12,028 research outputs found
Achieving a BCS transition in an atomic Fermi gas
We consider a gas of cold fermionic atoms having two spin components with
interactions characterized by their s-wave scattering length . At positive
scattering length the atoms form weakly bound bosonic molecules which can be
evaporatively cooled to undergo Bose-Einstein condensation, whereas at negative
scattering length BCS pairing can take place. It is shown that, by
adiabatically tuning the scattering length from positive to negative
values, one may transform the molecular Bose-Einstein condensate into a highly
degenerate atomic Fermi gas, with the ratio of temperature to Fermi temperature
. The corresponding critical final value of
which leads to the BCS transition is found to be about one half, where is
the Fermi momentum.Comment: 4 pages, 1 figure. Phys. Rev. Lett. in pres
Quantum Entangled Dark Solitons Formed by Ultracold Atoms in Optical Lattices
Inspired by experiments on Bose-Einstein condensates in optical lattices, we
study the quantum evolution of dark soliton initial conditions in the context
of the Bose-Hubbard Hamiltonian. An extensive set of quantum measures is
utilized in our analysis, including von Neumann and generalized quantum
entropies, quantum depletion, and the pair correlation function. We find that
quantum effects cause the soliton to fill in. Moreover, soliton-soliton
collisions become inelastic, in strong contrast to the predictions of
mean-field theory. These features show that the lifetime and collision
properties of dark solitons in optical lattices provide clear signals of
quantum effects.Comment: 4 pages, 4 figures; version appearing in PRL, only minor changes from
v
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Evaluating candidate reactions to selection practices using organisational justice theory
Objectives: This study aimed to examine candidate reactions to selection practices in postgraduate medical training using organisational justice theory.
Methods: We carried out three independent cross-sectional studies using samples from three consecutive annual recruitment rounds. Data were gathered from candidates applying for entry into UK general practice (GP) training during 2007, 2008 and 2009. Participants completed an evaluation questionnaire immediately after the short-listing stage and after the selection centre (interview) stage. Participants were doctors applying for GP training in the UK. Main outcome measures were participants’ evaluations of the selection methods and perceptions of the overall fairness of each selection stage (short-listing and selection centre).
Results: A total of 23 855 evaluation questionnaires were completed (6893 in 2007, 10 497 in 2008 and 6465 in 2009). Absolute levels of perceptions of fairness of all the selection methods at both the short-listing and selection centre stages were consistently high over the 3 years. Similarly, all selection methods were considered to be job-related by candidates. However, in general, candidates considered the selection centre stage to be significantly fairer than the short-listing stage. Of all the selection methods, the simulated patient consultation completed at the selection centre stage was rated as the most job-relevant.
Conclusions: This is the first study to use a model of organisational justice theory to evaluate candidate reactions during selection into postgraduate specialty training. The high-fidelity selection methods are consistently viewed as more job-relevant and fairer by candidates. This has important implications for the design of recruitment systems for all specialties and, potentially, for medical school admissions. Using this approach, recruiters can systematically compare perceptions of the fairness and job relevance of various selection methods
Alien Registration- Carr, Elizabeth V. (Houlton, Aroostook County)
https://digitalmaine.com/alien_docs/35889/thumbnail.jp
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
Mathematics, mastery and metacognition: how adding a creative approach can support children in maths
Background: Children who hold an incremental view of ability show greater perseverance, improved help-seeking skills and are better able to cope with unexpected challenges. Classroom instruction can influence how children view themselves as learners.
Aim: To explore how mastery-orientated classroom instruction, collaborative learning and metacognitive reflection can foster learners’ attitudes to their task performance. We hypothesised that using a mastery-oriented approach within a mathematics curriculum encourages metacognition, improves motivation and helps children achieve an underlying understanding of mathematical concepts thus improving mathematics
performance.
Method: This paper reports an 11-week project aiming to embed problem-solving strategies within a mastery-oriented whole-class environment. Children completed pre- and post-task semi-structured interviews and maths problems in addition to the 11-week collaborative maths project. Participants were 24 children from a rural primary school in East Sussex, 12 boys and 12 girls (mean age 8 years and 9 months). The interviews are presented qualitatively and a repeated measures analysis of variance on mathematics motivation and performance was conducted.
Findings: The learners showed increased metacognitive reflection on learning strategies as well as increases in girls’ motivation for mathematics.
Limitations: This is a small sample size and, being conducted within a typical everyday classroom, there were several uncontrolled variables. Although change was evident in both attitude and maths scores, it was difficult to apportion added value to the different variables contributing to the change in maths scores.
