14,144 research outputs found
Cage-size control of guest vibration and thermal conductivity in Sr8Ga16Si30-xGex
We present a systematic study of thermal conductivity, specific heat,
electrical resistivity, thermopower and x-ray diffraction measurements
performed on single-crystalline samples of the pseudoquaternary type-I
clathrate system Sr8Ga16Si30-xGex, in the full range of 0 < x < 30. All the
samples show metallic behavior with n-type majority carriers. However, the
thermal conductivity and specific heat strongly depend on x. Upon increasing x
from 0 to 30, the lattice parameter increases by 3%, from 10.446 to 10.726 A,
and the localized vibrational energies of the Sr guest ions in the
tetrakaidekahedron (dodecahedron) cages decrease from 59 (120) K to 35 (90) K.
Furthermore, the lattice thermal conductivity at low temperatures is largely
suppressed. In fact, a crystalline peak found at 15 K for x = 0 gradually
decreases and disappears for x > 20, evolving into the anomalous glass-like
behavior observed for x = 30. It is found that the increase of the free space
for the Sr guest motion directly correlates with a continuous transition from
on-center harmonic vibration to off-center anharmonic vibration, with
consequent increase in the coupling strength between the guest's low-energy
modes and the cage's acoustic phonon modes.Comment: 7 pages, 7 figures, submitted to PR
Star formation history in the solar neighborhood: the link between stars and cosmology
Using a cosmological galactic evolutionary approach to model the Milky Way,
we calculate the star formation history (SFH) of the solar neighborhood. The
good agreement we obtain with the observational inferences suggests that our
physical model describes accurately the long term/large spatial trends of the
local and global Milky Way SFH. In this model, star formation is triggered by
disk gravitational instabilities and self-regulated by an energy balance in the
ISM. The drivers of the SFH are the cosmological gas infall rate and the gas
surface density determined by the primordial spin parameter. A LambdaCDM
cosmology was used throughout.Comment: 8 pages, uses kluwer.cls. Invited talk, to appear in "New Quests in
Stellar Astrophysics: The link between Stars and Cosmology", eds. M. Chavez,
A. Bressan, A. Buzzoni & D. Mayya, Kluwer Academic Publisher
High-Energy Proton-Proton Forward Scattering and Derivative Analyticity Relations
We present the results of several parametrizations to two different ensemble
of data on total cross sections at the highest
center-of-mass energies (including cosmic-ray information). The results are
statistically consistent with two distinct scenarios at high energies. From one
ensemble the prediction for the LHC ( TeV) is mb and from the other, mb. From each
parametrization, and making use of derivative analyticity relations (DAR), we
determine (ratio between the forward real and imaginary parts of the
elastic scattering amplitude). A discussion on the optimization of the DAR in
terms of a free parameter is also presented.In all cases good descriptions of
the experimental data are obtained.Comment: One formula added, one unit changed, small misprints corrected, final
version to be published in Brazilian Journal of Physics; 13 pages, 8 figures,
aps-revte
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