40,840 research outputs found

### Molecular Dynamics Simulation of Collisions between Hydrogen and Graphite

Hydrogen adsorption by graphite is examined by classical molecular dynamics
simulation using a modified Brenner REBO potential. Such interactions are
typical in chemical sputtering experiments, and knowledge of the fundamental
behavior of hydrogen and graphene in collisional conditions is essential for
modeling the sputtering mechanism. The hydrogen adsorption rate is found to be
dependent on the incident hydrogen energy and not on graphene temperature.
Rather than destroying the graphene, hydrogen incidence at energies of less
than 100 eV can be classified into three regimes of adsorption, reflection and
penetration through one or more graphene layers. Incidence at the lowest
energies is shown to distort the graphene structure.Comment: 4 pages, 4 figures. Accepted for publication by j. Plasma Phys.
Proccedings for the joint conference of 19th International Conference on
Numerical Simulation of Plasmas and 7th Asia Pacific Plasma Theor

### A Canonical Ensemble Approach to the Fermion/Boson Random Point Processes and its Applications

We introduce the boson and the fermion point processes from the elementary
quantum mechanical point of view. That is, we consider quantum statistical
mechanics of canonical ensemble for a fixed number of particles which obey
Bose-Einstein, Fermi-Dirac statistics, respectively, in a finite volume.
Focusing on the distribution of positions of the particles, we have point
processes of the fixed number of points in a bounded domain. By taking the
thermodynamic limit such that the particle density converges to a finite value,
the boson/fermion processes are obtained. This argument is a realization of the
equivalence of ensembles, since resulting processes are considered to describe
a grand canonical ensemble of points. Random point processes corresponding to
para-particles of order two are discussed as an application of the formulation.
A statistics of a system of composite particles at zero temperature are also
considered as a model of determinantal random point processes.Comment: 26pages, Some typos are corrected, to be published in Commun. Math.
Phy

### Hawaii's Pelagic Fisheries

Hawaii's diverse pelagic fisheries supply the bulk of the State's total catch. The largest Hawaii fishery is the recently expanded longline fishery, which now lands about 4,400 metric tons (t) of broadbill swordfish, Xiphias gladius; 1,500 t of bigeye tuna, Thunnus obesus, and 3,000 t of other pelagic species annually. The increased catch of these other species has raised concerns regarding the continued availability of yellowfin tuna, T. albacares; blue marlin, Makaira mazara; and mahimahi, Coryphaena hippurus, in the small-vessel troll and handline fisheries which target those species.
Analysis of catch per unit effort (CPUE) statistics from Hawaii's fisheries did not provide strong evidence of recent declines in availability related to local fishery expansion. A more influential factor was variation in Pacific-wide CPUE, representing overall population abundance and catchability. Exogenous factors, including Pacific-wide fishing pressure, may overwhelm the influence of local fishing pressure on fish availability

### Efficient Implementations of Molecular Dynamics Simulations for Lennard-Jones Systems

Efficient implementations of the classical molecular dynamics (MD) method for
Lennard-Jones particle systems are considered. Not only general algorithms but
also techniques that are efficient for some specific CPU architectures are also
explained. A simple spatial-decomposition-based strategy is adopted for
parallelization. By utilizing the developed code, benchmark simulations are
performed on a HITACHI SR16000/J2 system consisting of IBM POWER6 processors
which are 4.7 GHz at the National Institute for Fusion Science (NIFS) and an
SGI Altix ICE 8400EX system consisting of Intel Xeon processors which are 2.93
GHz at the Institute for Solid State Physics (ISSP), the University of Tokyo.
The parallelization efficiency of the largest run, consisting of 4.1 billion
particles with 8192 MPI processes, is about 73% relative to that of the
smallest run with 128 MPI processes at NIFS, and it is about 66% relative to
that of the smallest run with 4 MPI processes at ISSP. The factors causing the
parallel overhead are investigated. It is found that fluctuations of the
execution time of each process degrade the parallel efficiency. These
fluctuations may be due to the interference of the operating system, which is
known as OS Jitter.Comment: 33 pages, 19 figures, add references and figures are revise

