. [J. Geophys. Res. 104, 19801 (1999)]. In the present discussion, an instability of Bernstein waves excited by a beam of energetic electrons is investigated. Special attention is paid to the regime where the ratio of plasma frequency, vpe , to electron gyrofrequency, Ve , is sufficiently higher than unity. An approximate but fairly accurate scheme is introduced to deal with the situation dictated by the condition, vpe 2 /Ve 2e1. The present investigation is motivated by the research in solar radiophysics. However, in this article the emphasis is placed on basic properties of the instability rather than its application
Collapse of the vortex-lattice inductance and shear modulus at the melting transition in untwinned
The complex resistivity of the vortex lattice in an
untwinned crystal of 93-K has been measured at frequencies
from 100 kHz to 20 MHz in a 2-Tesla field ,
using a 4-probe RF transmission technique that enables continuous measurements
versus and temperature . As is increased, the inductance increases steeply to a cusp
at the melting temperature , and then undergoes a steep collapse
consistent with vanishing of the shear modulus . We discuss in detail
the separation of the vortex-lattice inductance from the `volume' inductance,
and other skin-depth effects. To analyze the spectra, we consider a weakly
disordered lattice with a low pin density. Close fits are obtained to
over 2 decades in . Values of the pinning parameter
and shear modulus obtained show that collapses by
over 4 decades at , whereas remains finite.Comment: 11 pages, 8 figures, Phys. Rev. B, in pres
Rotating Black Holes in Metric-Affine Gravity
Within the framework of metric-affine gravity (MAG, metric and an independent
linear connection constitute spacetime), we find, for a specific gravitational
Lagrangian and by using {\it prolongation} techniques, a stationary axially
symmetric exact solution of the vacuum field equations. This black hole
solution embodies a Kerr-deSitter metric and the post-Riemannian structures of
torsion and nonmetricity. The solution is characterized by mass, angular
momentum, and shear charge, the latter of which is a measure for violating
Lorentz invariance.Comment: 32 pages latex, 3 table
Protein Bioinformatics Infrastructure for the Integration and Analysis of Multiple High-Throughput “omics” Data
High-throughput “omics” technologies bring new opportunities for biological and biomedical researchers to ask complex questions and gain new scientific insights. However, the voluminous, complex, and context-dependent data being maintained in heterogeneous and distributed environments plus the lack of well-defined data standard and standardized nomenclature imposes a major challenge which requires advanced computational methods and bioinformatics infrastructures for integration, mining, visualization, and comparative analysis to facilitate data-driven hypothesis generation and biological knowledge discovery. In this paper, we present the challenges in high-throughput “omics” data integration and analysis, introduce a protein-centric approach for systems integration of large and heterogeneous high-throughput “omics” data including microarray, mass spectrometry, protein sequence, protein structure, and protein interaction data, and use scientific case study to illustrate how one can use varied “omics” data from different laboratories to make useful connections that could lead to new biological knowledge
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