11,337 research outputs found
Simulations of stable compact proton beam acceleration from a two-ion-species ultrathin foil
We report stable laser-driven proton beam acceleration from ultrathin foils
consisting of two ion species: heavier carbon ions and lighter protons.
Multi-dimensional particle-in-cell (PIC) simulations show that the radiation
pressure leads to very fast and complete spatial separation of the species. The
laser pulse does not penetrate the carbon ion layer, avoiding the proton
Rayleigh-Taylor-like (RT) instability. Ultimately, the carbon ions are heated
and spread extensively in space. In contrast, protons always ride on the front
of the carbon ion cloud, forming a compact high quality bunch. We introduce a
simple three-interface model to interpret the instability suppression in the
proton layer. The model is backed by simulations of various compound foils such
as carbon-deuterium (C-D) and carbon-tritium (C-T) foils. The effects of the
carbon ions' charge state on proton acceleration are also investigated. It is
shown that with the decrease of the carbon ion charge state, both the RT-like
instability and the Coulomb explosion degrade the energy spectrum of the
protons. Finally, full 3D simulations are performed to demonstrate the
robustness of the stable two-ion-species regime.Comment: 14 pages, 10figures, to be published in PO
Local Periodicity-Based Beat Tracking for Expressive Classical Piano Music
To model the periodicity of beats, state-of-the-art beat tracking systems use
"post-processing trackers" (PPTs) that rely on several empirically determined
global assumptions for tempo transition, which work well for music with a
steady tempo. For expressive classical music, however, these assumptions can be
too rigid. With two large datasets of Western classical piano music, namely the
Aligned Scores and Performances (ASAP) dataset and a dataset of Chopin's
Mazurkas (Maz-5), we report on experiments showing the failure of existing PPTs
to cope with local tempo changes, thus calling for new methods. In this paper,
we propose a new local periodicity-based PPT, called predominant local
pulse-based dynamic programming (PLPDP) tracking, that allows for more flexible
tempo transitions. Specifically, the new PPT incorporates a method called
"predominant local pulses" (PLP) in combination with a dynamic programming (DP)
component to jointly consider the locally detected periodicity and beat
activation strength at each time instant. Accordingly, PLPDP accounts for the
local periodicity, rather than relying on a global tempo assumption. Compared
to existing PPTs, PLPDP particularly enhances the recall values at the cost of
a lower precision, resulting in an overall improvement of F1-score for beat
tracking in ASAP (from 0.473 to 0.493) and Maz-5 (from 0.595 to 0.838).Comment: Accepted to IEEE/ACM Transactions on Audio, Speech, and Language
Processing (July 2023
Modified London Equation, Abrikosov-Like Vortices and Knot Solitons in Two-Gap Superconductors
We derive the exact modified London equation for the two-gap superconductor,
compare it with its single-gap counterpart. We show that the vortices in the
two-gap superconductor are soft (or continuous) core vortices. In particular,
we discuss the topological structure of the finite energy vortices
(Abrikosov-like vortices), and find that they can be viewed as the incarnation
of the baby skyrmion stretched in the third direction. Besides, we point out
that the knot soliton in the two-gap superconductor is the twisted
Abrikosov-like vortex with its two periodic ends connected smoothly. The
relation between the magnetic monopoles and the Abrikosov-like vortices is also
discussed briefly.Comment: 3 pages, no figure
Induced magnetization in LaSrMnO/BiFeO superlattices
Using polarized neutron reflectometry (PNR), we observe an induced
magnetization of 75 25 kA/m at 10 K in a LaSrMnO
(LSMO)/BiFeO superlattice extending from the interface through several
atomic layers of the BiFeO (BFO). The induced magnetization in BFO is
explained by density functional theory, where the size of bandgap of BFO plays
an important role. Considering a classical exchange field between the LSMO and
BFO layers, we further show that magnetization is expected to extend throughout
the BFO, which provides a theoretical explanation for the results of the
neutron scattering experiment.Comment: 5 pages, 4 figures, with Supplemental Materials. To appear in
Physical Review Letter
Anomalous spin susceptibility and magnetic polaron formation in the double exchange systems
The magnetic susceptibility and spin-spin correlation of the double-exchange
model for doped manganites are investigated through the Monte Carlo
calculations on the three-dimensional lattice model. Deviations of the
