7,836 research outputs found
Statistical Inference for Partially Observed Markov Processes via the R Package pomp
Partially observed Markov process (POMP) models, also known as hidden Markov
models or state space models, are ubiquitous tools for time series analysis.
The R package pomp provides a very flexible framework for Monte Carlo
statistical investigations using nonlinear, non-Gaussian POMP models. A range
of modern statistical methods for POMP models have been implemented in this
framework including sequential Monte Carlo, iterated filtering, particle Markov
chain Monte Carlo, approximate Bayesian computation, maximum synthetic
likelihood estimation, nonlinear forecasting, and trajectory matching. In this
paper, we demonstrate the application of these methodologies using some simple
toy problems. We also illustrate the specification of more complex POMP models,
using a nonlinear epidemiological model with a discrete population,
seasonality, and extra-demographic stochasticity. We discuss the specification
of user-defined models and the development of additional methods within the
programming environment provided by pomp.Comment: In press at the Journal of Statistical Software. A version of this
paper is provided at the pomp package website: http://kingaa.github.io/pom
Magnetic Excitations in the High Tc Iron Pnictides
We calculate the expected finite frequency neutron scattering intensity based
on the two-sublattice collinear antiferromagnet found by recent neutron
scattering experiments as well as by theoretical analysis on the iron
oxypnictide LaOFeAs. We consider two types of superexchange couplings between
Fe atoms: nearest-neighbor coupling J1 and next-nearest-neighbor coupling J2.
We show how to distinguish experimentally between ferromagnetic and
antiferromagnetic J1. Whereas magnetic excitations in the cuprates display a
so-called resonance peak at (pi,pi) (corresponding to a saddlepoint in the
magnetic spectrum) which is at a wavevector that is at least close to nesting
Fermi-surface-like structures, no such corresponding excitations exist in the
iron pnictides. Rather, we find saddlepoints near (pi,pi/2) and (0,pi/2)(and
symmetry related points). Unlike in the cuprates, none of these vectors are
close to nesting the Fermi surfaces.Comment: 4 pages, 5 figure
Spin-chirality decoupling in the one-dimensional Heisenberg spin glass with long-range power-law interactions
We study the issue of the spin-chirality decoupling/coupling in the ordering
of the Heisenberg spin glass by performing large-scale Monte Carlo simulations
on a one-dimensional Heisenberg spin-glass model with a long-range power-law
interaction up to large system sizes. We find that the spin-chirality
decoupling occurs for an intermediate range of the power-law exponent.
Implications to the corresponding -dimensional short-range model is
discussed.Comment: 5 pages, 4 figures, to appear in Physical Review Letter
Monte Carlo studies of the chiral and spin orderings of the three-dimensional Heisenberg spin glass
The nature of the ordering of the three-dimensional isotropic Heisenberg spin
glass with nearest-neighbor random Gaussian coupling is studied by extensive
Monte Carlo simulations. Several independent physical quantities are measured
both for the spin and for the chirality, including the correlation-length
ratio, the Binder ratio, the glass order parameter, the overlap distribution
function and the non-self-averageness parameter. By controlling the effect of
the correction-to-scaling, we have obtained a numerical evidence for the
occurrence of successive chiral-glass and spin-glass transitions at nonzero
temperatures, T_{CG} > T_{SG} > 0. Hence, the spin and the chirality are
decoupled in the ordering of the model. The chiral-glass exponents are
estimated to be \nu_{CG}=1.4+-0.2 and \eta_{CG}=0.6+-0.2, indicating that the
chiral-glass transition lies in a universality class different from that of the
Ising spin glass. The possibility that the spin and chiral sectors undergo a
simultaneous Kosterlitz-Thouless-type transition is ruled out. The chiral-glass
state turns out to be non-self-averaging, possibly accompanying a one-step-like
peculiar replica-symmetry breaking. Implications to the chirality scenario of
experimental spin-glass transitions are discussed.Comment: 20 pages, 24 figures. The Chi^2-analysis of the transition point has
