9,205 research outputs found
Computing the lowest eigenvalues of the Fermion matrix by subspace iterations
Subspace iterations are used to minimise a generalised Ritz functional of a
large, sparse Hermitean matrix. In this way, the lowest eigenvalues are
determined. Tests with demonstrate that the computational
cost (no. of matrix multiplies) does not increase substantially with . This
implies that, as compared to the case of a , the additional eigenvalues
are obtained for free.Comment: Talk presented at LATTICE96(algorithms), 3 pages, 2 Postscript
figures, uses epsf.sty, espcrc2.st
The locality of the square-root method for improved staggered quarks
We study the effects of improvement on the locality of square-rooted
staggered Dirac operators in lattice QCD simulations. We find the localisation
lengths of the improved operators (FAT7TAD and ASQTAD) to be very similar to
that of the one-link operator studied by Bunk et al., being at least the
Compton wavelength of the lightest particle in the theory, even in the
continuum limit. We conclude that improvement has no effect. We discuss the
implications of this result for the locality of the nth-rooted fermion
determinant used to reduce the number of sea quark flavours, and for possible
staggered valence quark formulations
The Height of a Giraffe
A minor modification of the arguments of Press and Lightman leads to an
estimate of the height of the tallest running, breathing organism on a
habitable planet as the Bohr radius multiplied by the three-tenths power of the
ratio of the electrical to gravitational forces between two protons (rather
than the one-quarter power that Press got for the largest animal that would not
break in falling over, after making an assumption of unreasonable brittleness).
My new estimate gives a height of about 3.6 meters rather than Press's original
estimate of about 2.6 cm. It also implies that the number of atoms in the
tallest runner is very roughly of the order of the nine-tenths power of the
ratio of the electrical to gravitational forces between two protons, which is
about 3 x 10^32.Comment: 12 pages, LaTe
Determination of the zeta potential for highly charged colloidal suspensions
We compute the electrostatic potential at the surface, or zeta potential
, of a charged particle embedded in a colloidal suspension using a
hybrid mesoscopic model. We show that for weakly perturbing electric fields,
the value of obtained at steady state during electrophoresis is
statistically indistinguishable from in thermodynamic equilibrium. We
quantify the effect of counterions concentration on . We also evaluate
the relevance of the lattice resolution for the calculation of and
discuss how to identify the effective electrostatic radius.Comment: 8 pages, 3 figures with 2 panel
Dipole Excitation of Dipositronium
The energy interval between the ground and the P-wave excited states of the
recently discovered positronium molecule Ps_2 is evaluated, including the
relativistic and the leading logarithmic radiative corrections, E_P-E_S = 0.181
586 7(8) a.u. The P-state, decaying usually via annihilation, is found to decay
into the ground state by an electric dipole transition 19 percent of the time.
Anticipated observation of this transition will provide insight into this
exotic system.Comment: 5 page
Ab initio mass tensor molecular dynamics
Mass tensor molecular dynamics was first introduced by Bennett [J. Comput.
Phys. 19, 267 (1975)] for efficient sampling of phase space through the use of
generalized atomic masses. Here, we show how to apply this method to ab initio
molecular dynamics simulations with minimal computational overhead. Test
calculations on liquid water show a threefold reduction in computational effort
without making the fixed geometry approximation. We also present a simple
recipe for estimating the optimal atomic masses using only the first
derivatives of the potential energy.Comment: 19 pages, 5 figure
Non-Fourier heat transport in metal-dielectric core-shell nanoparticles under ultrafast laser pulse excitation
Relaxation dynamics of embedded metal nanoparticles after ultrafast laser
pulse excitation is driven by thermal phenomena of different origins the
accurate description of which is crucial for interpreting experimental results:
hot electron gas generation, electron-phonon coupling, heat transfer to the
particle environment and heat propagation in the latter. Regardingthis last
mechanism, it is well known that heat transport in nanoscale structures and/or
at ultrashort timescales may deviate from the predictions of the Fourier law.
In these cases heat transport may rather be described by the Boltzmann
transport equation. We present a numerical model allowing us to determine the
electron and lattice temperature dynamics in a spherical gold nanoparticle core
under subpicosecond pulsed excitation, as well as that of the surrounding shell
dielectric medium. For this, we have used the electron-phonon coupling equation
in the particle with a source term linked with the laser pulse absorption, and
the ballistic-diffusive equations for heat conduction in the host medium.
Either thermalizing or adiabatic boundary conditions have been considered at
the shell external surface. Our results show that the heat transfer rate from
the particle to the matrix can be significantly smaller than the prediction of
Fourier's law. Consequently, the particle temperature rise is larger and its
cooling dynamics might be slower than that obtained by using Fourier's law.
This difference is attributed to the nonlocal and nonequilibrium heat
conduction in the vicinity of the core nanoparticle. These results are expected
to be of great importance for analyzing pump-probe experiments performed on
single nanoparticles or nanocomposite media
Bubble statistics and positioning in superhelically stressed DNA
We present a general framework to study the thermodynamic denaturation of
double-stranded DNA under superhelical stress. We report calculations of
position- and size-dependent opening probabilities for bubbles along the
sequence. Our results are obtained from transfer-matrix solutions of the
Zimm-Bragg model for unconstrained DNA and of a self-consistent linearization
of the Benham model for superhelical DNA. The numerical efficiency of our
method allows for the analysis of entire genomes and of random sequences of
corresponding length ( base pairs). We show that, at physiological
conditions, opening in superhelical DNA is strongly cooperative with average
bubble sizes of base pairs (bp), and orders of magnitude higher
than in unconstrained DNA. In heterogeneous sequences, the average degree of
base-pair opening is self-averaging, while bubble localization and statistics
are dominated by sequence disorder. Compared to random sequences with identical
GC-content, genomic DNA has a significantly increased probability to open large
bubbles under superhelical stress. These bubbles are frequently located
directly upstream of transcription start sites.Comment: to be appeared in Physical Review
Stochastic nonlinear differential equation generating 1/f noise
Starting from the simple point process model of 1/f noise we derive a
stochastic nonlinear differential equation for the signal exhibiting 1/f noise
in any desirably wide range of frequency. A stochastic differential equation
(the general Langevin equation with a multiplicative noise) that gives 1/f
noise is derived for the first time. The solution of the equation exhibits the
power-law distribution. The process with 1/f noise is demonstrated by the
numerical solution of the derived equation with the appropriate restriction of
the diffusion of the signal in some finite interval.Comment: 3 figure
Effects of Line-tying on Resistive Tearing Instability in Slab Geometry
The effects of line-tying on resistive tearing instability in slab geometry
is studied within the framework of reduced magnetohydrodynamics
(RMHD).\citep{KadomtsevP1974,Strauss1976} It is found that line-tying has a
stabilizing effect. The tearing mode is stabilized when the system length
is shorter than a critical length , which is independent of the
resistivity . When is not too much longer than , the
growthrate is proportional to . When is sufficiently long,
the tearing mode scaling is recovered. The transition
from to occurs at a transition length
.Comment: Correct a typ
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