13,609 research outputs found
The potential for gamma-emitting radionuclides to contribute to an understanding of erosion processes in South Africa
Several research projects undertaken by the authors and others over the last 14 years have used fallout and geogenic radionuclides for understanding erosion processes and sediment yield dynamics in South Africa over the last 100–200 years as European settlers colonised the interior plains and plateaux of the country and imported new livestock and farming techniques to the region. These projects have used two fallout radionuclides (210Pb and 137Cs) to date sediments accumulating in reservoirs, farm dams, wetlands, alluvial fans and floodouts and have used other fallout nuclides (7Be) and long-lived geogenic radionuclides (e.g. 40K, 235U) as part of a composite fingerprint exploring contemporary sediment sources and changes to sources through time. While successful in many parts of the world, applying these techniques in Southern Africa has posed a number of challenges often not encountered elsewhere. Here we explore some of the benefits and challenges in using gamma-emitting radionuclides, especially 137Cs, in these landscapes. Benefits include the potential for discriminating gully sidewall from topsoil sources, which has helped to identify contemporary gully systems as sediment conduits, rather than sources, and for providing a time-synchronous marker horizon in a range of sedimentary environments that has helped to develop robust chronologies. Challenges include the spatial variability in soil cover on steep rocky hillslopes, which is likely to challenge assumptions about the uniformity of initial fallout nuclide distribution, the paucity of stable (non-eroding) sites in order to estimate atmospheric fallout inventories, and the limited success of 210Pb dating in some rapidly accumulating high altitude catchments where sediments often comprise significant amounts of sand and gravel. Despite these challenges we present evidence suggesting that the use of gamma-emitting radionuclides can make a significant contribution to our understanding of erosion processes and sediment yield dynamics. Future research highlighted in the conclusion will try to address current challenges and outline new projects established to address them more fully
Vortex pairing in two-dimensional Bose gases
Recent experiments on ultracold Bose gases in two dimensions have provided
evidence for the existence of the Berezinskii-Kosterlitz-Thouless (BKT) phase
via analysis of the interference between two independent systems. In this work
we study the two-dimensional quantum degenerate Bose gas at finite temperature
using the projected Gross-Pitaevskii equation classical field method. While
this describes the highly occupied modes of the gas below a momentum cutoff, we
have developed a method to incorporate the higher momentum states in our model.
We concentrate on finite-sized homogeneous systems in order to simplify the
analysis of the vortex pairing. We determine the dependence of the condensate
fraction on temperature and compare this to the calculated superfluid fraction.
By measuring the first order correlation function we determine the boundary of
the Bose-Einstein condensate and BKT phases, and find it is consistent with the
superfluid fraction decreasing to zero. We reveal the characteristic unbinding
of vortex pairs above the BKT transition via a coarse-graining procedure.
Finally, we model the procedure used in experiments to infer system
correlations [Hadzibabic et al., Nature 441, 1118 (2006)], and quantify its
level of agreement with directly calculated in situ correlation functions.Comment: published versio
Cloudbus Toolkit for Market-Oriented Cloud Computing
This keynote paper: (1) presents the 21st century vision of computing and
identifies various IT paradigms promising to deliver computing as a utility;
(2) defines the architecture for creating market-oriented Clouds and computing
atmosphere by leveraging technologies such as virtual machines; (3) provides
thoughts on market-based resource management strategies that encompass both
customer-driven service management and computational risk management to sustain
SLA-oriented resource allocation; (4) presents the work carried out as part of
our new Cloud Computing initiative, called Cloudbus: (i) Aneka, a Platform as a
Service software system containing SDK (Software Development Kit) for
construction of Cloud applications and deployment on private or public Clouds,
in addition to supporting market-oriented resource management; (ii)
internetworking of Clouds for dynamic creation of federated computing
environments for scaling of elastic applications; (iii) creation of 3rd party
Cloud brokering services for building content delivery networks and e-Science
applications and their deployment on capabilities of IaaS providers such as
Amazon along with Grid mashups; (iv) CloudSim supporting modelling and
simulation of Clouds for performance studies; (v) Energy Efficient Resource
Allocation Mechanisms and Techniques for creation and management of Green
Clouds; and (vi) pathways for future research.Comment: 21 pages, 6 figures, 2 tables, Conference pape
Steering in computational science: mesoscale modelling and simulation
