1,473 research outputs found
Thermal conductance of thin film YIG determined using Bayesian statistics
Thin film YIG (YFeO) is a prototypical material for
experiments on thermally generated pure spin currents and the spin Seebeck
effect. The 3-omega method is an established technique to measure the
cross-plane thermal conductance of thin films, but can not be used in YIG/GGG
(GaGdO) systems in its standard form. We use two-dimensional
modeling of heat transport and introduce a technique based on Bayesian
statistics to evaluate measurement data taken from the 3-omega method. Our
analysis method allows us to study materials systems that have not been
accessible with the conventionally used 3-omega analysis. Temperature dependent
thermal conductance data of thin film YIG are of major importance for
experiments in the field of spin-caloritronics. Here we show data between room
temperature and 10 K for films covering a wide thickness range as well as the
magnetic field effect on the thermal conductance between 10 K and 50 K
Construction of the Pauli-Villars-regulated Dirac vacuum in electromagnetic fields
Using the Pauli-Villars regularization and arguments from convex analysis, we
construct solutions to the classical time-independent Maxwell equations in
Dirac's vacuum, in the presence of small external electromagnetic sources. The
vacuum is not an empty space, but rather a quantum fluctuating medium which
behaves as a nonlinear polarizable material. Its behavior is described by a
Dirac equation involving infinitely many particles. The quantum corrections to
the usual Maxwell equations are nonlinear and nonlocal. Even if photons are
described by a purely classical electromagnetic field, the resulting vacuum
polarization coincides to first order with that of full Quantum
Electrodynamics.Comment: Final version to appear in Arch. Rat. Mech. Analysi
On nonlocal symmetries, nonlocal conservation laws and nonlocal transformations of evolution equations
We discuss nonlocal symmetries and nonlocal conservation laws that follow
from the systematic potentialisation of evolution equations. Those are the Lie
point symmetries of the auxiliary systems, also known as potential symmetries.
We define higher-degree potential symmetries which then lead to nonlocal
conservation laws and nonlocal transformations for the equations. We
demonstrate our approach by the Burgers' hierarchy and the
Calogero-Degasperis-Ibragimov-Shabat hierarchy
From Lagrangian to Quantum Mechanics with Symmetries
We present an old and regretfully forgotten method by Jacobi which allows one
to find many Lagrangians of simple classical models and also of nonconservative
systems. We underline that the knowledge of Lie symmetries generates Jacobi
last multipliers and each of the latter yields a Lagrangian. Then it is shown
that Noether's theorem can identify among those Lagrangians the physical
Lagrangian(s) that will successfully lead to quantization. The preservation of
the Noether symmetries as Lie symmetries of the corresponding Schr\"odinger
equation is the key that takes classical mechanics into quantum mechanics.
Some examples are presented.Comment: To appear in: Proceedings of Symmetries in Science XV, Journal of
Physics: Conference Series, (2012
Current and future technical, economic and environmental feasibility of maize and wheat residues supply for biomass energy application:Illustrated for South Africa
AbstractThis study assessed the feasibility of mobilising maize and wheat residues for large-scale bioenergy applications in South Africa by establishing sustainable residue removal rates and cost of supply based on different production regions. A key objective was to refine the methodology for estimating crop residue harvesting for bioenergy use, while maintaining soil productivity and avoiding displacement of competing residue uses. At current conditions, the sustainable bioenergy potential from maize and wheat residues was estimated to be about 104 PJ. There is potential to increase the amount of crop residues to 238 PJ through measures such as no till cultivation and adopting improved cropping systems. These estimates were based on minimum residues requirements of 2 t ha−1 for soil erosion control and additional residue amounts to maintain 2% SOC level.At the farm gate, crop residues cost between 0.9 and 1.7 GJ−1. In the improved scenario, up to 85% of the biomass is below 1.3 GJ−1 while in the optimised scenario, about 87% is delivered below 5$ GJ−1. Co-firing residues with coal results in lower cost of electricity compared to other renewables and significant GHG (CO2 eq) emissions reduction (up to 0.72 tons MWh−1). Establishing sustainable crop residue supply systems in South Africa could start by utilising the existing agricultural infrastructure to secure supply and develop a functional market. It would then be necessary to incentivise improvements across the value chain
Radiative Corrections to the Casimir Energy
The lowest radiative correction to the Casimir energy density between two
parallel plates is calculated using effective field theory. Since the
correlators of the electromagnetic field diverge near the plates, the
regularized energy density is also divergent. However, the regularized integral
of the energy density is finite and varies with the plate separation L as
1/L^7. This apparently paradoxical situation is analyzed in an equivalent, but
more transparent theory of a massless scalar field in 1+1 dimensions confined
to a line element of length L and satisfying Dirichlet boundary conditions.Comment: 7 pages, Late
Espécies florestais nativas prioritárias para produção de sementes e mudas no estado do Amapá.
O Brasil assumiu compromisso internacional de restaurar e reflorestar 12 milhões de hectares de florestas até 2030, nas suas Contribuições Nacionalmente Determinadas firmadas no Acordo de Paris. Embora o desmatamento ainda não seja um grande problema para o Amapá, sementes e mudas de espécies nativas possuem grande potencial como produto regional capaz de contribuir para geração de renda complementar das comunidades e agricultores que detém florestas e utilizam esses recursos como meio de vida. Por isso é necessário conhecer a importância ecológica e comercial de espécies nativas, para auxiliar a cadeia de produção de sementes e mudas na região. O objetivo do estudo foi realizar um levantamento de espécies nativas prioritárias no estado do Amapá, avaliando o grau de importância econômica e ambiental dessas espécies
Photon-Neutrino Interactions in Magnetic Field through Neutrino Magnetic Moment
We study the neutrino-photon processes like in
the presence of uniform external magnetic field for the case when neutrinos can
couple to the electromagnetic field directly through their dipole magnetic
moment and obtain the stellar energy loss. The process would be of special
relevance in astrophysical situations where standard left-handed neutrinos are
trapped and the right handed neutrinos produced through the spin flip
interaction induced by neutrino magnetic moment alone can freely stream out.Comment: LaTex2e file, 9 page
Thirty-two Goldbach Variations
We give thirty-two diverse proofs of a small mathematical gem--the
fundamental Euler sum identity zeta(2,1)=zeta(3) =8zeta(\bar 2,1). We also
discuss various generalizations for multiple harmonic (Euler) sums and some of
their many connections, thereby illustrating both the wide variety of
techniques fruitfully used to study such sums and the attraction of their
study.Comment: v1: 34 pages AMSLaTeX. v2: 41 pages AMSLaTeX. New introductory
material added and material on inequalities, Hilbert matrix and Witten zeta
functions. Errors in the second section on Complex Line Integrals are
corrected. To appear in International Journal of Number Theory. Title change
gamma nu -> gamma gamma nu and crossed processes at energies below m_W
The cross sections for the processes ,
and are
calculated for a range of center of mass energies from below to
considerably above , but much less than . This enables us to treat
the neutrino--electron coupling as a four--Fermi interaction and results in
amplitudes which are electron box diagrams with three real photons and one
virtual photon at their vertices. These calculations extend our previous
low--energy effective interaction results to higher energies and enable us to
determine where the effective theory is reliable.Comment: 12 pages, RevTex, 10 postscript figures include
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