6,768 research outputs found
Pseudoparticle Multipole Method: A Simple Method to Implement High-Accuracy Treecode
In this letter we describe the pseudoparticle multipole method (P2M2), a new
method to express multipole expansion by a distribution of pseudoparticles. We
can use this distribution of particles to calculate high order terms in both
the Barnes-Hut treecode and FMM. The primary advantage of P2M2 is that it works
on GRAPE. GRAPE is a special-purpose hardware for the calculation of
gravitational force between particles. Although the treecode has been
implemented on GRAPE, we could handle terms only up to dipole, since GRAPE can
calculate forces from point-mass particles only. Thus the calculation cost
grows quickly when high accuracy is required. With P2M2, the multipole
expansion is expressed by particles, and thus GRAPE can calculate high order
terms. Using P2M2, we implemented an arbitrary-order treecode on GRAPE-4.
Timing result shows GRAPE-4 accelerates the calculation by a factor between 10
(for low accuracy) to 150 (for high accuracy). Even on general-purpose
programmable computers, our method offers the advantage that the mathematical
formulae and therefore the actual program is much simpler than that of the
direct implementation of multipole expansion.Comment: 6 pages, 4 figures, latex, submitted to ApJ Letter
Evolution of Compact Groups of Galaxies I. Merging Rates
We discuss the merging rates in compact groups of 5 identical elliptical
galaxies. All groups have the same mass and binding energy. We consider both
cases with individual halos and cases where the halo is common to all galaxies
and enveloping the whole group. In the latter situation the merging rate is
slower if the halo is more massive. The mass of individual halos has little
influence on the merging rates, due to the fact that all galaxies in our
simulations have the same mass, and so the more extended ones have a smaller
velocity dispersion. Groups with individual halos merge faster than groups with
common halos if the configuration is centrally concentrated, like a King
distribution of index 10. On the other hand for less concentrated
configurations the merging is initially faster for individual halo cases, and
slower after part of the group has merged. In cases with common halo, centrally
concentrated configurations merge faster for high halo-to-total mass ratios and
slower for low halo-to-total mass ratios. Groups whose virial ratio is
initially less than one merge faster, while groups that have initially
cylindrical rotation merge slower than groups starting in virial equilibrium.
In order to test how long a virialised group can survive before merging we
followed the evolution of a group with a high halo-to-total mass ratio and a
density distribution with very little central concentration. We find that the
first merging occurred only after a large number of crossing times, which with
areasonable calibration should be larger than a Hubble time. Hence, at least
for appropriate initial conditions, the longevity of compact groups is not
necessarily a problem, which is an alternative explanation to why we observe so
many compact groups despite the fact that their lifetimes seem short.Comment: 15 pages Latex, with 12 figures included, requires mn.sty, accepted
for publication in MNRA
Analysis on reflection spectra in strained ZnO thin films
Thin films of laser molecular-beam epitaxy grown ZnO films were studied with
respect to their optical properties. 4-K reflectivity was used to analyze
various samples grown at different biaxial in-plane strain. The spectra show
two structures at 3.37 eV corresponding to the A-free exciton transition and at
3.38 eV corresponding to the B-free exciton transition. Theoretical
reflectivity spectra were calculated using the spatial dispersion model. Thus,
the transverse energies, the longitudinal transversal splitting (ELT,), the
oscillator strengths, and the damping parameters were determined for both the
A- and B-free excitons of ZnO. As a rough trend, the strain dependence of the
energy E_LT for the A-excitons is characterized by a negatively-peaking
behavior with a minimum around the zero strain, while ELT for the B-excitons is
an increasing function of the strain field values.Comment: 4 pages, 2 figures, 1 table, conference: ICMAT2005 (Singapore), to
appear in an issue of J. Cryst. Growt
Long-Term Evolution of Massive Black Hole Binaries. II. Binary Evolution in Low-Density Galaxies
We use direct-summation N-body integrations to follow the evolution of binary
black holes at the centers of galaxy models with large, constant-density cores.
Particle numbers as large as 400K are considered. The results are compared with
the predictions of loss-cone theory, under the assumption that the supply of
stars to the binary is limited by the rate at which they can be scattered into
the binary's influence sphere by gravitational encounters. The agreement
between theory and simulation is quite good; in particular, we are able to
quantitatively explain the observed dependence of binary hardening rate on N.
We do not verify the recent claim of Chatterjee, Hernquist & Loeb (2003) that
the hardening rate of the binary stabilizes when N exceeds a particular value,
or that Brownian wandering of the binary has a significant effect on its
evolution. When scaled to real galaxies, our results suggest that massive black
hole binaries in gas-poor nuclei would be unlikely to reach gravitational-wave
coalescence in a Hubble time.Comment: 13 pages, 8 figure
Hole Transport in p-Type ZnO
A two-band model involving the A- and B-valence bands was adopted to analyze
the temperature dependent Hall effect measured on N-doped \textit{p}-type ZnO.
The hole transport characteristics (mobilities, and effective Hall factor) are
calculated using the ``relaxation time approximation'' as a function of
temperature. It is shown that the lattice scattering by the acoustic
deformation potential is dominant. In the calculation of the scattering rate
for ionized impurity mechanism, the activation energy of 100 or 170 meV is used
at different compensation ratios between donor and acceptor concentrations. The
theoretical Hall mobility at acceptor concentration of
cm is about 70 cmVs with the activation energy of 100 meV
and the compensation ratio of 0.8 at 300 K. We also found that the compensation
ratios conspicuously affected the Hall mobilities.Comment: 5page, 5 figures, accepted for publication in Jpn. J. Appl. Phy
Gallium concentration dependence of room-temperature near-bandedge luminescence in n-type ZnO:Ga
We investigated the optical properties of epitaxial \textit{n}-type ZnO films
grown on lattice-matched ScAlMgO substrates. As the Ga doping concentration
increased up to cm, the absorption edge showed a
systematic blueshift, consistent with the Burstein-Moss effect. A bright
near-bandedge photoluminescence (PL) could be observed even at room
temperature, the intensity of which increased monotonically as the doping
concentration was increased except for the highest doping level. It indicates
that nonradiative transitions dominate at a low doping density. Both a Stokes
shift and broadening in the PL band are monotonically increasing functions of
donor concentration, which was explained in terms of potential fluctuations
caused by the random distribution of donor impurities.Comment: accepted for publication for Applied Physics Letters 4 figure
Monte-Carlo simulation of localization dynamics of excitons in ZnO and CdZnO quantum well structures
Localization dynamics of excitons was studied for ZnO/MgZnO and CdZnO/MgZnO
quantum wells (QW). The experimental photoluminescence (PL) and absorption data
were compared with the results of Monte Carlo simulation in which the excitonic
hopping was modeled. The temperature-dependent PL linewidth and Stokes shift
were found to be in a qualitatively reasonable agreement with the hopping
model, with accounting for an additional inhomogeneous broadening for the case
of linewidth. The density of localized states used in the simulation for the
CdZnO QW was consistent with the absorption spectrum taken at 5 K.Comment: 4 figures, to appear in J. Appl. Phy
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