716 research outputs found
Exact and Approximate Stochastic Simulation of Intracellular Calcium Dynamics
In simulations of chemical systems, the main task is to find an exact or approximate solution of the chemical master equation (CME) that satisfies certain constraints with respect to computation time and accuracy. While Brownian motion simulations of single molecules are often too time consuming to represent the mesoscopic level, the classical Gillespie algorithm is a stochastically exact algorithm that provides satisfying results in the representation of calcium microdomains. Gillespie's algorithm can be approximated via
the tau-leap method and the chemical Langevin equation (CLE). Both methods lead to a substantial acceleration in computation time and a relatively small decrease in accuracy. Elimination of the noise terms leads to the classical, deterministic reaction rate equations (RRE). For complex multiscale systems, hybrid simulations are increasingly
proposed to combine the advantages of stochastic and deterministic algorithms. An often used exemplary cell type in this context are striated muscle cells (e.g., cardiac and skeletal muscle cells).
The properties of these cells are well described and they express many common calcium-dependent signaling
pathways. The purpose of the present paper is to provide an overview of the aforementioned simulation approaches and their mutual relationships in the spectrum ranging from stochastic to deterministic algorithms
One-dimensional dynamics of the d-electrons in -NaVO
We have studied the electronic properties of the ladder compound
-NaVO, adopting a joint experimental and theoretical
approach. The momentum-dependent loss function was measured using electron
energy-loss spectroscopy in transmission. The optical conductivity derived from
the loss function by a Kramers-Kronig analysis agrees well with our results
from LSDA+U band-structure calculations upon application of an
antiferromagnetic alignment of the V~3 spins along the legs and an
on-site Coulomb interaction U of between 2 and 3 eV. The decomposition of the
calculated optical conductivity into contributions from transitions between
selected energy regions of the DOS reveals the origin of the observed
anisotropy of the optical conductivity. In addition, we have investigated the
plasmon excitations related to transitions between the vanadium states within
an effective 16 site vanadium cluster model. Good agreement between the
theoretical and experimental loss function was obtained using the hopping
parameters derived from the tight binding fit to the band-structure and
moderate Coulomb interactions between the electrons within the ab plane.Comment: 23 pages, 8 figures; submitted to PR
Enhancement of Anisotropy due to Fluctuations in Quasi-One-Dimensional Antiferromagnets
It is shown that the observed anisotropy of magnetization at high magnetic
fields in RbMnBr3 , a quasi-one-dimensional antiferromagnet on a distorted
stacked triangular lattice, is due to quantum and thermal fluctuations. These
fluctuations are taken into account in the framework of linear spin-wave theory
in the region of strong magnetic fields. In this region the divergent
one-dimensional integrals are cut off by magnetic field and the bare easy-plane
anisotropy. Logarithmical dependence on the cutoff leads to the "enhancement"
of the anisotropy in magnetization. Comparison between magnetization data and
our theory with parameters obtained from neutron scattering experiments has
been done.Comment: 15 pages + 5 postscript figures available upon request, RevTex
Charge order and low frequency spin dynamics in lanthanum cuprates revealed by Nuclear Magnetic Resonance
We report detailed 17O, 139La, and 63Cu Nuclear Magnetic Resonance (NMR) and
Nuclear Quadrupole Resonance (NQR) measurements in a stripe ordered
La1.875Ba0.125CuO4 single crystal and in oriented powder samples of
La1.8-xEu0.2SrxCuO4. We observe a partial wipeout of the 17O NMR intensity and
a simultaneous drop of the 17O electric field gradient (EFG) at low
temperatures where the spin stripe order sets in. In contrast, the 63Cu
intensity is completely wiped out at the same temperature. The drop of the 17O
quadrupole frequency is compatible with a charge stripe order. The 17O spin
lattice relaxation rate shows a peak similar to that of the 139La, which is of
magnetic origin. This peak is doping dependent and is maximal at x ~ 1/8.Comment: submitted to European Physical Journal Special Topic
Rate-equation calculations of the current flow through two-site molecular device and DNA-based junction
Here we present the calculations of incoherent current flowing through the
two-site molecular device as well as the DNA-based junction within the
rate-equation approach. Few interesting phenomena are discussed in detail.
Structural asymmetry of two-site molecule results in rectification effect,
which can be neutralized by asymmetric voltage drop at the molecule-metal
contacts due to coupling asymmetry. The results received for poly(dG)-poly(dC)
DNA molecule reveal the coupling- and temperature-independent saturation effect
of the current at high voltages, where for short chains we establish the
inverse square distance dependence. Besides, we document the shift of the
conductance peak in the direction to higher voltages due to the temperature
decrease.Comment: 12 pages, 6 figure
All electron and pseudopotential study of the spin polarization of the V (001) surface: LDA versus GGA
The spin-polarization at the V(001) surface has been studied by using
different local (LSDA) and semilocal (GGA) approximations to the
exchange-correlation potential of DFT within two ab initio methods: the
all-electron TB-LMTO-ASA and the pseudopotential LCAO code SIESTA (Spanish
Initiative for Electronic Simulations with Thousands of Atoms). A comparative
analysis is performed first for the bulk and then for a N-layer V(001) film (7
< N < 15). The LSDA approximation leads to a non magnetic V(001) surface with
both theoretical models in agreement (disagreement) with magneto-optical Kerr
(electron-capture spectroscopy) experiments. The GGA within the pseudopotential
method needs thicker slabs than the LSDA to yield zero moment at the central
layer, giving a high surface magnetization (1.70 Bohr magnetons), in contrast
with the non magnetic solution obtained by means of the all-electron code.Comment: 12 pages, 1 figure. Latex gzipped tar fil
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