847 research outputs found
The effects of stress on benthic algal communities
The effects of stress on both microalgal and macroalgal communities are considered. On one hand the contrasting approaches of studies of these two communities reflect intrinsic differences in plant size, longevity and ease of handling. On the other hand they reveal that biological monitoring of the potentially deleterious effects of man's activities has focused largely on freshwater environments in which macroalgae only occasionally dominate. Large conspicuous plants can be readily investigated as individuals, whereas it is virtually impossible to trace effects of stress on an individual cell of a vegetatively-reproducing microalga; a population approach is almost inevitably necessary. However, rapid turnover rates, a spectrum of ecological characteristics distributed between many taxa, and the potential for statistical analysis, have facilitated the use of microalgae in environmental impact studies. Failure to extend such investigations into marine systems rests as much on man's ability to ignore environmental deterioration until it affects his quality of life as on the visual dominance of seaweeds around our coasts. However, large gaps remain in our knowledge of both large and small algae; some reported community changes over time are suspect, and the causes of even blatant changes are not always apparent
The application of optical coherence tomography to image subsurface tissue structure of Antarctic krill Euphausia superba
Many small open ocean animals, such as Antarctic krill, are an important part of marine ecosystems. To discover what will happen to animals such as krill in a changing ocean, experiments are run in aquaria where conditions can be controlled to simulate water characteristics predicted to occur in the future. The response of individual animals to changing water conditions can be hard to observe, and with current observation techniques it is very difficult to follow the progress of an individual animal through its life. Optical coherence tomography (OCT) is an optical imaging technique that allows images at high resolution to be obtained from depths up to a few millimeters inside biological specimens. It is compatible with in vivo imaging and can be used repeatedly on the same specimens. In this work, we show how OCT may be applied to post mortem krill samples and how important physiological data such as shell thickness and estimates of organ volume can be obtained. Using OCT we find an average value for the thickness of krill exoskeleton to be (30±4) µm along a 1 cm length of the animal body. We also show that the technique may be used to provide detailed imagery of the internal structure of a pleopod joint and provide an estimate for the heart volume of (0.73±0.03) mm3
Anderson impurity model at finite Coulomb interaction U: generalized Non-crossing Approximation
We present an extension of the non-crossing approximation (NCA), which is
widely used to calculate properties of Anderson impurity models in the limit of
infinite Coulomb repulsion , to the case of finite . A
self-consistent conserving pseudo-particle representation is derived by
symmetrizing the usual NCA diagrams with respect to empty and doubly occupied
local states. This requires an infinite summation of skeleton diagrams in the
generating functional thus defining the ``Symmetrized finite-U NCA'' (SUNCA).
We show that within SUNCA the low energy scale (Kondo temperature) is
correctly obtained, in contrast to other simpler approximations discussed in
the literature.Comment: 7 pages, 6 figure
Restricted three body problems at the nanoscale
In this paper, we investigate some of the classical restricted three body
problems at the nanoscale, such as the circular planar restricted problem for
three C60 fullerenes, and a carbon atom and two C60 fullerenes. We model the
van der Waals forces between the fullerenes by the Lennard-Jones potential. In
particular, the pairwise potential energies between the carbon atoms on the
fullerenes are approximated by the continuous approach, so that the total
molecular energy between two fullerenes can be determined analytically. Since
we assume that such interactions between the molecules occur at sufficiently
large distance, the classical three body problems analysis is legitimate to
determine the collective angular velocity of the two and three C60 fullerenes
at the nanoscale. We find that the maximum angular frequency of the two and
three fullerenes systems reach the terahertz range and we determine the
stationary points and the points which have maximum velocity for the carbon
atom for the carbon atom and the two fullerenes system
Spin Seebeck effect in polycrystalline yttrium iron garnet pellets prepared by the solid-state method [Letter]
We study the properties of polycrystalline bulk yttrium iron garnet (YIG) pellets
prepared by the solid-state method, where the choice of the sintering temperature can lead to
mixed phases of yttrium iron perovskite (YIP) and YIG or single phase YIG. Magnetometry
shows multiple switching regimes in the mixed-phase pellets where the saturation magnetization
is dominated by the proportion of YIG present. Ferromagnetic resonance was used to corroborate
the saturation magnetization from magnetometry and to extract the spin wave damping α. The
lowest damping was observed for the YIG pellet, which resulted in a spin Seebeck effect (SSE)
coefficient that was approximately 55% of single crystal YIG. This demonstrates that macroscale
crystallization does not play a major role in the SSE and paves the way for utilising polycrystalline
samples for thermomagnetic applications
A new non-Fermi liquid fixed point
We study a new exchange interaction in which the conduction electrons with
pseudo spin interact with the impurity spin . Due to the
overscreening of the impurity spin by higher conduction electron spin, a new
non-trivial intermediate coupling strength fixed point is realized. Using the
numerical renormalization group (NRG), we show that the low-energy spectra are
described by a non-Fermi liquid excitation spectrum. A conformal field theory
analysis is compared with NRG results and excellent agreement is obtained.