### A numerical algorithm for optimal feedback gains in high dimensional LQR problems

A hybrid method for computing the feedback gains in linear quadratic regulator problems is proposed. The method, which combines the use of a Chandrasekhar type system with an iteration of the Newton-Kleinman form with variable acceleration parameter Smith schemes, is formulated so as to efficiently compute directly the feedback gains rather than solutions of an associated Riccati equation. The hybrid method is particularly appropriate when used with large dimensional systems such as those arising in approximating infinite dimensional (distributed parameter) control systems (e.g., those governed by delay-differential and partial differential equations). Computational advantage of the proposed algorithm over the standard eigenvector (Potter, Laub-Schur) based techniques are discussed and numerical evidence of the efficacy of our ideas presented

### Dynamical breakdown of the Ising spin-glass order under a magnetic field

The dynamical magnetic properties of an Ising spin glass
Fe$_{0.55}$Mn$_{0.45}$TiO$_3$ are studied under various magnetic fields. Having
determined the temperature and static field dependent relaxation time
$\tau(T;H)$ from ac magnetization measurements under a dc bias field by a
general method, we first demonstrate that these data provide evidence for a
spin-glass (SG) phase transition only in zero field. We next argue that the
data $\tau(T;H)$ of finite $H$ can be well interpreted by the droplet theory
which predicts the absence of a SG phase transition in finite fields.Comment: 4 pages, 5 figure

### A unified framework for approximation in inverse problems for distributed parameter systems

A theoretical framework is presented that can be used to treat approximation techniques for very general classes of parameter estimation problems involving distributed systems that are either first or second order in time. Using the approach developed, one can obtain both convergence and stability (continuous dependence of parameter estimates with respect to the observations) under very weak regularity and compactness assumptions on the set of admissible parameters. This unified theory can be used for many problems found in the recent literature and in many cases offers significant improvements to existing results

### Evolution of non-thermal emission from shell associated with AGN jets

We explore the evolution of the emissions by accelerated electrons in shocked
shells driven by jets in active galactic nuclei (AGNs). Focusing on powerful
sources which host luminous quasars, we evaluated the broadband emission
spectra by properly taking into account adiabatic and radiative cooling effects
on the electron distribution. The synchrotron radiation and inverse Compton
(IC) scattering of various photons that are mainly produced in the accretion
disc and dusty torus are considered as radiation processes. We show that the
resultant radiation is dominated by the IC emission for compact sources (<
10kpc), whereas the synchrotron radiation is more important for larger sources.
We also compare the shell emissions with those expected from the lobe under the
assumption that a fractions of the energy deposited in the shell and lobe
carried by the non-thermal electrons are $\epsilon_e \sim 0.01$ and
$\epsilon_{e, lobe} \sim 1$, respectively. Then, we find that the shell
emissions are brighter than the lobe ones at infra-red and optical bands when
the source size is > 10kpc, and the IC emissions from the shell at > 10 GeV can
be observed with the absence of contamination from the lobe irrespective of the
source size. In particular, it is predicted that, for most powerful nearby
sources ($L_j \sim 10^{47} ergs s^{-1}$), TeV gamma-rays produced via the IC
emissions can be detected by the modern Cherenkov telescopes such as MAGIC,
HESS and VERITAS.Comment: 13 pages, 5 figures, accepted for publication in Ap

### Instanton Calculus in R-R 3-form Background and Deformed N=2 Super Yang-Mills Theory

We study the ADHM construction of instantons in N=2 supersymmetric Yang-Mills
theory deformed in constant Ramond-Ramond (R-R) 3-form field strength
background in type IIB superstrings. We compare the deformed instanton
effective action with the effective action of fractional D3/D(-1) branes at the
orbifold singularity of C^2/Z_2 in the same R-R background. We find discrepancy
between them at the second order in deformation parameters, which comes from
the coupling of the translational zero modes of the D(-1)-branes to the R-R
background. We improve the deformed action by adding a term with space-time
dependent gauge coupling. Although the space-time action differs from the
action in the omega-background, both actions lead to the same instanton
equations of motion at the lowest order in gauge coupling.Comment: 27 pages, version to appear in JHE

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