889 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
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
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
On the Theory of Superfluidity in Two Dimensions
The superfluid phase transition of the general vortex gas, in which the
circulations may be any non-zero integer, is studied. When the net circulation
of the system is not zero the absence of a superfluid phase is shown. When the
net circulation of the vortices vanishes, the presence of off-diagonal long
range order is demonstrated and the existence of an order parameter is
proposed. The transition temperature for the general vortex gas is shown to be
the Kosterlitz---Thouless temperature. An upper bound for the average vortex
number density is established for the general vortex gas and an exact
expression is derived for the Kosterlitz---Thouless ensemble.Comment: 22 pages, one figure, written in plain TeX, published in J. Phys. A24
(1991) 502
Lower Spectral Branches of a Particle Coupled to a Bose Field
The structure of the lower part (i.e. -away below the two-boson
threshold) spectrum of Fr\"ohlich's polaron Hamiltonian in the weak coupling
regime is obtained in spatial dimension . It contains a single polaron
branch defined for total momentum , where is a bounded domain, and, for any , a
manifold of polaron + one-boson states with boson momentum in a bounded
domain depending on . The polaron becomes unstable and dissolves into the
one boson manifold at the boundary of . The dispersion laws and
generalized eigenfunctions are calculated
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