775 research outputs found
Methods of electron beam bunching
A review of electron beam bunching methods and new bunching methods are
presented. Linear accelerator, storage ring and buncher technologies offer
high-beam quality and short bunches necessary for operation of high efficiency,
high-power, high-degree monochromatic prebunched FELs in optical and X-ray
regions.Comment: 4 pages, LaTe
Perspectives of research on detritus: Do factors controlling the availability of detritus to macroconsumers depend on its source?
Various factors have been suggested as regulating the availability of detritus to detritivores, e.g., particle-size and microbial dynamics, nutritional composition of detritus per se, and chemical inhibitors/ enhancers. These factors might vary in importance depending on detritus source: fecal pellets, vascular plants, or seaweed-derived detritus. We review the literature to evaluate the above factors and suggest that future research on detritus should consider regulatory mechanisms which depend on detritus type
On Parity-Violating Three-Nucleon Interactions and the Predictive Power of Few-Nucleon EFT at Very Low Energies
We address the typical strengths of hadronic parity-violating three-nucleon
interactions in "pion-less" Effective Field Theory in the nucleon-deuteron
(iso-doublet) system. By analysing the superficial degree of divergence of loop
diagrams, we conclude that no such interactions are needed at leading order.
The only two linearly independent parity-violating three-nucleon structures
with one derivative mix two-S and two-P-half waves with iso-spin transitions
Delta I = 0 or 1. Due to their structure, they cannot absorb any divergence
ostensibly appearing at next-to-leading order. This observation is based on the
approximate realisation of Wigner's combined SU(4) spin-isospin symmetry in the
two-nucleon system, even when effective-range corrections are included.
Parity-violating three-nucleon interactions thus only appear beyond
next-to-leading order. This guarantees renormalisability of the theory to that
order without introducing new, unknown coupling constants and allows the direct
extraction of parity-violating two-nucleon interactions from three-nucleon
experiments.Comment: 20 pages LaTeX2e, including 9 figures as .eps file embedded with
includegraphicx. Minor modifications and stylistic corrections. Version
accepted for publication in Eur. Phys. J.
Twisted k-graph algebras associated to Bratteli diagrams
Given a system of coverings of k-graphs, we show that the cohomology of the
resulting (k+1)-graph is isomorphic to that of any one of the k-graphs in the
system. We then consider Bratteli diagrams of 2-graphs whose twisted
C*-algebras are matrix algebras over noncommutative tori. For such systems we
calculate the ordered K-theory and the gauge-invariant semifinite traces of the
resulting 3-graph C*-algebras. We deduce that every simple C*-algebra of this
form is Morita equivalent to the C*-algebra of a rank-2 Bratteli diagram in the
sense of Pask-Raeburn-R{\o}rdam-Sims.Comment: 28 pages, pictures prepared using tik
Parity-violating neutron spin rotation in hydrogen and deuterium
We calculate the (parity-violating) spin rotation angle of a polarized
neutron beam through hydrogen and deuterium targets, using pionless effective
field theory up to next-to-leading order. Our result is part of a program to
obtain the five leading independent low-energy parameters that characterize
hadronic parity-violation from few-body observables in one systematic and
consistent framework. The two spin-rotation angles provide independent
constraints on these parameters. Using naive dimensional analysis to estimate
the typical size of the couplings, we expect the signal for standard target
densities to be 10^-7 to 10^-6 rad/m for both hydrogen and deuterium targets.
We find no indication that the nd observable is enhanced compared to the np
one. All results are properly renormalized. An estimate of the numerical and
systematic uncertainties of our calculations indicates excellent convergence.
