1,614 research outputs found
Charge and Magnetic Moment of the Neutrino in the Background Field Method and in the Linear R_xi^L Gauge
We present a computation of the charge and the magnetic moment of the
neutrino in the recently developed electro-weak Background Field Method and in
the linear gauge. First, we deduce a formal Ward-Takahashi identity
which implies the immediate cancellation of the neutrino electric charge. This
Ward-Takahashi identity is as simple as that for QED. The computation of the
(proper and improper) one loop vertex diagrams contributing to the neutrino
electric charge is also presented in an arbitrary gauge, checking in this way
the Ward-Takahashi identity previously obtained. Finally, the calculation of
the magnetic moment of the neutrino, in the minimal extension of the Standard
Model with massive Dirac neutrinos, is presented, showing its gauge parameter
and gauge structure independence explicitly.Comment: Latex, 19 pages, 9 PS and 10 EPS figures. One reference added.
Appendix B modified and Appendices C-E eliminated. To be published in Eur.
Phys. J.
A short note on the nested-sweep polarized traces method for the 2D Helmholtz equation
We present a variant of the solver in Zepeda-N\'u\~nez and Demanet (2014),
for the 2D high-frequency Helmholtz equation in heterogeneous acoustic media.
By changing the domain decomposition from a layered to a grid-like partition,
this variant yields improved asymptotic online and offline runtimes and a lower
memory footprint. The solver has online parallel complexity that scales
\emph{sub linearly} as , where is
the number of volume unknowns, and is the number of processors, provided
that . The variant in Zepeda-N\'u\~nez and Demanet
(2014) only afforded . Algorithmic scalability is a
prime requirement for wave simulation in regimes of interest for geophysical
imaging.Comment: 5 pages, 5 figure
GoldâCopper Nano-Alloy, âTumbagaâ, in the era of nano: phase diagram and segregation
Goldâcopper (AuâCu) phases were employed already by pre-Columbian civilizations, essentially in decorative arts, whereas nowadays, they emerge in nanotechnology as an important catalyst. The knowledge of the phase diagram is critical to understanding the performance of a material. However, experimental determination of nanophase diagrams is rare because calorimetry remains quite challenging at the nanoscale; theoretical investigations, therefore, are welcomed. Using nanothermodynamics, this paper presents the phase diagrams of various polyhedral nanoparticles (tetrahedron, cube, octahedron, decahedron, dodecahedron, rhombic dodecahedron, truncated octahedron, cuboctahedron, and icosahedron) at sizes 4 and 10 nm. One finds, for all the shapes investigated, that the congruent melting point of these nanoparticles is shifted with respect to both size and composition (copper enrichment). Segregation reveals a gold enrichment at the surface, leading to a kind of coreâshell structure, reminiscent of the historical artifacts. Finally, the most stable structures were determined to be the dodecahedron, truncated octahedron, and icosahedron with a Cu-rich core/Au-rich surface. The results of the thermodynamic approach are compared and supported by molecular-dynamics simulations and by electron-microscopy (EDX) observations
Assembly of Pt nanoparticles on graphitized carbon nanofibers as hierarchically structured electrodes
Carbon-based nanofibers decorated with metallic nanoparticles (NPs) as hierarchically structured electrodes offer significant opportunities for use in low-temperature fuel cells, electrolyzers, flow and air batteries, and electrochemical sensors. We present a facile and scalable method for preparing nanostructured electrodes composed of Pt NPs on graphitized carbon nanofibers. Electrospinning directly addresses the issues related to large-scale production of Pt-based fuel cell electrocatalysts. Through precursors containing polyacrylonitrile and Pt salt electrospinning along with an annealing protocol, we obtain approximately 180 nm thick graphitized nanofibers decorated with approximately 5 nm Pt NPs. By in situ annealing scanning transmission electron microscopy, we qualitatively resolve and quantitatively analyze the unique dynamics of Pt NP formation and movement. Interestingly, by very efficient thermal-induced segregation of all Pt from the inside to the surface of the nanofibers, we increase overall Pt utilization as electrocatalysis is a surface phenomenon. The obtained nanomaterials are also investigated by spatially resolved Raman spectroscopy, highlighting the higher structural order in nanofibers upon doping with Pt precursors. The rationalization of the observed phenomena of segregation and ordering mechanisms in complex carbon-based nanostructured systems is critically important for the effective utilization of all metal-containing catalysts, such as electrochemical oxygen reduction reactions, among many other applications
Fourier Method for Approximating Eigenvalues of Indefinite Stekloff Operator
We introduce an efficient method for computing the Stekloff eigenvalues
associated with the Helmholtz equation. In general, this eigenvalue problem
requires solving the Helmholtz equation with Dirichlet and/or Neumann boundary
condition repeatedly. We propose solving the related constant coefficient
Helmholtz equation with Fast Fourier Transform (FFT) based on carefully
designed extensions and restrictions of the equation. The proposed Fourier
method, combined with proper eigensolver, results in an efficient and clear
approach for computing the Stekloff eigenvalues.Comment: 12 pages, 4 figure
DRIVERS FOR EFFICIENT WATER USE IN AGRICULTURE: AN EMPIRICAL ANALYSIS OF FAMILY FARMS IN ALMERĂA, SPAIN
This paper aims to determine the influence of family farming features on efficient use of water. The analysis focusses on a random sample of family farms in the province of AlmerĂa, southeast Spain. A hierarchical regression model was conducted to determine how the water efficiency is related to environmental awareness and certain characteristics of farmers, decision-makers and the family farm itself. The results show that these family farms strive to be more efficient in their use of water when they are going to be inherited, when there are younger decision-makers who have received a better education, and also when women are involved. Moreover, this efficiency is positively related to more ecological production and to the farmer's habitual behaviour with respect to water economy. The study provides evidence regarding the influences of socio-economic and environmental features of family farming on water use efficiency that may prove useful for other analyses and policy makers on water management in agriculture
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