263 research outputs found
Charged inclusion in nematic liquid crystals
We present a general theory of liquid crystals under inhomogeneous electric
field in a Ginzburg-Landau scheme. The molecular orientation can be deformed by
electric field when the dielectric tensor is orientation-dependent. We then
investigate the influence of a charged particle on the orientation order in a
nematic state. The director is aligned either along or perpendicular to the
local electric field around the charge, depending on the sign of the dielectric
anisotropy. The deformation becomes stronger with increasing the ratio ,
where is the charge and is the radius of the particle. Numerical
analysis shows the presence of defects around the particle for large .
They are nanometer-scale defects for microscopic ions. If the dielectric
anisotropy is positive, a Saturn ring defect appears. If it is negative, a pair
of point defects appear apart from the particle surface, each being connected
to the surface by a disclination line segment.Comment: 12 figure
The crossover from propagating to strongly scattered acoustic modes of glasses observed in densified silica
Spectroscopic results on low frequency excitations of densified silica are
presented and related to characteristic thermal properties of glasses. The end
of the longitudinal acoustic branch is marked by a rapid increase of the
Brillouin linewidth with the scattering vector. This rapid growth saturates at
a crossover frequency Omega_co which nearly coincides with the center of the
boson peak. The latter is clearly due to additional optic-like excitations
related to nearly rigid SiO_4 librations as indicated by hyper-Raman
scattering. Whether the onset of strong scattering is best described by
hybridization of acoustic modes with these librations, by their elastic
scattering (Rayleigh scattering) on the local excitations, or by soft
potentials remains to be settled.Comment: 14 pages, 6 figures, to be published in a special issue of J. Phys.
Condens. Matte
Numerical study of anharmonic vibrational decay in amorphous and paracrystalline silicon
The anharmonic decay rates of atomic vibrations in amorphous silicon (a-Si)
and paracrystalline silicon (p-Si), containing small crystalline grains
embedded in a disordered matrix, are calculated using realistic structural
models. The models are 1000-atom four-coordinated networks relaxed to a local
minimum of the Stillinger-Weber interatomic potential. The vibrational decay
rates are calculated numerically by perturbation theory, taking into account
cubic anharmonicity as the perturbation. The vibrational lifetimes for a-Si are
found to be on picosecond time scales, in agreement with the previous
perturbative and classical molecular dynamics calculations on a 216-atom model.
The calculated decay rates for p-Si are similar to those of a-Si. No modes in
p-Si reside entirely on the crystalline cluster, decoupled from the amorphous
matrix. The localized modes with the largest (up to 59%) weight on the cluster
decay primarily to two diffusons. The numerical results are discussed in
relation to a recent suggestion by van der Voort et al. [Phys. Rev. B {\bf 62},
8072 (2000)] that long vibrational relaxation inferred experimentally may be
due to possible crystalline nanostructures in some types of a-Si.Comment: 9 two-column pages, 13 figure
Static and Dynamic Properties of a Viscous Silica Melt Molecular Dynamics Computer Simulations
We present the results of a large scale molecular dynamics computer
simulation in which we investigated the static and dynamic properties of a
silica melt in the temperature range in which the viscosity of the system
changes from O(10^-2) Poise to O(10^2) Poise. We show that even at temperatures
as high as 4000 K the structure of this system is very similar to the random
tetrahedral network found in silica at lower temperatures. The temperature
dependence of the concentration of the defects in this network shows an
Arrhenius law. From the partial structure factors we calculate the neutron
scattering function and find that it agrees very well with experimental neutron
scattering data. At low temperatures the temperature dependence of the
diffusion constants shows an Arrhenius law with activation energies which
are in very good agreement with the experimental values. With increasing
temperature we find that this dependence shows a cross-over to one which can be
described well by a power-law, D\propto (T-T_c)^gamma. The critical temperature
T_c is 3330 K and the exponent gamma is close to 2.1. Since we find a similar
cross-over in the viscosity we have evidence that the relaxation dynamics of
the system changes from a flow-like motion of the particles, as described by
the ideal version of mode-coupling theory, to a hopping like motion. We show