Using the double fusion rule to generate the operator spectrum with the
conformal theory, we find that the specific heat coefficient and magnetic
susceptibility will diverge as , that the scaling dimension of an
applied magnetic field is , and that exchange anisotropy is always
relevant. We discuss the possible relevance of our work to two-level system
Kondo materials and dilute cerium alloys, and we point out a paradox in
understanding the Bethe-Ansatz solutions to the multichannel Kondo model.Comment: Revised. 20 page
FANCD2 re-expression is associated with glioma grade and chemical inhibition of the Fanconi Anaemia pathway sensitises gliomas to chemotherapeutic agents.
Brain tumours kill more children and adults under 40 than any other cancer. Around half of primary brain tumours are glioblastoma multiforme (GBMs) where treatment remains a significant challenge. GBM survival rates have improved little over the last 40 years, thus highlighting an unmet need for the identification/development of novel therapeutic targets and agents to improve GBM treatment. Using archived and fresh glioma tissue, we show that in contrast to normal brain or benign schwannomas GBMs exhibit re-expression of FANCD2, a key protein of the Fanconi Anaemia (FA) DNA repair pathway, and possess an active FA pathway. Importantly, FANCD2 expression levels are strongly associated with tumour grade, revealing a potential exploitable therapeutic window to allow inhibition of the FA pathway in tumour cells, whilst sparing normal brain tissue. Using several small molecule inhibitors of the FA pathway in combination with isogenic FA-proficient/deficient glioma cell lines as well as primary GBM cultures, we demonstrate that inhibition of the FA pathway sensitises gliomas to the chemotherapeutic agents Temozolomide and Carmustine. Our findings therefore provide a strong rationale for the development of novel and potent inhibitors of the FA pathway to improve the treatment of GBMs, which may ultimately impact on patient outcome
What are communities of practice? A comparative review of four seminal works
This paper is a comparative review of four seminal works on communities of practice. It is argued that the ambiguities of the terms community and practice are a source of the concept's reusability allowing it to be reappropriated for different purposes, academic and practical. However, it is potentially confusing that the works differ so markedly in their conceptualizations of community, learning, power and change, diversity and informality. The three earlier works are underpinned by a common epistemological view, but Lave and Wenger's 1991 short monograph is often read as primarily about the socialization of newcomers into knowledge by a form of apprenticeship, while the focus in Brown and Duguid's article of the same year is, in contrast, on improvising new knowledge in an interstitial group that forms in resistance to management. Wenger's 1998 book treats communities of practice as the informal relations and understandings that develop in mutual engagement on an appropriated joint enterprise, but his focus is the impact on individual identity. The applicability of the concept to the heavily individualized and tightly managed work of the twenty-first century is questionable. The most recent work by Wenger – this time with McDermott and Snyder as coauthors – marks a distinct shift towards a managerialist stance. The proposition that managers should foster informal horizontal groups across organizational boundaries is in fact a fundamental redefinition of the concept. However it does identify a plausible, if limited, knowledge management (KM) tool. This paper discusses different interpretations of the idea of 'co-ordinating' communities of practice as a management ideology of empowerment
P-Wave Charmonium Production in B-Meson Decays
We calculate the decay rates of mesons into P-wave charmonium states
using new factorization formulas that are valid to leading order in the
relative velocity of the charmed quark and antiquark and to all orders in the
running coupling constant of QCD. We express the production rates for all four
P states in terms of two nonperturbative parameters, the derivative of the
wavefunction at the origin and another parameter related to the probability for
a charmed-quark-antiquark pair in a color-octet S-wave state to radiate a soft
gluon and form a P-wave bound state. Using existing data on meson decays
into to estimate the color-octet parameter, we find that the
color-octet mechanism may account for a significant fraction of the
production rate and that mesons should decay into at a similar
rate.Comment: 14 page
Auxiliary particle theory of threshold singularities in photoemission and X-ray absorption spectra: Test of a conserving T-matrix approximation
We calculate the exponents of the threshold singularities in the
photoemission spectrum of a deep core hole and its X-ray absorption spectrum in
the framework of a systematic many-body theory of slave bosons and
pseudofermions (for the empty and occupied core level). In this representation,
photoemission and X-ray absorption can be understood on the same footing; no
distinction between orthogonality catastrophe and excitonic effects is
necessary. We apply the conserving slave particle T-matrix approximation
(CTMA), recently developed to describe both Fermi and non-Fermi liquid behavior
systems with strong local correlations, to the X-ray problem as a test case.
The numerical results for both photoemission and X-ray absorption are found to
be in agreement with the exact infrared powerlaw behavior in the weak as well
as in the strong coupling regions. We point out a close relation of the CTMA
with the parquet equation approach of Nozi{\`e}res et al.Comment: 10 pages, 9 figures, published versio
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