262 research outputs found
A discrete time-dependent method for metastable atoms in intense fields
The full-dimensional time-dependent Schrodinger equation for the electronic
dynamics of single-electron systems in intense external fields is solved
directly using a discrete method.
Our approach combines the finite-difference and Lagrange mesh methods. The
method is applied to calculate the quasienergies and ionization probabilities
of atomic and molecular systems in intense static and dynamic electric fields.
The gauge invariance and accuracy of the method is established. Applications to
multiphoton ionization of positronium and hydrogen atoms and molecules are
presented. At very high intensity above saturation threshold, we extend the
method using a scaling technique to estimate the quasienergies of metastable
states of the hydrogen molecular ion. The results are in good agreement with
recent experiments.Comment: 10 pages, 9 figure, 4 table
Collective vibrational states with fast iterative QRPA method
An iterative method we previously proposed to compute nuclear strength
functions is developed to allow it to accurately calculate properties of
individual nuclear states. The approach is based on the
quasi-particle-random-phase approximation (QRPA) and uses an iterative
non-hermitian Arnoldi diagonalization method where the QRPA matrix does not
have to be explicitly calculated and stored. The method gives substantial
advantages over conventional QRPA calculations with regards to the
computational cost. The method is used to calculate excitation energies and
decay rates of the lowest lying 2+ and 3- states in Pb, Sn, Ni and Ca isotopes
using three different Skyrme interactions and a separable gaussian pairing
force.Comment: 10 pages, 11 figure
Extreme multiplicity in cylindrical Rayleigh-Benard convection: II. Bifurcation diagram and symmetry classification
A large number of flows with distinctive patterns have been observed in
experiments and simulations of Rayleigh-Benard convection in a water-filled
cylinder whose radius is twice the height. We have adapted a time-dependent
pseudospectral code, first, to carry out Newton's method and branch
continuation and, second, to carry out the exponential power method and Arnoldi
iteration to calculate leading eigenpairs and determine the stability of the
steady states. The resulting bifurcation diagram represents a compromise
between the tendency in the bulk towards parallel rolls, and the requirement
imposed by the boundary conditions that primary bifurcations be towards states
whose azimuthal dependence is trigonometric. The diagram contains 17 branches
of stable and unstable steady states. These can be classified geometrically as
roll states containing two, three, and four rolls; axisymmetric patterns with
one or two tori; three-fold symmetric patterns called mercedes, mitubishi,
marigold and cloverleaf; trigonometric patterns called dipole and pizza; and
less symmetric patterns called CO and asymmetric three-rolls. The convective
branches are connected to the conductive state and to each other by 16 primary
and secondary pitchfork bifurcations and turning points. In order to better
understand this complicated bifurcation diagram, we have partitioned it
according to azimuthal symmetry. We have been able to determine the
bifurcation-theoretic origin from the conductive state of all the branches
observed at high Rayleigh number
Bogoliubov modes of a dipolar condensate in a cylindrical trap
The calculation of properties of Bose-Einstein condensates with dipolar
interactions has proven a computationally intensive problem due to the long
range nature of the interactions, limiting the scope of applications. In
particular, the lowest lying Bogoliubov excitations in three dimensional
harmonic trap with cylindrical symmetry were so far computed in an indirect
way, by Fourier analysis of time dependent perturbations, or by approximate
variational methods. We have developed a very fast and accurate numerical
algorithm based on the Hankel transform for calculating properties of dipolar
Bose-Einstein condensates in cylindrically symmetric traps. As an application,
we are able to compute many excitation modes by directly solving the
Bogoliubov-De Gennes equations. We explore the behavior of the excited modes in
different trap geometries. We use these results to calculate the quantum
depletion of the condensate by a combination of a computation of the exact
modes and the use of a local density approximation
Kinematic dynamo action in a sphere. I. Effects of differential rotation and meridional circulation on solutions with axial dipole symmetry