This paper outlines the benefits of computational steering for high
performance computing applications. Lattice-Boltzmann mesoscale fluid
simulations of binary and ternary amphiphilic fluids in two and three
dimensions are used to illustrate the substantial improvements which
computational steering offers in terms of resource efficiency and time to
discover new physics. We discuss details of our current steering
implementations and describe their future outlook with the advent of
computational grids.Comment: 40 pages, 11 figures. Accepted for publication in Contemporary
Physic
Two dimensional self-avoiding walk with hydrogen-like bonding: Phase diagram and critical behaviour
The phase diagram for a two-dimensional self-avoiding walk model on the
square lattice incorporating attractive short-ranged interactions between
parallel sections of walk is derived using numerical transfer matrix
techniques. The model displays a collapse transition. In contrast to the
standard -point model, the transition is first order. The phase diagram
in the full fugacity-temperature plane displays an additional transition line,
when compared to the -point model, as well as a critical transition at
finite temperature in the hamiltonian walk limit.Comment: 22 pages, 13 figures. To appear in Journal of Physics
A grid-based infrastructure for distributed retrieval
In large-scale distributed retrieval, challenges of latency, heterogeneity, and dynamicity emphasise the importance of infrastructural support in reducing the development costs of state-of-the-art solutions. We present a service-based infrastructure for distributed retrieval which blends middleware facilities and a design framework to ‘lift’ the resource sharing approach and the computational services of a European Grid platform into the domain of e-Science applications. In this paper, we give an overview of the DILIGENT Search Framework and illustrate its exploitation in the field of Earth Science
Confidence trick: the interpretation of confidence intervals
The frequent misinterpretation of the nature of confidence intervals by students has been well documented. This article examines the problem as an aspect of the learning of mathematical definitions and considers the tension between parroting mathematically rigorous, but essentially uninternalized, statements on the one hand and expressing imperfect but developing understandings on the other. A small-scale study among schoolteachers sought comments on four definitions expressing differing understandings of confidence intervals, and these are examined and discussed. The article concludes that some student wordings could be regarded as less inaccurate than they might seem at first sight and presents a case for accepting a wider range of more intuitive understandings as a work in progress
Interferon Alpha Induces Sustained Changes in NK Cell Responsiveness to Hepatitis B Viral Load Suppression In Vivo
This work was supported by funding from
The NIHR Academic Clinical Fellowship scheme and
a Wellcome Trust Clinical Research Training
fellowship (107389/Z/15/Z) awarded to USG; a
Wellcome Trust Senior Investigator award (101848/Z/
13/Z) to MKM and a Barts and The London Charity
award (No. 723/1795) to PTFK
Slow imbalance relaxation and thermoelectric transport in graphene
We compute the electronic component of the thermal conductivity (TC) and the
thermoelectric power (TEP) of monolayer graphene, within the hydrodynamic
regime, taking into account the slow rate of carrier population imbalance
relaxation. Interband electron-hole generation and recombination processes are
inefficient due to the non-decaying nature of the relativistic energy spectrum.
As a result, a population imbalance of the conduction and valence bands is
generically induced upon the application of a thermal gradient. We show that
the thermoelectric response of a graphene monolayer depends upon the ratio of
the sample length to an intrinsic length scale l_Q, set by the imbalance
relaxation rate. At the same time, we incorporate the crucial influence of the
metallic contacts required for the thermopower measurement (under open circuit
boundary conditions), since carrier exchange with the contacts also relaxes the
imbalance. These effects are especially pronounced for clean graphene, where
the thermoelectric transport is limited exclusively by intercarrier collisions.
For specimens shorter than l_Q, the population imbalance extends throughout the
sample; the TC and TEP asymptote toward their zero imbalance relaxation limits.
In the opposite limit of a graphene slab longer than l_Q, at non-zero doping
the TC and TEP approach intrinsic values characteristic of the infinite
imbalance relaxation limit. Samples of intermediate (long) length in the doped
(undoped) case are predicted to exhibit an inhomogeneous temperature profile,
whilst the TC and TEP grow linearly with the system size. In all cases except
for the shortest devices, we develop a picture of bulk electron and hole number
currents that flow between thermally conductive leads, where steady-state
recombination and generation processes relax the accumulating imbalance.Comment: 14 pages, 4 figure
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