8,275 research outputs found
Synthetic Mudscapes: Human Interventions in Deltaic Land Building
In order to defend infrastructure, economy, and settlement in Southeast Louisiana, we must construct new land to
mitigate increasing risk. Links between urban environments and economic drivers have constrained the dynamic delta
landscape for generations, now threatening to undermine the ecological fitness of the entire region. Static methods of
measuring, controlling, and valuing land fail in an environment that is constantly in flux; change and indeterminacy are
denied by traditional inhabitation.
Multiple land building practices reintroduce deltaic fluctuation and strategic deposition of fertile material to form the
foundations of a multi-layered defence strategy. Manufactured marshlands reduce exposure to storm surge further
inland. Virtual monitoring and communication networks inform design decisions and land use becomes determined
by its ecological health. Mudscapes at the threshold of land and water place new value on former wastelands. The
social, economic, and ecological evolution of the region are defended by an expanded web of growing land
Spin excitations in the skymion host Cu2OSeO3
We have used inelastic neutron scattering to measure the magnetic excitation
spectrum along the high-symmetry directions of the first Brillouin zone of the
magnetic skyrmion hosting compound CuOSeO. The majority of our
scattering data are consistent with the expectations of a recently proposed
model for the magnetic excitations in CuOSeO, and we report best-fit
parameters for the dominant exchange interactions. Important differences exist,
however, between our experimental findings and the model expectations. These
include the identification of two energy scales that likely arise due to
neglected anisotropic interactions. This feature of our work suggests that
anisotropy should be considered in future theoretical work aimed at the full
microscopic understanding of the emergence of the skyrmion state in this
material.Comment: 5 pages, 6 figure
Breaking the Redshift Deadlock - I: Constraining the star formation history of galaxies with sub-millimetre photometric redshifts
Future extragalactic sub-millimetre and millimetre surveys have the potential
to provide a sensitive census of the level of obscured star formation in
galaxies at all redshifts. While in general there is good agreement between the
source counts from existing SCUBA (850um) and MAMBO (1.25mm) surveys of
different depths and areas, it remains difficult to determine the redshift
distribution and bolometric luminosities of the sub-millimetre and millimetre
galaxy population. This is principally due to the ambiguity in identifying an
individual sub-millimetre source with its optical, IR or radio counterpart
which, in turn, prevents a confident measurement of the spectroscopic redshift.
Additionally, the lack of data measuring the rest-frame FIR spectral peak of
the sub-millimetre galaxies gives rise to poor constraints on their rest-frame
FIR luminosities and star formation rates. In this paper we describe
Monte-Carlo simulations of ground-based, balloon-borne and satellite
sub-millimetre surveys that demonstrate how the rest-frame FIR-sub-millimetre
spectral energy distributions (250-850um) can be used to derive photometric
redshifts with an r.m.s accuracy of +/- 0.4 over the range 0 < z < 6. This
opportunity to break the redshift deadlock will provide an estimate of the
global star formation history for luminous optically-obscured galaxies [L(FIR)
> 3 x 10^12 Lsun] with an accuracy of 20 per cent.Comment: 14 pages, 22 figures, submitted to MNRAS, replaced with accepted
versio
Mathematical analysis of a model of river channel formation.
The study of overland flow of water over an erodible sediment leads to a coupled model describing the evolution of the topographic elevation and the depth of the overland water film. The spatially uniform solution of this model is unstable, and this instability corresponds to the formation of rills, which in reality then grow and coalesce to form large-scale river channels. In this paper we consider the deduction and mathematical analysis of a deterministic model describing river channel formation and the evolution of its depth. The model involves a degenerate nonlinear parabolic equation (satisfied on the interior of the support of the solution) with a super-linear source term and a prescribed constant mass. We propose here a global formulation of the problem (formulated in the whole space, beyond the support of the solution) which allows us to show the existence of a solution and leads to a suitable numerical scheme for its approximation. A particular novelty of the model is that the evolving channel self-determines its own width, without the need to pose any extra conditions at the channel margin
Incommensurate spin-density wave order in electron-doped BaFe2As2 superconductors
Neutron diffraction studies of Ba(Fe[1-x]Co[x])2As2 reveal that commensurate
antiferromagnetic order gives way to incommensurate magnetic order for Co
compositions between 0.056 < x < 0.06. The incommensurability has the form of a
small transverse splitting (0, +-e, 0) from the nominal commensurate
antiferromagnetic propagation vector Q[AFM] = (1, 0, 1) (in orthorhombic
notation) where e = 0.02-0.03 and is composition dependent. The results are
consistent with the formation of a spin-density wave driven by Fermi surface
nesting of electron and hole pockets and confirm the itinerant nature of
magnetism in the iron arsenide superconductors.Comment: 14 pages, 3 figure
Electron-phonon interaction via Pekar mechanism in nanostructures
We consider an electron-acoustic phonon coupling mechanism associated with
the dependence of crystal dielectric permittivity on the strain (the so-called
Pekar mechanism) in nanostructures characterized by strong confining electric
fields. The efficiency of Pekar coupling is a function of both the absolute
value and the spatial distribution of the electric field. It is demonstrated
that this mechanism exhibits a phonon wavevector dependence similar to that of
piezoelectricity and must be taken into account for electron transport
calculations in an extended field distribution. In particular, we analyze the
role of Pekar coupling in energy relaxation in silicon inversion layers.
Comparison with the recent experimental results is provided to illustrate its
potential significance
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