1,125 research outputs found
Role of Li_2B_(12)H_(12) for the Formation and Decomposition of LiBH_4
By in situ X-ray diffraction (XRD) and nuclear magnetic resonance (NMR) spectroscopy, the role
of Li_2B_(12)H_(12) for the sorption of LiBH_4 is analyzed. We demonstrate that Li_2B_(12)H_(12) and an
amorphous Li_2B_(10)H_(10) phase are formed by the reaction of LiBH_4 with diborane (B_2H_6) at 200 °C.
Based on our present results, we propose that the Li -2B - (12)H_(12) formation in the desorption of LiBH_4 can
be explained as a result of reaction of diborane and LiBH_4. This reaction of the borohydride with
diborane may also be observed for other borohydrides, where B_(12)H_(12) phases are found during
decomposition
Symmetric hyperbolic systems for Bianchi equations
We obtain a family of first-order symmetric hyperbolic systems for the
Bianchi equations. They have only physical characteristics: the light cone and
timelike hypersurfaces. In the proof of the hyperbolicity, new positivity
properties of the Bel tensor are used.Comment: latex, 7 pages, accepted for publication in Class. Quantum Gra
An evaluation of ocean color model estimates of marine primary productivity in coastal and pelagic regions across the globe
Nearly half of the earth\u27s photosynthetically fixed carbon derives from the oceans. To determine global and region specific rates, we rely on models that estimate marine net primary productivity (NPP) thus it is essential that these models are evaluated to determine their accuracy. Here we assessed the skill of 21 ocean color models by comparing their estimates of depth-integrated NPP to 1156 in situ C-14 measurements encompassing ten marine regions including the Sargasso Sea, pelagic North Atlantic, coastal Northeast Atlantic, Black Sea, Mediterranean Sea, Arabian Sea, subtropical North Pacific, Ross Sea, West Antarctic Peninsula, and the Antarctic Polar Frontal Zone. Average model skill, as determined by root-mean square difference calculations, was lowest in the Black and Mediterranean Seas, highest in the pelagic North Atlantic and the Antarctic Polar Frontal Zone, and intermediate in the other six regions. The maximum fraction of model skill that may be attributable to uncertainties in both the input variables and in situ NPP measurements was nearly 72%. On average, the simplest depth/wavelength integrated models performed no worse than the more complex depth/wavelength resolved models. Ocean color models were not highly challenged in extreme conditions of surface chlorophyll-a and sea surface temperature, nor in high-nitrate low-chlorophyll waters. Water column depth was the primary influence on ocean color model performance such that average skill was significantly higher at depths greater than 250 m, suggesting that ocean color models are more challenged in Case-2 waters (coastal) than in Case-1 (pelagic) waters. Given that in situ chlorophyll-a data was used as input data, algorithm improvement is required to eliminate the poor performance of ocean color NPP models in Case-2 waters that are close to coastlines. Finally, ocean color chlorophyll-a algorithms are challenged by optically complex Case-2 waters, thus using satellite-derived chlorophyll-a to estimate NPP in coastal areas would likely further reduce the skill of ocean color models
Polynomials for Crystal Frameworks and the Rigid Unit Mode Spectrum
To each discrete translationally periodic bar-joint framework \C in \bR^d
we associate a matrix-valued function \Phi_\C(z) defined on the d-torus. The
rigid unit mode spectrum \Omega(\C) of \C is defined in terms of the
multi-phases of phase-periodic infinitesimal flexes and is shown to correspond
to the singular points of the function z \to \rank \Phi_\C(z) and also to the
set of wave vectors of harmonic excitations which have vanishing energy in the
long wavelength limit. To a crystal framework in Maxwell counting equilibrium,
which corresponds to \Phi_\C(z) being square, the determinant of \Phi_\C(z)
gives rise to a unique multi-variable polynomial p_\C(z_1,\dots,z_d). For
ideal zeolites the algebraic variety of zeros of p_\C(z) on the d-torus
coincides with the RUM spectrum. The matrix function is related to other
aspects of idealised framework rigidity and flexibility and in particular leads
to an explicit formula for the number of supercell-periodic floppy modes. In
the case of certain zeolite frameworks in dimensions 2 and 3 direct proofs are
given to show the maximal floppy mode property (order ). In particular this
is the case for the cubic symmetry sodalite framework and some other idealised
zeolites.Comment: Final version with new examples and figures, and with clearer
streamlined proof
Assimilating bio-optical glider data during a phytoplankton bloom in the southern Ross Sea
