15,542 research outputs found
A Deformable Model for Magnetic Vortex Pinning
A two-parameter analytical model of the magnetic vortex in a thin disk of
soft magnetic material is constructed. The model is capable of describing the
change in evolution of net vortex state magnetization and of core position when
the vortex core interacts with a magnetic pinning site. The model employs a
piecewise, physically continuous, magnetization distribution obtained by the
merger of two extensively used one-parameter analytical models of the vortex
state in a disk. Through comparison to numerical simulations of ideal disks
with and without pinning sites, the model is found to accurately predict the
magnetization, vortex position, hysteretic transitions, and 2-D displacement of
the vortex in the presence of pinning sites. The model will be applicable to
the quantitative determination of vortex pinning energies from measurements of
magnetization.Comment: 27 pages, 7 figures, including supplementary information, ancillary
files:3 supplementary movie
On the Naturalness of Higgs Inflation
We critically examine the recent claim that the Standard Model Higgs boson
could drive inflation in agreement with observations if has a strong coupling to the Ricci curvature scalar. We
first show that the effective theory approach upon which that claim is based
ceases to be valid beyond a cutoff scale , where is the
reduced Planck mass. We then argue that knowing the Higgs potential profile for
the field values relevant for inflation () requires knowledge of the ultraviolet completion of the SM beyond
. In absence of such microscopic theory, the extrapolation of the pure
SM potential beyond is unwarranted and the scenario is akin to other
ad-hoc inflaton potentials afflicted with significant fine-tuning. The
appealing naturalness of this minimal proposal is therefore lost.Comment: 9 pages. Replaced with published version, plus a footnote clarifying
the use of power counting estimate
Specialization of the rostral prefrontal cortex for distinct analogy processes
Analogical reasoning is central to learning and abstract thinking. It involves using a more familiar situation (source) to make inferences about a less familiar situation (target). According to the predominant cognitive models, analogical reasoning includes 1) generation of structured mental representations and 2) mapping based on structural similarities between them. This study used functional magnetic resonance imaging to specify the role of rostral prefrontal cortex (PFC) in these distinct processes. An experimental paradigm was designed that enabled differentiation between these processes, by temporal separation of the presentation of the source and the target. Within rostral PFC, a lateral subregion was activated by analogy task both during study of the source (before the source could be compared with a target) and when the target appeared. This may suggest that this subregion supports fundamental analogy processes such as generating structured representations of stimuli but is not specific to one particular processing stage. By contrast, a dorsomedial subregion of rostral PFC showed an interaction between task (analogy vs. control) and period (more activated when the target appeared). We propose that this region is involved in comparison or mapping processes. These results add to the growing evidence for functional differentiation between rostral PFC subregions
An overview of tea research in Tanzania - with special reference to the Southern Highlands.
The history of tea development in Tanzania from the early part of this century
to the present is summarised. Average yields of made tea from well managed
estates in the Mufindi district have increased from around 600 kg ha-1 in the
late 1950s to 3000 kg ha-1 at the present time: by comparison, yields from
smallholder farms have remained much lower, averaging only 400-500 kg ha-1.
There have been a large number of technical, economic and other changes over the
last 30 to 40 years. The removal of shade trees, the use of herbicides, the
application of NPK compound fertilisers, the introduction of irrigation (on some
estates) and changes in harvesting policy have all contributed to the increases
in yield. Financial and infrastructural problems have contributed to the low
yields from many smallholders and others, and have limited the uptake of new
technology. The contribution of research is reviewed, from the start of the Tea
Research Institute of East Africa in Kenya in 1951, through to the development
of the Marikitanda Tea Research Centre in Amani in 1967; the Ngwazi Tea Research
Unit in Mufindi (1967 to 1970, and from 1986), and lastly the Kifyulilo Tea
Research Station, also in Mufindi in 1986. The yield potential of well
fertilized and irrigated clonal tea, grown at an altitude of 1800 m, is around
6000 kg ha-1. This potential is reduced by drought, lack of fertilizer, bush
vacancies and inefficient harvesting practices. The corresponding potential
yields at high (2200 m) and low (1200 m) altitude sites range from 3000-3500 kg
ha-1 up to 9000-10000 kg ha-1 and are largely a function of temperature. The
opportunities for increasing yields of existing tea, smallholder and estate, are
enormous. Tea production in the Southern Highlands of Tanzania is about to
expand rapidly. Good, appropriate research is needed to sustain this development
over the long term, and suggestions on how best this is done in order to assist
the large scale producers as well as the smallholders, are discussed
Multi-Throat Brane Inflation
We present a scenario where brane inflation arises more generically. We start
with D3 and anti-D3-branes at the infrared ends of two different throats. This
setup is a natural consequence of the assumption that in the beginning we have
a multi-throat string compactification with many wandering anti-D3-branes. A
long period of inflation is triggered when D3-branes slowly exit the highly
warped infrared region, under a potential generically arising from the moduli
stabilization. In this scenario, the usual slow-roll conditions are not
required, and a large warping is allowed to incorporate the Randall-Sundrum
model.Comment: 11 pages; v3: minor revision, PRD versio
Cosmology and Static Spherically Symmetric solutions in D-dimensional Scalar Tensor Theories: Some Novel Features
We consider scalar tensor theories in D-dimensional spacetime, D \ge 4. They
consist of metric and a non minimally coupled scalar field, with its non
minimal coupling characterised by a function. The probes couple minimally to
the metric only. We obtain vacuum solutions - both cosmological and static
spherically symmetric ones - and study their properties. We find that, as seen
by the probes, there is no singularity in the cosmological solutions for a
class of functions which obey certain constraints. It turns out that for the
same class of functions, there are static spherically symmetric solutions which
exhibit novel properties: {\em e.g.} near the ``horizon'', the gravitational
force as seen by the probe becomes repulsive.Comment: Revtex. 21 pages. Version 2: More references added. Version 3: Issues
raised by the referee are addressed. Results unchanged. Title modified; a new
subsection and more references added. Verison to appear in Physical Review
A Microscopic Derivation of the SO(5)-Symmetric Landau-Ginzburg Potential
We construct a microscopic model of electron interactions which gives rise to
both superconductivity and antiferromagnetism, and which admits an approximate
SO(5) symmetry that relates these two phases. The symmetry can be exact, or it
may exist only in the long-wavelength limit, depending on the detailed form of
the interactions. We compute the macroscopic Landau-Ginzburg free energy for
this model as a function of temperature and doping, by explicitly integrating
out the fermions. We find that the resulting phase diagram can resemble that
observed for the cuprates, with the antiferromagnetism realized as a spin
density wave, whose wavelength might be incommensurate with the lattice spacing
away from half filling.Comment: 29 pp., plain TeX, 7 figures, uses macros.tex (included) and
epsf.tex; added subject clas
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