A sphere containing electrically conducting fluid can generate a magnetic field by dynamo action, provided the flow is sufficiently complicated and vigorous. The dynamo mechanism is thought to sustain magnetic fields in planets and stars. The kinematic dynamo problem tests steady flows for magnetic instability, but rather few dynamos have been found so far because of severe numerical difficulties. Dynamo action might, therefore, be quite unusual, at least for large-scale steady flows. We address this question by testing a two-parameter class of flows for dynamo generation of magnetic fields containing an axial dipole. The class of flows includes two completely different types of known dynamos, one dominated by differential rotation (D) and one with none. We find that 36% of the flows in seven distinct zones in parameter space act as dynamos, while the remaining 64% either fail to generate this type of magnetic field or generate fields that are too small in scale to be resolved by our numerical method. The two previously known dynamo types lie in the same zone, and it is therefore possible to change the flow continuously from one to the other without losing dynamo action. Differential rotation is found to promote large-scale axisymmetric toroidal magnetic fields, while meridional circulation (M) promotes large-scale axisymmetric poloidal fields concentrated at high latitudes near the axis. Magnetic fields resembling that of the Earth are generated by D > 0, corresponding to westward flow at the surface, and M of either sign but not zero. Very few oscillatory solutions are found
Clustered bottlenecks in mRNA translation and protein synthesis
We construct an algorithm that generates large, band-diagonal transition
matrices for a totally asymmetric exclusion process (TASEP) with local hopping
rate inhomogeneities. The matrices are diagonalized numerically to find
steady-state currents of TASEPs with local variations in hopping rate. The
results are then used to investigate clustering of slow codons along mRNA.
Ribosome density profiles near neighboring clusters of slow codons interact,
enhancing suppression of ribosome throughput when such bottlenecks are closely
spaced. Increasing the slow codon cluster size, beyond , does not
significantly reduce ribosome current. Our results are verified by extensive
Monte-Carlo simulations and provide a biologically-motivated explanation for
the experimentally-observed clustering of low-usage codons
Spatial and spectral properties of the pulsed second-harmonic generation in a PP-KTP waveguide
Spatial and spectral properties of the pulsed second harmonic generation in a
periodically-poled KTP waveguide exploiting simultaneously the first, second,
and third harmonics of periodic nonlinear modulation are analyzed. Experimental
results are interpreted using a model based on finite elements method.
Correlations between spatial and spectral properties of the fundamental and
second-harmonic fields are revealed. Individual nonlinear processes can be
exploited combining spatial and spectral filtering. Also the influence of
waveguide parameters to the second-harmonic spectra is addressed.Comment: 13 pages, 8 figure
Time delay between photoemission from the 2p and 2s subshells of Neon
The R-Matrix incorporating Time (RMT) method is a new method for solving the
time-dependent Schroedinger equation for multi-electron atomic systems exposed
to intense short-pulse laser light. We have employed the RMT method to
investigate the time delay in the photoemission of an electron liberated from a
2p orbital in a neon atom with respect to one released from a 2s orbital
following absorption of an attosecond XUV pulse. Time delays due to XUV pulses
in the range 76-105 eV are presented. For an XUV pulse at the experimentally
relevant 105.2 eV, we calculate the time delay to be 10.2 +/- 1.3 attoseconds,
somewhat larger than estimated by other theoretical calculations, but still a
factor two smaller than experiment. We repeated the calculation for a photon
energy of 89.8 eV with a larger basis set capable of modelling
correlated-electron dynamics within the neon atom and the residual Ne(+) ion. A
time delay of 14.5 +/- 1.5 attoseconds was observed, compared to a 16.7 +/- 1.5
attosecond result using a single-configuration representation of the residual
Ne(+) ion.Comment: 4 pages, 3 figures, 1 tabl
Diffusion with critically correlated traps and the slow relaxation of the longest wavelength mode
We study diffusion on a substrate with permanent traps distributed with
critical positional correlation, modeled by their placement on the perimeters
of a critical percolation cluster. We perform a numerical analysis of the
vibrational density of states and the largest eigenvalue of the equivalent
scalar elasticity problem using the method of Arnoldi and Saad. We show that
the critical trap correlation increases the exponent appearing in the stretched
exponential behavior of the low frequency density of states by approximately a
factor of two as compared to the case of no correlations. A finite size scaling
hypothesis of the largest eigenvalue is proposed and its relation to the
density of states is given. The numerical analysis of this scaling postulate
leads to the estimation of the stretch exponent in good agreement with the
density of states result.Comment: 15 pages, LaTeX (RevTeX
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