that such a change of the transport mechanism is also observed in the product
of the diffusion constant and the life time of a Si-O bond, or the space and
time dependence of the van Hove correlation functions.Comment: 30 pages of Latex, 14 figure
Harmonic Vibrational Excitations in Disordered Solids and the "Boson Peak"
We consider a system of coupled classical harmonic oscillators with spatially
fluctuating nearest-neighbor force constants on a simple cubic lattice. The
model is solved both by numerically diagonalizing the Hamiltonian and by
applying the single-bond coherent potential approximation. The results for the
density of states are in excellent agreement with each other. As
the degree of disorder is increased the system becomes unstable due to the
presence of negative force constants. If the system is near the borderline of
stability a low-frequency peak appears in the reduced density of states
as a precursor of the instability. We argue that this peak
is the analogon of the "boson peak", observed in structural glasses. By means
of the level distance statistics we show that the peak is not associated with
localized states
Potential of multispectral synergism for observing ozone pollution by combining IASI-NG and UVNS measurements from the EPS-SG satellite
Present and future satellite observations offer great potential for
monitoring air quality on a daily and global basis. However, measurements from
currently orbiting satellites do not allow a single sensor to accurately probe
surface concentrations of gaseous pollutants such as tropospheric
ozone. Combining information from IASI (Infrared Atmospheric Sounding
Interferometer) and GOME-2 (Global Ozone Monitoring Experiment-2)
respectively in the TIR and UV spectra, a recent multispectral method
(referred to as IASI+GOME-2) has shown enhanced sensitivity for probing
ozone in the lowermost troposphere (LMT, below 3âŻkm altitude) with
maximum sensitivity down to 2.20âŻkmâŻa.s.l. over land, while sensitivity for
IASI or GOME-2 alone only peaks at 3 to 4âŻkm at the lowest.In this work we develop a pseudo-observation simulator and evaluate the
potential of future EPS-SG (EUMETSAT Polar System â Second Generation)
satellite observations, from new-generation sensors IASI-NG (Infrared
Atmospheric Sounding Interferometer â New Generation) and UVNS (Ultraviolet
Visible Near-infrared Shortwave-infrared), to observe near-surface O3
through the IASI-NG+UVNS multispectral method. The pseudo-real state of
the atmosphere is provided by the MOCAGE (MOdÚle de Chimie Atmosphérique
Ă Grande Ăchelle) chemical transport model. We perform full and
accurate forward and inverse radiative transfer calculations for a period of
4Â days (8â11 July 2010) over Europe.In the LMT, there is a remarkable agreement in the geographical distribution
of O3 partial columns between IASI-NG+UVNS pseudo-observations and the corresponding
MOCAGE pseudo-reality. With respect to synthetic IASI+GOME-2 products,
IASI-NG+UVNS shows a higher correlation between pseudo-observations and
pseudo-reality, which is enhanced by about 12âŻ%. The bias on high ozone retrieval
is reduced and the average accuracy increases by 22âŻ%. The sensitivity to
LMT ozone is also enhanced. On average, the degree of freedom for
signal is higher by 159âŻ% over land (from 0.29 to 0.75)
and 214âŻ% over ocean (from 0.21 to 0.66). The mean height of maximum sensitivity for the LMT peaks at 1.43âŻkm
over land and 2.02âŻkm over ocean, respectively 1.03 and 1.30âŻkm below
that of IASI+GOME-2. IASI-NG+UVNS also shows good retrieval skill in the
surfaceâ2âŻkm altitude range. It is one of a kind for retrieving ozone layers of 2â3âŻkm thickness, in the
first 2â3âŻkm of the atmosphere. IASI-NG+UVNS is expected to largely
enhance the capacity to observe ozone pollution from space
Potential of multispectral synergism for observing ozone pollution by combining IASI-NG and UVNS measurements from the EPS-SG satellite
Present and future satellite observations offer great potential for
monitoring air quality on a daily and global basis. However, measurements from
currently orbiting satellites do not allow a single sensor to accurately probe
surface concentrations of gaseous pollutants such as tropospheric
ozone. Combining information from IASI (Infrared Atmospheric Sounding
Interferometer) and GOME-2 (Global Ozone Monitoring Experiment-2)
respectively in the TIR and UV spectra, a recent multispectral method
(referred to as IASI+GOME-2) has shown enhanced sensitivity for probing
ozone in the lowermost troposphere (LMT, below 3âŻkm altitude) with
maximum sensitivity down to 2.20âŻkmâŻa.s.l. over land, while sensitivity for
IASI or GOME-2 alone only peaks at 3 to 4âŻkm at the lowest.In this work we develop a pseudo-observation simulator and evaluate the
potential of future EPS-SG (EUMETSAT Polar System â Second Generation)
