7,358 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 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
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
Numerical simulations of generic singuarities
Numerical simulations of the approach to the singularity in vacuum spacetimes
are presented here. The spacetimes examined have no symmetries and can be
regarded as representing the general behavior of singularities. It is found
that the singularity is spacelike and that as it is approached, the spacetime
dynamics becomes local and oscillatory.Comment: typos correcte
Quantum dynamics of an Ising spin-chain in a random transverse field
We consider an Ising spin-chain in a random transverse magnetic field and
compute the zero temperature wave vector and frequency dependent dynamic
structure factor numerically by using Jordan-Wigner transformation. Two types
of distributions of magnetic fields are introduced. For a rectangular
distribution, a dispersing branch is observed, and disorder tends to broaden
the dispersion peak and close the excitation gap. For a binary distribution, a
non-dispersing branch at almost zero energy is recovered. We discuss the
relationship of our work to the neutron scattering measurement in
.Comment: 4 pages and 6 eps figures; minor clarifications were made; the text
was shortened to add an additional figur
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