9,233 research outputs found
Phonon Properties of Knbo3 and Ktao3 from First-Principles Calculations
The frequencies of transverse-optical phonons in KNbO and
KTaO are calculated in the frozen-phonon scheme making use of the
full-potential linearized muffin-tin orbital method. The calculated frequencies
in the cubic phase of KNbO and in the tetragonal ferroelectric phase are in
good agreement with experimental data. For KTaO, the effect of lattice
volume was found to be substantial on the frequency of the soft mode, but
rather small on the relative displacement patterns of atoms in all three modes
of the symmetry. The TO frequencies in KTaO are found to be of the
order of, but somehow higher than, the corresponding frequencies in cubic
KNbO.Comment: 8 pages + 1 LaTeX figure, Revtex 3.0, SISSA-CM-94-00
Ergodicity breaking in strong and network-forming glassy system
The temperature dependence of the non-ergodicity factor of vitreous GeO,
, as deduced from elastic and quasi-elastic neutron scattering
experiments, is analyzed. The data are collected in a wide range of
temperatures from the glassy phase, up to the glass transition temperature, and
well above into the undercooled liquid state. Notwithstanding the investigated
system is classified as prototype of strong glass, it is found that the
temperature- and the -behavior of follow some of the predictions
of Mode Coupling Theory. The experimental data support the hypothesis of the
existence of an ergodic to non-ergodic transition occurring also in network
forming glassy systems
Deep R-band counts of z~3 Lyman break galaxy candidates with the LBT
Aims. We present a deep multiwavelength imaging survey (UGR) in 3 different
fields, Q0933, Q1623, and COSMOS, for a total area of ~1500arcmin^2. The data
were obtained with the Large Binocular Camera on the Large Binocular Telescope.
Methods. To select our Lyman break galaxy (LBG) candidates, we adopted the well
established and widely used color-selection criterion (U-G vs. G-R). One of the
main advantages of our survey is that it has a wider dynamic color range for
U-dropout selection than in previous studies. This allows us to fully exploit
the depth of our R-band images, obtaining a robust sample with few interlopers.
In addition, for 2 of our fields we have spectroscopic redshift information
that is needed to better estimate the completeness of our sample and interloper
fraction. Results. Our limiting magnitudes reach 27.0(AB) in the R band
(5\sigma) and 28.6(AB) in the U band (1\sigma). This dataset was used to derive
LBG candidates at z~3. We obtained a catalog with a total of 12264 sources down
to the 50% completeness magnitude limit in the R band for each field. We find a
surface density of ~3 LBG candidates arcmin^2 down to R=25.5, where
completeness is >=95% for all 3 fields. This number is higher than the original
studies, but consistent with more recent samples.Comment: in press by A&A, full LBG candidates' catalog will be available in
electronic form at the CD
Unveiling obscured accretion in the Chandra Deep Field South
A large population of heavily obscured, Compton Thick AGNs is predicted by
models of galaxy formation, models of Cosmic X-ray Background and by the
``relic'' super-massive black-hole mass function measured from local bulges.
However, so far only a handful of Compton thick AGNs have been possibly
detected using even the deepest Chandra and XMM surveys. Compton-thick AGNs can
be recovered thanks to the reprocessing of the AGN UV emission in the infrared
by selecting sources with AGN luminosity's in the mid-infrared and faint
near-infrared and optical emission. To this purpose, we make use of deep HST,
VLT, Spitzer and Chandra data on the Chandra Deep Field South to constrain the
number of Compton thick AGN in this field. We show that sources with high
24m to optical flux ratios and red colors form a distinct source
population, and that their infrared luminosity is dominated by AGN emission.
Analysis of the X-ray properties of these extreme sources shows that most of
them (80) are indeed likely to be highly obscured, Compton thick AGNs.
