784 research outputs found
Drought and disaster in a revolutionary age: colonial Antigua during the American Independence War
The American War of Independence (1775-1783) spelled crisis for the British West Indies. Trade embargos between rebelling and loyal territories, losses to American pirates and hostilities with other European states left the Crown’s tropical Atlantic colonies short of the imported supplies that normally sustained their populations and commerce. Historians have studied the dynamics and consequences of these developments in considerable detail, at both regional and local scales, but have tended to focus on economic, social and political dimensions of the subject matter. Although some investigations have highlighted that climate variability compounded agricultural and subsistence problems in certain locations, the role of climate has rarely been subject to the same level of scrutiny. The present paper addresses this theme by focusing on the Lesser Antillean island of Antigua and the severe drought which gripped the colony during the war period. Through extensive analysis of original, largely unpublished archival sources, the implications of deficient rainfall for human livelihoods, fiscal stability and governmental crisis management are examined. By supplementing findings with evidence from other episodes of warfare which coincided with extreme climate phenomena in the late 1700s and early 1800s, it is argued that successive years of drought were pivotal in defining the severe human and economic losses sustained in Antigua during the American independence conflict. The critical agency of this weather event must, however, be understood as the product of its dynamic interaction with the precarious backdrop of a colonial regime under profound socio-economic and geopolitical stress
Lattice thermal conductivity of TiZrHfNiSn half-Heusler alloys calculated from first principles: Key role of nature of phonon modes
In spite of their relatively high lattice thermal conductivity
, the XNiSn (X=Ti, Zr or Hf) half-Heusler compounds are good
thermoelectric materials. Previous studies have shown that can
be reduced by sublattice-alloying on the X-site. To cast light on how the alloy
composition affects , we study this system using the phonon
Boltzmann-transport equation within the relaxation time approximation in
conjunction with density functional theory.The effect of alloying through
mass-disorder scattering is explored using the virtual crystal approximation to
screen the entire ternary TiZrHfNiSn phase diagram. The
lowest lattice thermal conductivity is found for the TiHfNiSn
compositions; in particular, there is a shallow minimum centered at
TiHfNiSn with taking values between 3.2 and 4.1 W/mK
when the Ti content varies between 20 and 80\%. Interestingly, the overall
behavior of mass-disorder scattering in this system can only be understood from
a combination of the nature of the phonon modes and the magnitude of the mass
variance. Mass-disorder scattering is not effective at scattering acoustic
phonons of low energy. By using a simple model of grain boundary scattering, we
find that nanostructuring these compounds can scatter such phonons effectively
and thus further reduce the lattice thermal conductivity; for instance,
TiHfNiSn with a grain size of nm experiences a 42\%
reduction of compared to that of the single crystal
Structure and binding in crystals of cage-like molecules: hexamine and platonic hydrocarbons
In this paper, we show that first-principle calculations using a van der
Waals density functional (vdW-DF), [Phys. Rev. Lett. , 246401
(2004)] permits determination of molecular crystal structure. We study the
crystal structures of hexamine and the platonic hydrocarbons (cubane and
dodecahedrane). The calculated lattice parameters and cohesion energy agree
well with experiments. Further, we examine the asymptotic accounts of the van
der Waals forces by comparing full vdW-DF with asymptotic atom-based pair
potentials extracted from vdW-DF. The character of the binding differ in the
two cases, with vdW-DF giving a significant enhancement at intermediate and
relevant binding separations. We analyze consequences of this result for
methods such as DFT-D, and question DFT-D's transferability over the full range
of separations
Understanding adhesion at as-deposited interfaces from ab initio thermodynamics of deposition growth: thin-film alumina on titanium carbide
We investigate the chemical composition and adhesion of chemical vapour
deposited thin-film alumina on TiC using and extending a recently proposed
nonequilibrium method of ab initio thermodynamics of deposition growth (AIT-DG)
[Rohrer J and Hyldgaard P 2010 Phys. Rev. B 82 045415]. A previous study of
this system [Rohrer J, Ruberto C and Hyldgaard P 2010 J. Phys.: Condens. Matter
22 015004] found that use of equilibrium thermodynamics leads to predictions of
a non-binding TiC/alumina interface, despite the industrial use as a
wear-resistant coating. This discrepancy between equilibrium theory and
experiment is resolved by the AIT-DG method which predicts interfaces with
strong adhesion. The AIT-DG method combines density functional theory
calculations, rate-equation modelling of the pressure evolution of the
deposition environment and thermochemical data. The AIT-DG method was
previously used to predict prevalent terminations of growing or as-deposited
surfaces of binary materials. Here we extent the method to predict surface and
interface compositions of growing or as-deposited thin films on a substrate and
find that inclusion of the nonequilibrium deposition environment has important
implications for the nature of buried interfaces.Comment: 8 pages, 6 figures, submitted to J. Phys.: Condens. Matte
Stochastic B-series and order conditions for exponential integrators
We discuss stochastic differential equations with a stiff linear part and
their approximation by stochastic exponential integrators. Representing the
exact and approximate solutions using B-series and rooted trees, we derive the
order conditions for stochastic exponential integrators. The resulting general
order theory covers both It\^{o} and Stratonovich integration
On post-Lie algebras, Lie--Butcher series and moving frames
Pre-Lie (or Vinberg) algebras arise from flat and torsion-free connections on
differential manifolds. They have been studied extensively in recent years,
both from algebraic operadic points of view and through numerous applications
in numerical analysis, control theory, stochastic differential equations and
renormalization. Butcher series are formal power series founded on pre-Lie
algebras, used in numerical analysis to study geometric properties of flows on
euclidean spaces. Motivated by the analysis of flows on manifolds and
homogeneous spaces, we investigate algebras arising from flat connections with
constant torsion, leading to the definition of post-Lie algebras, a
generalization of pre-Lie algebras. Whereas pre-Lie algebras are intimately
associated with euclidean geometry, post-Lie algebras occur naturally in the
differential geometry of homogeneous spaces, and are also closely related to
Cartan's method of moving frames. Lie--Butcher series combine Butcher series
with Lie series and are used to analyze flows on manifolds. In this paper we
show that Lie--Butcher series are founded on post-Lie algebras. The functorial
relations between post-Lie algebras and their enveloping algebras, called
D-algebras, are explored. Furthermore, we develop new formulas for computations
in free post-Lie algebras and D-algebras, based on recursions in a magma, and
we show that Lie--Butcher series are related to invariants of curves described
by moving frames.Comment: added discussion of post-Lie algebroid
Toll-like Receptor 7-Dependent Loss of B Cell Tolerance in Pathogenic Autoantibody Knockin Mice
SummarySystemic lupus erythematosus (SLE) is characterized by the production of autoantibodies that are frequently directed against nucleic acid-associated antigens. To better understand how B cells reactive with such antigens are regulated, we generated a model system in which heavy and light chain genes encoding 564 immunoglobulin have been targeted to the heavy and light chain loci of the nonautoimmune C57BL/6 mouse strain. This antibody recognizes RNA, single-stranded DNA, and nucleosomes. We show that B cells expressing this immunoglobulin were activated, producing class-switched autoantibody in vivo despite the apparently normal induction of anergy. This autoantibody production was largely dependent on Toll-like receptor 7 (TLR7). We further show that production of these autoantibodies was sufficient to cause kidney pathology in these mice. These results demonstrate that the particular threat of nucleic acid-containing autoantigens lies in their ability to bind both antigen receptor and TLR7
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