satellite observations, from new-generation sensors IASI-NG (Infrared
Atmospheric Sounding Interferometer â New Generation) and UVNS (Ultraviolet
Visible Near-infrared Shortwave-infrared), to observe near-surface O3
through the IASI-NG+UVNS multispectral method. The pseudo-real state of
the atmosphere is provided by the MOCAGE (MOdÚle de Chimie Atmosphérique
Ă Grande Ăchelle) chemical transport model. We perform full and
accurate forward and inverse radiative transfer calculations for a period of
4Â days (8â11 July 2010) over Europe.In the LMT, there is a remarkable agreement in the geographical distribution
of O3 partial columns between IASI-NG+UVNS pseudo-observations and the corresponding
MOCAGE pseudo-reality. With respect to synthetic IASI+GOME-2 products,
IASI-NG+UVNS shows a higher correlation between pseudo-observations and
pseudo-reality, which is enhanced by about 12âŻ%. The bias on high ozone retrieval
is reduced and the average accuracy increases by 22âŻ%. The sensitivity to
LMT ozone is also enhanced. On average, the degree of freedom for
signal is higher by 159âŻ% over land (from 0.29 to 0.75)
and 214âŻ% over ocean (from 0.21 to 0.66). The mean height of maximum sensitivity for the LMT peaks at 1.43âŻkm
over land and 2.02âŻkm over ocean, respectively 1.03 and 1.30âŻkm below
that of IASI+GOME-2. IASI-NG+UVNS also shows good retrieval skill in the
surfaceâ2âŻkm altitude range. It is one of a kind for retrieving ozone layers of 2â3âŻkm thickness, in the
first 2â3âŻkm of the atmosphere. IASI-NG+UVNS is expected to largely
enhance the capacity to observe ozone pollution from space
The evolution of vibrational excitations in glassy systems
The equations of the mode-coupling theory (MCT) for ideal liquid-glass
transitions are used for a discussion of the evolution of the
density-fluctuation spectra of glass-forming systems for frequencies within the
dynamical window between the band of high-frequency motion and the band of
low-frequency-structural-relaxation processes. It is shown that the strong
interaction between density fluctuations with microscopic wave length and the
arrested glass structure causes an anomalous-oscillation peak, which exhibits
the properties of the so-called boson peak. It produces an elastic modulus
which governs the hybridization of density fluctuations of mesoscopic wave
length with the boson-peak oscillations. This leads to the existence of
high-frequency sound with properties as found by X-ray-scattering spectroscopy
of glasses and glassy liquids. The results of the theory are demonstrated for a
model of the hard-sphere system. It is also derived that certain schematic MCT
models, whose spectra for the stiff-glass states can be expressed by elementary
formulas, provide reasonable approximations for the solutions of the general
MCT equations.Comment: 50 pages, 17 postscript files including 18 figures, to be published
in Phys. Rev.
Tropospheric Ozone Assessment Report: Tropospheric ozone from 1877 to 2016, observed levels, trends and uncertainties
From the earliest observations of ozone in the lower atmosphere in the 19th century, both measurement methods and the portion of the globe observed have evolved and changed. These methods have different uncertainties and biases, and the data records differ with respect to coverage (space and time), information content, and representativeness. In this study, various ozone measurement methods and ozone datasets are reviewed and selected for inclusion in the historical record of background ozone levels, based on relationship of the measurement technique to the modern UV absorption standard, absence of interfering pollutants, representativeness of the well-mixed boundary layer and expert judgement of their credibility. There are significant uncertainties with the 19th and early 20th-century measurements related to interference of other gases. Spectroscopic methods applied before 1960 have likely underestimated ozone by as much as 11% at the surface and by about 24% in the free troposphere, due to the use of differing ozone absorption coefficients.
There is no unambiguous evidence in the measurement record back to 1896 that typical mid-latitude background surface ozone values were below about 20 nmol molâ1, but there is robust evidence for increases in the temperate and polar regions of the northern hemisphere of 30â70%, with large uncertainty, between the period of historic observations, 1896â1975, and the modern period (1990â2014). Independent historical observations from balloons and aircraft indicate similar changes in the free troposphere. Changes in the southern hemisphere are much less. Regional representativeness of the available observations remains a potential source of large errors, which are difficult to quantify
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