The Ross Sea is a region characterized by high primary productivity in comparison to other Antarctic coastal regions, and its productivity is marked by considerable variability both spatially (1-50 km) and temporally (days to weeks). This variability presents a challenge for inferring phytoplankton dynamics from observations that are limited in time or space, which is often the case due to logistical limitations of sampling. To better understand the spatio-temporal variability in Ross Sea phytoplankton dynamics and to determine how restricted sampling may skew dynamical interpretations, high-resolution bio-optical glider measurements were assimilated into a one-dimensional biogeochemical model adapted for the Ross Sea. The assimilation of data from the entire glider track using the micro-genetic and local search algorithms in the Marine Model Optimization Testbed improves the model-data fit by similar to 50 %, generating rates of integrated primary production of 104 g C m(-2) yr(-1) and export at 200 m of 27 g C m(-2) yr(-1). Assimilating glider data from three different latitudinal bands and three different longitudinal bands results in minimal changes to the simulations, improves the model-data fit with respect to unassimilated data by similar to 35 %, and confirms that analyzing these glider observations as a time series via a one-dimensional model is reasonable on these scales. Whereas assimilating the full glider data set produces well-constrained simulations, assimilating subsampled glider data at a frequency consistent with cruise-based sampling results in a wide range of primary production and export estimates. These estimates depend strongly on the timing of the assimilated observations, due to the presence of high mesoscale variability in this region. Assimilating surface glider data subsampled at a frequency consistent with available satellite-derived data results in 40% lower carbon export, primarily resulting from optimized rates generating more slowly sinking diatoms. This analysis highlights the need for the strategic consideration of the impacts of data frequency, duration, and coverage when combining observations with biogeochemical modeling in regions with strong mesoscale variability
Binary black hole spacetimes with a helical Killing vector
Binary black hole spacetimes with a helical Killing vector, which are
discussed as an approximation for the early stage of a binary system, are
studied in a projection formalism. In this setting the four dimensional
Einstein equations are equivalent to a three dimensional gravitational theory
with a sigma model as the material source. The sigma
model is determined by a complex Ernst equation. 2+1 decompositions of the
3-metric are used to establish the field equations on the orbit space of the
Killing vector. The two Killing horizons of spherical topology which
characterize the black holes, the cylinder of light where the Killing vector
changes from timelike to spacelike, and infinity are singular points of the
equations. The horizon and the light cylinder are shown to be regular
singularities, i.e. the metric functions can be expanded in a formal power
series in the vicinity. The behavior of the metric at spatial infinity is
studied in terms of formal series solutions to the linearized Einstein
equations. It is shown that the spacetime is not asymptotically flat in the
strong sense to have a smooth null infinity under the assumption that the
metric tends asymptotically to the Minkowski metric. In this case the metric
functions have an oscillatory behavior in the radial coordinate in a
non-axisymmetric setting, the asymptotic multipoles are not defined. The
asymptotic behavior of the Weyl tensor near infinity shows that there is no
smooth null infinity.Comment: to be published in Phys. Rev. D, minor correction
Well-posedness of boundary layer equations for time-dependent flow of non-Newtonian fluids
We consider the flow of an upper convected Maxwell fluid in the limit of high
Weissenberg and Reynolds number. In this limit, the no-slip condition cannot be
imposed on the solutions. We derive equations for the resulting boundary layer
and prove the well-posedness of these equations. A transformation to Lagrangian
coordinates is crucial in the argument
Ultracoherence and Canonical Transformations
The (in)finite dimensional symplectic group of homogeneous canonical
transformations is represented on the bosonic Fock space by the action of the
group on the ultracoherent vectors, which are generalizations of the coherent
states.Comment: 24 page
On hybrid states of two and three level atoms
We calculate atom-photon resonances in the Wigner-Weisskopf model, admitting
two photons and choosing a particular coupling function. We also present a
rough description of the set of resonances in a model for a three-level atom
coupled to the photon field. We give a general picture of matter-field
resonances these results fit into.Comment: 33 pages, 12 figure
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