An appendix contains the relevant partial-wave projectors of the three-nucleon
system.Comment: 44 pages, 17 figures; minor corrections; to be published in EPJ
Semilattices of groups and nonstable K-theory of extended Cuntz limits
We give an elementary characterization of those abelian monoidsM
that are direct limits of countable sequences of finite direct sums of monoids
of the form either (Z/nZ) ⊔ {0} or Z ⊔ {0}. This characterization involves the
Riesz refinement property together with lattice-theoretical properties of the
collection of all subgroups of M (viewed as a semigroup), and it makes it pos-
sible to express M as a certain submonoid of a direct product ×G, where
is a distributive semilattice with zero and G is an abelian group. When applied
to the monoids V (A) appearing in the nonstable K-theory of C*-algebras, our
results yield a full description of V (A) for C*-inductive limits A of finite sums
of full matrix algebras over either Cuntz algebras On, where 2 ≤ n < ∞, or
corners of O1 by projections, thus extending to the case including O1 earlier
work by the authors together with K.R. Goodearl
Investigation of structure and hydrogen bonding of super-hydrous phase B (HT) under pressure using first principles density functional calculations
High pressure behaviour of superhydrous phase B(HT) of Mg10Si3O14(OH)4 (Shy
B) is investigated with the help of density functional theory based first
principles calculations. In addition to the lattice parameters and equation of
state, we use these calculations to determine the positional parameters of
atoms as a function of pressure. Our results show that the compression induced
structural changes involve cooperative distortions in the full geometry of the
hydrogen bonds. The bond bending mechanism proposed by Hofmeister et al [1999]
for hydrogen bonds to relieve the heightened repulsion due to short H--H
contacts is not found to be effective in Shy B. The calculated O-H bond
contraction is consistent with the observed blue shift in the stretching
frequency of the hydrogen bond. These results establish that one can use first
principles calculations to obtain reliable insights into the pressure induced
bonding changes of complex minerals.Comment: 16 pages, 4 figure
Localization by disorder in the infrared conductivity of (Y,Pr)Ba2Cu3O7 films
The ab-plane reflectivity of (Y{1-x}Prx)Ba2Cu3O7 thin films was measured in
the 30-30000 cm-1 range for samples with x = 0 (Tc = 90 K), x = 0.4 (Tc = 35 K)
and x = 0.5 (Tc = 19 K) as a function of temperature in the normal state. The
effective charge density obtained from the integrated spectral weight decreases
with increasing x. The variation is consistent with the higher dc resistivity
for x = 0.4, but is one order of magnitude smaller than what would be expected
for x = 0.5. In the latter sample, the conductivity is dominated at all
temperatures by a large localization peak. Its magnitude increases as the
temperature decreases. We relate this peak to the dc resistivity enhancement. A
simple localization-by-disorder model accounts for the optical conductivity of
the x = 0.5 sample.Comment: 7 pages with (4) figures include
Percepção ambiental dos produtores e qualidade do solo em propriedades orgânicas e convencionais.
O sistema orgânico de produção deve resultar na utilização mais racional dos recursos naturais, sobretudo do solo. Os produtores orgânicos devem adotar práticas conservacionistas, além de atender a outros princípios deste sistema de produção. Neste trabalho foram avaliadas as práticas de manejo e conservação do solo adotadas por produtores orgânicos e convencionais nas regiões dos municípios de Socorro (Serra da Mantiqueira e Mogiana) e Ibiúna, situados no Estado de São Paulo. O objetivo dessa pesquisa foi verificar o grau de adoção de práticas conservacionistas pelos produtores e avaliar a qualidade do solo na agricultura orgânica e convencional. Foi elaborado e aplicado um questionário, com visitas locais para verificação das práticas conservacionistas e ocorrência de erosão, em 30 propriedades. Os resultados foram expressos em índices indicadores de diversidade de uso do solo, atitude conservacionista, percepção de erosão e do impacto sobre os recursos hídricos. Foram também realizadas análises de atributos químicos, físicos e biológicos indicadores da qualidade do solo. Concluiu-se que os produtores orgânicos têm maior percepção quanto à atitude conservacionista e nas propriedades orgânicas há maior diversidade de cultivos, embora não haja maior diversidade geral de uso do solo. A produção orgânica de alface, em Ibiúna proporcionou maior atividade biológica no solo e maior colonização por fungos micorrízicos arbusculares. O sistema orgânico possui maiores teores de matéria orgânica e menores de potássio e não houve diferença entre os atributos físicos do solo dos sistemas orgânico e convencional
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