464 research outputs found
Photoelasticity as a Research Technique for Analyzing Stresses in Dental Structures
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/67063/2/10.1177_00220345550340060601.pd
Delta Baryon Magnetic Moments From Lattice QCD
Theoretical predictions for the magnetic moments of the physical Delta
baryons are extracted from lattice QCD calculations. We utilize finite-range
regulated effective field theory that is constructed to have the correct Dirac
moment mass dependence in the region where the up and down quark masses are
heavy. Of particular interest is the chiral nonanalytic behaviour encountered
as the nucleon-pion decay channel opens. We find a Delta^++ magnetic moment (at
the Delta pole) of 4.99 \pm 0.56 \mu_N. This result is within the Particle Data
Group range of 3.7-7.5 \mu_N and compares well with the experimental result of
Bosshard et al. of 4.52 \pm 0.51 \pm 0.45 \mu_N. The interplay between the
different pion-loop contributions to the Delta^+ magnetic moment leads to the
surprising result that the proton moment may exceed that of the Delta^+,
contrary to conventional expectations.Comment: 7 pages, 7 figures, RevTex 4; Updated to include a recent
experimental resul
Electromagnetic Response of Layered Superconductors with Broken Lattice Inversion Symmetry
We investigate the macroscopic effects of charge density waves (CDW) and
superconductivity in layered superconducting systems with broken lattice
inversion symmetry (allowing for piezoelectricity) such as two dimensional (2D)
transition metal dichalcogenides (TMD). We work with the low temperature time
dependent Ginzburg-Landau theory and study the coupling of lattice distortions
and low energy CDW collective modes to the superconducting order parameter in
the presence of electromagnetic fields. We show that superconductivity and
piezoelectricity can coexist in these singular metals. Furthermore, our study
indicates the nature of the quantum phase transition between a commensurate CDW
phase and the stripe phase that has been observed as a function of applied
pressure.Comment: 9 pages, 1 figure. Final version. Accepted in Phys.Rev.
Relativistic treatment of harmonics from impurity systems in quantum wires
Within a one particle approximation of the Dirac equation we investigate a
defect system in a quantum wire. We demonstrate that by minimally coupling a
laser field of frequency omega to such an impurity system, one may generate
harmonics of multiples of the driving frequency. In a multiple defect system
one may employ the distance between the defects in order to tune the cut-off
frequency.Comment: 9 pages Latex, 8 eps figures, section added, numerics improve
Schroedingers equation with gauge coupling derived from a continuity equation
We consider a statistical ensemble of particles of mass m, which can be
described by a probability density \rho and a probability current \vec{j} of
the form \rho \nabla S/m. The continuity equation for \rho and \vec{j} implies
a first differential equation for the basic variables \rho and S. We further
assume that this system may be described by a linear differential equation for
a complex state variable \chi. Using this assumptions and the simplest possible
Ansatz \chi(\rho,S) Schroedingers equation for a particle of mass m in an
external potential V(q,t) is deduced. All calculations are performed for a
single spatial dimension (variable q) Using a second Ansatz \chi(\rho,S,q,t)
which allows for an explict q,t-dependence of \chi, one obtains a generalized
Schroedinger equation with an unusual external influence described by a
time-dependent Planck constant. All other modifications of Schroeodingers
equation obtained within this Ansatz may be eliminated by means of a gauge
transformation. Thus, this second Ansatz may be considered as a generalized
gauging procedure. Finally, making a third Ansatz, which allows for an
non-unique external q,t-dependence of \chi, one obtains Schroedingers equation
with electromagnetic potentials \vec{A}, \phi in the familiar gauge coupling
form. A possible source of the non-uniqueness is pointed out.Comment: 25 pages, no figure
Observational constraints on the nature of very short gamma-ray bursts
We discuss a very peculiar subgroup of gamma-ray bursts among the BATSE
sources. These bursts are very short (0.1 s), hard, and came
predominantly from a restricted direction of the sky (close to the Galactic
anti-center). We analyze their arrival times and possible correlations, as well
as the profiles of individual bursts. We find no peculiarities in the arrival
times of Very Short Bursts (VSBs) despite their highly non-uniform spatial
distribution. There is no dependence in the burst shapes on location. Bursts
coming both from the burst-enhancement Galactic Anticenter region and from all
other directions show considerable dispersion in their rise and fall times.
Significant fraction of VSBs have multiple peaks despite their extremely short
duration. Burst time properties are most likely to be consistent with two
origin mechanisms: either with binary NS-NS mergers with low total masses
passing through a phase of hypermassive neutron star, or with evaporation of
the primordial black holes in the scenario of no photosphere formation.Comment: 25 pages, 14 figures; accepted to New Astronom
Optimised mixing and flow resistance during shear flow over a rib roughened boundary
A series of numerical investigations has been performed to study the effect of lower boundary roughness on turbulent flow in a two-dimensional channel. The roughness spacing to height ratio, w/k, has been investigated over the range 0.12 to 402 by varying the horizontal rib spacing. The square roughness elements each have a cross-sectional area of (0.05 H)2, where H is the full channel height. The Reynolds number, Reτ is fixed based on the value of the imposed pressure gradient, dp/dx, and is in the range 6.3 × 103 − 4.5 × 104. A Reynolds Averaged Navier–Stokes (RANS) based turbulence modelling approach is adopted using a commercial CFD code, ANSYS-CFX 14.0. Measurements of eddy viscosity and friction factor have been made over this range to establish the optimum spacings to produce maximum turbulence enhancement, mixing and resistance to flow. These occur when w/k is approximately 7. It is found that this value is only weakly dependent on Reynolds number, and the decay rate of turbulence enhancement as a function of w/k ratio beyond this optimum spacing is slow. The implications for heat transfer design optimisation and particle transport are considered
Impacts of salvage logging on biodiversity: A meta-analysis
Logging to "salvage" economic returns from forests affected by natural disturbances has become increasingly prevalent globally. Despite potential negative effects on biodiversity, salvage logging is often conducted, even in areas otherwise excluded from logging and reserved for nature conservation, inter alia because strategic priorities for post-disturbance management are widely lacking. A review of the existing literature revealed that most studies investigating the effects of salvage logging on biodiversity have been conducted less than 5 years following natural disturbances, and focused on non-saproxylic organisms. A meta-analysis across 24 species groups revealed that salvage logging significantly decreases numbers of species of eight taxonomic groups. Richness of dead wood dependent taxa (i.e. saproxylic organisms) decreased more strongly than richness of non-saproxylic taxa. In contrast, taxonomic groups typically associated with open habitats increased in the number of species after salvage logging. By analysing 134 original species abundance matrices, we demonstrate that salvage logging significantly alters community composition in 7 of 17 species groups, particularly affecting saproxylic assemblages. Synthesis and applications. Our results suggest that salvage logging is not consistent with the management objectives of protected areas. Substantial changes, such as the retention of dead wood in naturally disturbed forests, are needed to support biodiversity. Future research should investigate the amount and spatio-temporal distribution of retained dead wood needed to maintain all components of biodiversity
Estimating cassava yield in future IPCC climate scenarios for the Rio Grande do Sul State, Brazil
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