The number of infrared selected, Compton thick AGNs with 5.8m luminosity
higher than erg s turns out to be similar to that of X-ray
selected, unobscured and moderately obscured AGNs with 2-10 keV luminosity
higher than erg s in the redshift bin 1.2-2.6. This ``factor
of 2'' source population is exactly what it is needed to solve the
discrepancies between model predictions and X-ray AGN selection.Comment: Revised version, to be published by The Astrophysical Journa
Thermodynamic Scaling of the Viscosity of Van Der Waals, H-Bonded, and Ionic Liquids
Viscosities and their temperature, T, and volume, V, dependences are reported
for 7 molecular liquids and polymers. In combination with literature viscosity
data for 5 other liquids, we show that the superpositioning of relaxation times
for various glass-forming materials when expressed as a function of TV^g, where
the exponent g is a material constant, can be extended to the viscosity. The
latter is usually measured to higher temperatures than the corresponding
relaxation times, demonstrating the validity of the thermodynamic scaling
throughout the supercooled and higher T regimes. The value of g for a given
liquid principally reflects the magnitude of the intermolecular forces (e.g.,
steepness of the repulsive potential); thus, we find decreasing g in going from
van der Waals fluids to ionic liquids. For strongly H-bonded materials, such as
low molecular weight polypropylene glycol and water, the superpositioning
fails, due to the non-trivial change of chemical structure (degree of
H-bonding) with thermodynamic conditions.Comment: 16 pages 7 figure
Acoustic Nature of the Boson Peak in Vitreous Silica
New temperature dependent inelastic x-ray (IXS) and Raman (RS) scattering
data are compared to each other and with existing inelastic neutron scattering
data in vitreous silica (v-SiO_2), in the 300 - 1775 K region. The IXS data
show collective propagating excitations up to Q=3.5 nm^-1. The temperature
behaviour of the excitations at Q=1.6 nm^-1 matches that of the boson peak
found in INS and RS. This supports the acoustic origin of the excess of
vibrational states giving rise to the boson peak in this glass.Comment: 10 pages and 4 figure
Metallicity evolution, metallicity gradients and gas fractions at z~3.4
We used near-infrared integral field spectroscopic observations from the
AMAZE and LSD programs to constrain the metallicity in a sample of 40 star
forming galaxies at 3<z<5 (most of which at z~3.4). We measure metallicities by
exploiting strong emission line diagnostics. We found that a significant
fraction of star-forming galaxies at z~3.4 deviate from the Fundamental
Metallicity Relation (FMR), with a metallicity up to a factor of ten lower than
expected according to the FMR. This deviation does not correlate with the
dynamical properties of the galaxy or with the presence of interactions. To
investigate the origin of the metallicity deviations in more detail, we also
infer information on the gas content, by inverting the Schmidt-Kennicutt
relation. In agreement with recent CO observational data, we found that, in
contrast with the steeply rising trend at 0<z<2, the gas fraction in massive
galaxies remains constant, with indication of a marginal decline, at 2<z<3.5.
When combined with the metallicity information, we infer that to explain both
the low metallicity and gas content in z~3.4 galaxies, both prominent outflows
and massive pristine gas inflows are needed. In ten galaxies we can also
spatially resolve the metallicity distribution. We found that the metallicity
generally anticorrelates with the distribution of star formation and with the
gas surface density. We discuss these findings in terms of pristine gas inflows
towards the center, and outflows of metal rich gas from the center toward the
external regions. (Abridged)Comment: Replaced to match the published versio
Efficient CO2 electroreduction on tin modified cuprous oxide synthesized via a one-pot microwave-assisted route
Bimetallic copper-tin catalysts are considered cost-effective and suitable for large-scale electrochemical conversion of CO2 to valuable products. In this work, a class of tin (Sn) modified cuprous oxide (Cu2O) is simply synthesized through a one-pot microwave-assisted solvothermal method and thoroughly characterized by various techniques. Sn is uniformly distributed on the Cu2O crystals showing a cube-within-cube structure, and CuSn alloy phase emerges at high Sn contents. The atomic ratio of Cu to Sn is found to be crucially important for the selectivity of the CO2 reduction reaction, and a ratio of 11.6 leads to the optimal selectivity for CO. This electrode shows a high current density of 47.2 mA cm−2 for CO formation at −1.0 V vs. the reversible hydrogen electrode and also displays good CO selectivity of 80–90% in a wide potential range. In particular, considerable CO selectivity of 72–81% is achieved at relatively low overpotentials from 240 mV to 340 mV. During the long-term tests, satisfactory stability is observed for the optimal electrode in terms of both electrode activity and CO selectivity. The relatively low price, the fast and scalable synthesis, and the encouraging performance of the proposed material implies its good potential to be implemented in large-scale CO2 electrolyzers
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