749 research outputs found
Origin of the large phonon band-gap in SrTiO3 and the vibrational signatures of ferroelectricity in ATiO3 perovskite: First principles lattice dynamics and inelastic neutron scattering of PbTiO3, BaTiO3 and SrTiO3
We report first principles density functional perturbation theory
calculations and inelastic neutron scattering measurements of the phonon
density of states, dispersion relations and electromechanical response of
PbTiO3, BaTiO3 and SrTiO3. The phonon density-of-states of the quantum
paraelectric SrTiO3 is found to be fundamentally distinct from that of
ferroelectric PbTiO3 and BaTiO3 with a large 70-90 meV phonon band-gap. The
phonon dispersion and electromechanical response of PbTiO3 reveal giant
anisotropies. The interplay of covalent bonding and ferroelectricity, strongly
modulates the electromechanical response and give rise to spectacular
signatures in the phonon spectra. The computed charge densities have been used
to study the bonding in these perovskites. Distinct bonding characteristics in
the ferroelectric and paraelectric phases give rise to spectacular vibrational
signatures. While a large phonon band-gap in ATiO3 perovskites seems a
characteristic of quantum paraelectrics, anisotropy of the phonon spectra
correlates well with ferroelectric strength. These correlations between the
phonon spectra and ferroelectricity, can guide future efforts at custom
designing still more effective piezoelectrics for applications. These results
suggest that vibrational spectroscopy can help design novel materials.Comment: 11 pages, 4 color figures and 2 Table
Sociodemographic and clinical characteristics of patients with asthma who were prescribed ≥3 SABA canisters/year in the SABINA International study
Peer reviewedPostprin
Over-the-counter (OTC) short-acting β2-agonist (SABA) purchase and asthma outcomes in SABINA III
Peer reviewedPostprin
Ursinus College Alumni Journal, July 1957
Bachelor\u27s degrees awarded to 135 • Letters to the editor • Ursinus evaluated • Note from the Admissions Office • Commencement 1957 • Ursinus astronomers record celestial event • The faculty cornered • York County alumni meet • April election results • New York alumni honor H. Dean Steward \u2707 • H. Ober Hess \u2733 speaks at college • History Department advises saving of documents • \u27Id al-Jalus, Ta\u27izz, Yemen, 1953 • May Day festivities • Washington alumni meet • Seventeen annual prizes awarded at commencement • Ursinus Woman\u27s Club • Mrs. William U. Helfferich feted • 1957 track season • 1957 varsity baseball • 1957 varsity tennis • 1957 top athletes • Women\u27s basketball • Blazer award • Memorial trophy for Ken Walker • Tennis and badminton undefeated season • News about ourselves • Weddings • Births • Necrologyhttps://digitalcommons.ursinus.edu/alumnijournal/1059/thumbnail.jp
Galactic winds driven by cosmic-ray streaming
Galactic winds are observed in many spiral galaxies with sizes from dwarfs up
to the Milky Way, and they sometimes carry a mass in excess of that of newly
formed stars by up to a factor of ten. Multiple driving processes of such winds
have been proposed, including thermal pressure due to supernova-heating, UV
radiation pressure on dust grains, or cosmic ray (CR) pressure. We here study
wind formation due to CR physics using a numerical model that accounts for CR
acceleration by supernovae, CR thermalization, and advective CR transport. In
addition, we introduce a novel implementation of CR streaming relative to the
rest frame of the gas. We find that CR streaming drives powerful and sustained
winds in galaxies with virial masses M_200 < 10^{11} Msun. In dwarf galaxies
(M_200 ~ 10^9 Msun) the winds reach a mass loading factor of ~5, expel ~60 per
cent of the initial baryonic mass contained inside the halo's virial radius and
suppress the star formation rate by a factor of ~5. In dwarfs, the winds are
spherically symmetric while in larger galaxies the outflows transition to
bi-conical morphologies that are aligned with the disc's angular momentum axis.
We show that damping of Alfven waves excited by streaming CRs provides a means
of heating the outflows to temperatures that scale with the square of the
escape speed. In larger haloes (M_200 > 10^{11} Msun), CR streaming is able to
drive fountain flows that excite turbulence. For halo masses M_200 > 10^{10}
Msun, we predict an observable level of H-alpha and X-ray emission from the
heated halo gas. We conclude that CR-driven winds should be crucial in
suppressing and regulating the first epoch of galaxy formation, expelling a
large fraction of baryons, and - by extension - aid in shaping the faint end of
the galaxy luminosity function. They should then also be responsible for much
of the metal enrichment of the intergalactic medium.Comment: 25 pages, 14 figures, accepted by MNRA
Communication Research
Contains reports on eight research projects.Bell Telephone Laboratories, IncorporatedCarnegie FoundationRockefeller FoundationOffice of Naval Researc
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
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