Conclusions: Challenging children’s perceptions of mathematics encouraged greater self-reflection and increased motivation for girls
Magnetic cloaking by a paramagnet/superconductor cylindrical tube in the critical state
Cloaking of static magnetic fields by a finite thickness type-II
superconductor tube being in the full critical state and surrounded by a
coaxial paramagnet shell is studied. On the basis of exact solutions to the
Maxwell equations, it is shown that, additionally to previous studies assuming
the Meissner state of the superconductor constituent, perfect cloaking is still
realizable at fields higher than the field of full flux penetration into the
superconductor and for arbitrary geometrical parameters of both constituents.
It is also proven that simultaneously the structure is fully undetectable under
the cloaking conditions. Differently from the case of the Meissner state the
cloaking properties in the application relevant critical state are realized,
however, only at a certain field magnitude.Comment: 5 pages, 4 figures; to be published in Applied Physics Letters. arXiv
admin note: substantial text overlap with arXiv:1401.356
Quantum Many-Body Dynamics of Dark Solitons in Optical Lattices
We present a fully quantum many-body treatment of dark solitons formed by
ultracold bosonic atoms in one-dimensional optical lattices. Using
time-evolving block decimation to simulate the single-band Bose-Hubbard
Hamiltonian, we consider the quantum dynamics of density and phase engineered
dark solitons as well as the quantum evolution of mean-field dark solitons
injected into the quantum model. The former approach directly models how one
may create quantum entangled dark solitons in experiment. While we have already
presented results regarding the latter approach elsewhere [Phys. Rev. Lett.
{\bf 103}, 140403 (2009)], we expand upon those results in this work. In both
cases, quantum fluctuations cause the dark soliton to fill in and may induce an
inelasticity in soliton-soliton collisions. Comparisons are made to the
Bogoliubov theory which predicts depletion into an anomalous mode that fills in
the soliton. Our many-body treatment allows us to go beyond the Bogoliubov
approximation and calculate explicitly the dynamics of the system's natural
orbitals.Comment: 14 pages, 11 figures -- v3 has only minor changes from v2 -- this is
the print versio
Stability criterion for self-similar solutions with a scalar field and those with a stiff fluid in general relativity
A stability criterion is derived in general relativity for self-similar
solutions with a scalar field and those with a stiff fluid, which is a perfect
fluid with the equation of state . A wide class of self-similar
solutions turn out to be unstable against kink mode perturbation. According to
the criterion, the Evans-Coleman stiff-fluid solution is unstable and cannot be
a critical solution for the spherical collapse of a stiff fluid if we allow
sufficiently small discontinuity in the density gradient field in the initial
data sets. The self-similar scalar-field solution, which was recently found
numerically by Brady {\it et al.} (2002 {\it Class. Quantum. Grav.} {\bf 19}
6359), is also unstable. Both the flat Friedmann universe with a scalar field
and that with a stiff fluid suffer from kink instability at the particle
horizon scale.Comment: 15 pages, accepted for publication in Classical and Quantum Gravity,
typos correcte
A comparison of the responses of mature and young clonal tea to drought.
To assist commercial producers with optimising the use of irrigation water, the
responses to drought of mature and young tea crops (22 and 5 years after field
planting respectively) were compared using data from two adjacent long-term
irrigation experiments in Southern Tanzania. Providing the maximum potential
soil water deficit was below about 400-500 mm for mature, and 200-250 mm for
young plants (clone 6/8), annual yields of dry tea from rainfed or partially
irrigated crops were similar to those from the corresponding well-watered crops.
At deficits greater than this, annual yields declined rapidly in young tea (up
to 22 kg (ha mm)-1) but relatively slowly in mature tea (up to 6.5 kg (ha mm)-
1). This apparent insensitivity of the mature crop to drought was due
principally to compensation that occurred during the rains for yield lost in the
dry season. Differences in dry matter distribution and shoot to root ratios
contributed to these contrasting responses. Thus, the total above ground dry
mass of well-irrigated, mature plants was about twice that for young plants.
Similarly, the total mass of structural roots (>1 mm diameter), to 3 m depth,
was four times greater in the mature crop than in the young crop and, for fine
roots (<1 mm diameter), eight times greater. The corresponding shoot to root
ratios (dry mass) were about 1:1 and 2:1 respectively. In addition, each unit
area of leaf in the canopy of a mature plant had six times more fine roots (by
weight) available to extract and supply water than did a young plant. Despite
the logistical benefits resulting from more even crop distribution during the
year when crops are fully irrigated, producers currently prefer to save water
and energy costs by allowing a substantial soil water deficit to develop prior
to the start of the rains, up to 250 mm in mature tea, knowing that yield
compensation will occur later
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