4,027 research outputs found
Spinning Solitons of a Modified Non-Linear Schroedinger equation
We study soliton solutions of a modified non-linear Schroedinger (MNLS)
equation. Using an Ansatz for the time and azimuthal angle dependence
previously considered in the studies of the spinning Q-balls, we construct
multi-node solutions of MNLS as well as spinning generalisations.Comment: 8 Revtex pages, 5 ps figures; v2: minor change
Synthesis of liquid menthol by hydrogenation of dementholized peppermint oil over Ni catalysts
Hydrogenation of (-)-menthone and (+)-isomenthone was studied at 2.7 MPa and 100 ºC. The objective was to produce a liquid menthol mixture rich in (-)-menthol from dementholized peppermint oil. Ni-based catalysts were tested and compared for this reaction: a) 6 and 12% Ni dispersed into a nonstoichiometric magnesium aluminate (Ni-Mg-Al) with spinel structure; b) Ni-Raney catalyst. Both types of catalysts were active for (-)-menthone and (+)-isomenthone hydrogenation. Lower conversion but higher selectivity to (-)-menthol was obtained with Ni-Mg-Al catalysts. However, they rapidly lost their activity. Instead Ni-Raney catalysts kept its original activity even after several hydrogenation runs
Surface spin-flop transition in a uniaxial antiferromagnetic Fe/Cr superlattice induced by a magnetic field of arbitrary direction
We studied the transition between the antiferromagnetic and the surface
spin-flop phases of a uniaxial antiferromagnetic [Fe(14 \AA)/Cr(11 \AA] superlattice. For external fields applied parallel to the in-plane easy
axis, the layer-by-layer configuration, calculated in the framework of a
mean-field one-dimensional model, was benchmarked against published polarized
neutron reflectivity data. For an in-plane field applied at an angle with the easy axis, magnetometry shows that the magnetization
vanishes at H=0, then increases slowly with increasing . At a critical value
of , a finite jump in is observed for , while a
smooth increase of is found for . A dramatic
increase in the full width at half maximum of the magnetic susceptibility is
observed for . The phase diagram obtained from
micromagnetic calculations displays a first-order transition to a surface
spin-flop phase for low values, while the transition becomes continuous
for greater than a critical angle, . This is in fair agreement with the experimentally observed results.Comment: 24 pages, 7 figure
Ocupação nodular com estirpes de Bradyrhizobium spp. em cultivares de soja sob diferentes disponibilidades hídricas a campo.
A ocupação nodular com estirpes de Bradyrhizobium spp. em cultivares de soja sob diferentes disponibilidades hídricas foi analisada na safra 2007/08 em Londrina-PR. O delineamento experimental utilizado foi o de blocos ao acaso com parcelas sub-divididas, com quatro repetições. As parcelas principais receberam três tratamentos (1- déficit hídrico nos estádios reprodutivos - DHER, 2- condições normais de campo - CNC e 3- condições ótimas de umidade - IRR); e as sub-parcelas, dez cultivares de soja. Os nódulos foram tipificados pelo método de ELISA, com anti-soros das estirpes SEMIA 5079, SEMIA 5080, SEMIA 587 e SEMIA 5019. Não houve diferença significativa para a ocupação nodular em relação à disponibilidade hídrica, porém, estes resultados podem estar relacionados ao fato da indução de déficit hídrico ter ocorrido na fase reprodutiva, quando a nodulação já estava estabelecida. As estirpes inoculadas, quando consideradas em conjunto, foram mais competitivas na nodulação do que as estirpes naturalizadas no solo, sendo a SEMIA 587 a que apresentou maior ocupação nodular. Não foi possível identificar uma estirpe mais tolerante ao déficit hídrico
The Definition of Mach's Principle
Two definitions of Mach's principle are proposed. Both are related to gauge
theory, are universal in scope and amount to formulations of causality that
take into account the relational nature of position, time, and size. One of
them leads directly to general relativity and may have relevance to the problem
of creating a quantum theory of gravity.Comment: To be published in Foundations of Physics as invited contribution to
Peter Mittelstaedt's 80th Birthday Festschrift. 30 page
Tachyons on Dp-branes from Abelian Higgs sphalerons
We consider the Abelian Higgs model in a (p+2)-dimensional space time with
topology M^{p+1} x S^1 as a field theoretical toy model for tachyon
condensation on Dp-branes. The theory has periodic sphaleron solutions with the
normal mode equations resembling Lame-type equations. These equations are
quasi-exactly solvable (QES) for specific choices of the Higgs- to gauge boson
mass ratio and hence a finite number of algebraic normal modes can be computed
explicitely. We calculate the tachyon potential for two different values of the
Higgs- to gauge boson mass ratio and show that in comparison to previously
studied pure scalar field models an exact cancellation between the negative
energy contribution at the minimum of the tachyon potential and the brane
tension is possible for the simplest truncation in the expansion about the
field around the sphaleron. This gives further evidence for the correctness of
Sen's conjecture.Comment: 14 Latex pages including 3 eps-figure
Static non-reciprocity in mechanical metamaterials
Reciprocity is a fundamental principle governing various physical systems,
which ensures that the transfer function between any two points in space is
identical, regardless of geometrical or material asymmetries. Breaking this
transmission symmetry offers enhanced control over signal transport, isolation
and source protection. So far, devices that break reciprocity have been mostly
considered in dynamic systems, for electromagnetic, acoustic and mechanical
wave propagation associated with spatio-temporal variations. Here we show that
it is possible to strongly break reciprocity in static systems, realizing
mechanical metamaterials that, by combining large nonlinearities with suitable
geometrical asymmetries, and possibly topological features, exhibit vastly
different output displacements under excitation from different sides, as well
as one-way displacement amplification. In addition to extending non-reciprocity
and isolation to statics, our work sheds new light on the understanding of
energy propagation in non-linear materials with asymmetric crystalline
structures and topological properties, opening avenues for energy absorption,
conversion and harvesting, soft robotics, prosthetics and optomechanics.Comment: 19 pages, 3 figures, Supplementary information (11 pages and 5
figures
Shock Ignitiion: A New Approach to High Gain/Yield Targets for the National Ignition Facility
There is an obstetrical dilemma: Misconceptions about the evolution of human childbirth and pelvic form
Compared to other primates, modern humans face high rates of maternal and neonatal morbidity and mortality during childbirth. Since the early 20th century, this “difficulty” of human parturition has prompted numerous evolutionary explanations, typically assuming antagonistic selective forces acting on maternal and fetal traits, which has been termed the “obstetrical dilemma.” Recently, there has been a growing tendency among some anthropologists to question the difficulty of human childbirth and its evolutionary origin in an antagonistic selective regime. Partly, this stems from the motivation to combat increasing pathologization and overmedicalization of childbirth in industrialized countries. Some authors have argued that there is no obstetrical dilemma at all, and that the difficulty of childbirth mainly results from modern lifestyles and inappropriate and patriarchal obstetric practices. The failure of some studies to identify biomechanical and metabolic constraints on pelvic dimensions is sometimes interpreted as empirical support for discarding an obstetrical dilemma. Here we explain why these points are important but do not invalidate evolutionary explanations of human childbirth. We present robust empirical evidence and solid evolutionary theory supporting an obstetrical dilemma, yet one that is much more complex than originally conceived in the 20th century. We argue that evolutionary research does not hinder appropriate midwifery and obstetric care, nor does it promote negative views of female bodies. Understanding the evolutionary entanglement of biological and sociocultural factors underlying human childbirth can help us to understand individual variation in the risk factors of obstructed labor, and thus can contribute to more individualized maternal care
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