been added with new Fig.12. Some references also adde
Solar generation and storage of O2 (a 1 delta g)
An investigation was performed of the technical steps required to design a solar powered oxygen-iodine laser. Singlet delta oxygen is formed upon transfer of energy from selected photoexcited dye molecules to ground state molecular oxygen and then is concentrated and stored as an endoperoxide by reaction with an aromatic hydrocarbon. The endoperoxide, when heated, releases singlet oxygen in high yield thus providing a regenerable source of laser fuel. Energy transfer from dye molecules to molecular oxygen was investigated. When dye molecules were adsorbed to polymer substrates it was observed that the dye became embedded in the polymer matrix. Porphin dyes were incorporated into films of 1,4-dimethyl-2-poly(vinylnaphthalene), 2PVN. An endoperoxide was formed when porphin-doped 2PVN was exposed to visible radiation. This demonstrates the possibility of generating singlet oxygen using solar energy and concentrating and storing it in one simple step. Transport of energy by exciton migration in polycrystalline dye films was also investigated
Detection of bottom ferromagnetic electrode oxidation in magnetic tunnel junctions by magnetometry measurements
Surface oxidation of the bottom ferromagnetic (FM) electrode, one of the
major detrimental factors to the performance of a Magnetic Tunnel Junction
(MTJ), is difficult to avoid during the fabrication process of the MTJ's tunnel
barrier. Since Co rich alloys are commonly used for the FM electrodes in MTJs,
over-oxidation of the tunnel barrier results in the formation of a CoO
antiferromagnetic (AF) interface layer which couples with the bottom FM
electrode to form a typical AF/FM exchange bias (EB) system. In this work,
surface oxidation of the CoFe and CoFeB bottom electrodes was detected via
magnetometry measurements of exchange-bias characterizations including the EB
field, training effect, uncompensated spin density, and coercivity. Variations
of these parameters were found to be related to the surface oxidation of the
bottom electrode, among them the change of coercivity is most sensitive.
Annealed samples show evidence for an oxygen migration back to the MgO tunnel
barrier by annealing.Comment: 5 pages, 4 figues, submitted to J. Appl. Phy
Neutron transition strengths of states in the neutron rich Oxygen isotopes determined from inelastic proton scattering
A coupled-channel analysis of the O data has been
performed to determine the neutron transition strengths of 2 states in
Oxygen targets, using the microscopic optical potential and inelastic form
factor calculated in the folding model. A complex density- and \emph{isospin}
dependent version of the CDM3Y6 interaction was constructed, based on the
Brueckner-Hatree-Fock calculation of nuclear matter, for the folding model
input. Given an accurate isovector density dependence of the CDM3Y6
interaction, the isoscalar () and isovector () deformation
lengths of 2 states in O have been extracted from the
folding model analysis of the data. A specific -dependence of
and has been established which can be linked to the
neutron shell closure occurring at approaching 16. The strongest isovector
deformation was found for 2 state in O, with about 2.5
times larger than , which indicates a strong core polarization by the
valence neutrons in O. The ratios of the neutron/proton transition
matrix elements () determined for 2 states in O have
been compared to those deduced from the mirror symmetry, using the measured
values of 2 states in the proton rich Ne and Mg
nuclei, to discuss the isospin impurity in the excitation of the
and isobars.Comment: Version accepted for publication in Physical Review
Measuring the one-particle excitations of ultracold fermionic atoms by stimulated Raman spectroscopy
We propose a Raman spectroscopy technique which is able to probe the
one-particle Green's function, the Fermi surface, and the quasiparticles of a
gas of strongly interacting ultracold atoms. We give quantitative examples of
experimentally accessible spectra. The efficiency of the method is validated by
means of simulated images for the case of a usual Fermi liquid as well as for
more exotic states: specific signatures of e.g. a d-wave pseudo-gap are clearly
visible.Comment: 5 pages, 3 figures accepted for publication at Phys. Rev. Letter
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