susceptibility from the Curie-Weiss behavior above the ferromagnetic ordering
temperature seem to indicate a formation of local ferromagnetic clusters
in the vicinity of , which is consistent with recent electron paramagnetic
resonance experiments for LaCaMnO. A further analysis of
the spin-spin correlations show the ferromagnetic cluster size to be
three-to-four lattice spacings, suggesting that the charge carriers may form
magnetic polarons.Comment: 5 pages, 5 figures, Late
Positive and negative streamers in ambient air: modeling evolution and velocities
We simulate short positive and negative streamers in air at standard
temperature and pressure. They evolve in homogeneous electric fields or emerge
from needle electrodes with voltages of 10 to 20 kV. The streamer velocity at
given streamer length depends only weakly on the initial ionization seed,
except in the case of negative streamers in homogeneous fields. We characterize
the streamers by length, head radius, head charge and field enhancement. We
show that the velocity of positive streamers is mainly determined by their
radius and in quantitative agreement with recent experimental results both for
radius and velocity. The velocity of negative streamers is dominated by
electron drift in the enhanced field; in the low local fields of the present
simulations, it is little influenced by photo-ionization. Though negative
streamer fronts always move at least with the electron drift velocity in the
local field, this drift motion broadens the streamer head, decreases the field
enhancement and ultimately leads to slower propagation or even extinction of
the negative streamer.Comment: 18 pages, 10 figure
Pair production of neutralinos via gluon-gluon collisions
The production of a neutralino pair via gluon-gluon fusion is studied in the
minimal supersymmetric model(MSSM) at proton-proton colliders. The numerical
analysis of their production rates are carried out in the mSUGRA scenario. The
results show that this cross section may reach about 80 femto barn for
pair production and 23 femto barn
for pair production with suitable
input parameters at the future LHC collider. It shows that this loop mediated
process can be competitive with the quark-antiquark annihilation process at the
LHC.Comment: LaTex file, l4 pages, 5 EPS figure
Extended Smoothed Boundary Method for Solving Partial Differential Equations with General Boundary Conditions on Complex Boundaries
In this article, we describe an approach for solving partial differential
equations with general boundary conditions imposed on arbitrarily shaped
boundaries. A continuous function, the domain parameter, is used to modify the
original differential equations such that the equations are solved in the
region where a domain parameter takes a specified value while boundary
conditions are imposed on the region where the value of the domain parameter
varies smoothly across a short distance. The mathematical derivations are
straightforward and generically applicable to a wide variety of partial
differential equations. To demonstrate the general applicability of the
approach, we provide four examples herein: (1) the diffusion equation with both
Neumann and Dirichlet boundary conditions; (2) the diffusion equation with both
surface diffusion and reaction; (3) the mechanical equilibrium equation; and
(4) the equation for phase transformation with the presence of additional
boundaries. The solutions for several of these cases are validated against
corresponding analytical and semi-analytical solutions. The potential of the
approach is demonstrated with five applications: surface-reaction-diffusion
kinetics with a complex geometry, Kirkendall-effect-induced deformation,
thermal stress in a complex geometry, phase transformations affected by
substrate surfaces, and a self-propelled droplet.Comment: This document is the revised version of arXiv:0912.1288v
Time-Resolved Photoluminescence of Quaternary AlInGaN-Based Multiple Quantum Wells
Time-resolvedphotoluminescence(PL)dynamics has been studied in AlInGaN/AlInGaN multiple quantum wells(MQWs) grown by a pulsed metalorganic chemical vapor deposition (PMOCVD) procedure. The PL decay kinetics was found to be sensitive to the emission energy and temperature. The PL decay time increases with decreasing emission energy, which is a characteristic of localized carrier/exciton recombination due to alloy fluctuations. Its temperature dependence shows radiative recombination to be the dominant process at low temperatures, indicating a high quality of PMOCVD grown quaternary AlInGaN MQWs and establishing them as promising structures for the active region of deep ultraviolet light emitting diodes
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