7,693 research outputs found
The random phase approximation applied to ice
Standard density functionals without van der Waals interactions yield an
unsatisfactory description of ice phases, specifically, high density phases
occurring under pressure are too unstable compared to the common low density
phase I observed at ambient conditions. Although the description is
improved by using functionals that include van der Waals interactions, the
errors in relative volumes remain sizable. Here we assess the random phase
approximation (RPA) for the correlation energy and compare our results to
experimental data as well as diffusion Monte Carlo data for ice. The RPA yields
a very balanced description for all considered phases, approaching the accuracy
of diffusion Monte Carlo in relative energies and volumes. This opens a route
towards a concise description of molecular water phases on surfaces and in
cavities
Cohesion of BaReH and BaMnH: Density Functional Calculations and Prediction of (MnH Salts
Density functional calculations are used to calculate the structural and
electronic properties of BaReH and to analyze the bonding in this compound.
The high coordination in BaReH is due to bonding between Re 5 states and
states of -like symmetry formed from combinations of H orbitals in the
H cage. This explains the structure of the material, its short bond lengths
and other physical properties, such as the high band gap. We compare with
results for hypothetical BaMnH, which we find to have similar bonding and
cohesion to the Re compound. This suggests that it may be possible to
synthesize (MnH salts. Depending on the particular cation, such salts
may have exceptionally high hydrogen contents, in excess of 10 weight
Radio observations of two intermittent pulsars: PSRs J1832+0029 and J1841-0500
We present long-term observations of two intermittent pulsars,
PSRs~J1832+0029 and J18410500 using the Parkes 64\,m radio telescope. The
radio emission for these pulsars switches "off" for year-long durations. Our
new observations have enabled us to improve the determination of the on-off
timescales and the spin down rates during those emission states. In general our
results agree with previous studies of these pulsars, but we now have
significantly longer data spans. We have identified two unexpected signatures
in the data. Weak emission was detected in a single observation of
PSR~J18320029 during an "off" emission state. For PSR~J18410500, we
identified a quasi-periodic fluctuation in the intensities of the detectable
single pulses, with a modulation period between 21 and 36 pulse periods.Comment: 7 pages, 7 figures, accepted for publication in Ap
PSR J1016-5857: a young radio pulsar with possible supernova remnant, X-ray, and gamma-ray associations
We report the discovery of a young and energetic pulsar in the Parkes
multibeam survey of the Galactic plane. PSR J1016-5857 has a rotation period of
107 ms and period derivative of 8e-14, implying a characteristic age of 21 kyr
and spin-down luminosity of 2.6e36 erg/s. The pulsar is located just outside,
and possibly interacting with, the shell supernova remnant G284.3-1.8. Archival
X-ray data show a source near the pulsar position which is consistent with
emission from a pulsar wind nebula. The pulsar is also located inside the error
box of the unidentified EGRET source 3EG J1013-5915, for which it represents a
plausible counterpart.Comment: 5 pages, 3 included figures, accepted for publication by ApJ Letter
Interaction of quasilocal harmonic modes and boson peak in glasses
The direct proportionality relation between the boson peak maximum in
glasses, , and the Ioffe-Regel crossover frequency for phonons,
, is established. For several investigated materials . At the frequency the mean free path of the
phonons becomes equal to their wavelength because of strong resonant
scattering on quasilocal harmonic oscillators. Above this frequency phonons
cease to exist. We prove that the established correlation between
and holds in the general case and is a direct consequence of
bilinear coupling of quasilocal oscillators with the strain field.Comment: RevTex, 4 pages, 1 figur
Observations of radio pulses from CU Virginis
The magnetic chemically peculiar star CU Virginis is a unique astrophysical
laboratory for stellar magnetospheres and coherent emission processes. It is
the only known main sequence star to emit a radio pulse every rotation period.
Here we report on new observations of the CU Virginis pulse profile in the 13
and 20\,cm radio bands. The profile is known to be characterised by two peaks
of 100 circularly polarised emission that are thought to arise in an
electron-cyclotron maser mechanism. We find that the trailing peak is stable at
both 13 and 20\,cm, whereas the leading peak is intermittent at 13\,cm. Our
measured pulse arrival times confirm the discrepancy previously reported
between the putative stellar rotation rates measured with optical data and with
radio observations. We suggest that this period discrepancy might be caused by
an unknown companion or by instabilities in the emission region. Regular
long-term pulse timing and simultaneous multi-wavelength observations are
essential to clarify the behaviour of this emerging class of transient radio
source.Comment: Accepted by MNRAS Letters; 5 pages, 2 figures, 3 table
Abrupt grain boundary melting in ice
The effect of impurities on the grain boundary melting of ice is investigated
through an extension of Derjaguin-Landau-Verwey-Overbeek theory, in which we
include retarded potential effects in a calculation of the full frequency
dependent van der Waals and Coulombic interactions within a grain boundary. At
high dopant concentrations the classical solutal effect dominates the melting
behavior. However, depending on the amount of impurity and the surface charge
density, as temperature decreases, the attractive tail of the dispersion force
interaction begins to compete effectively with the repulsive screened Coulomb
interaction. This leads to a film-thickness/temperature curve that changes
depending on the relative strengths of these interactions and exhibits a
decrease in the film thickness with increasing impurity level. More striking is
the fact that at very large film thicknesses, the repulsive Coulomb interaction
can be effectively screened leading to an abrupt reduction to zero film
thickness.Comment: 8 pages, 1 figur
Detecting the stochastic gravitational wave background using pulsar timing
The direct detection of gravitational waves is a major goal of current
astrophysics. We provide details of a new method for detecting a stochastic
background of gravitational waves using pulsar timing data. Our results show
that regular timing observations of 40 pulsars each with a timing accuracy of
100 nano-seconds will be able to make a direct detection of the predicted
stochastic background from coalescing black holes within five years. With an
improved pre-whitening algorithm, or if the background is at the upper end of
the predicted range, a significant detection should be possible with only 20
pulsars.Comment: Accepted by ApJ Letter
Lithium-6: A Probe of the Early Universe
I consider the synthesis of 6Li due to the decay of relic particles, such as
gravitinos or moduli, after the epoch of Big Bang Nucleosynthesis. The
synthesized 6Li/H ratio may be compared to 6Li/H in metal-poor stars which, in
the absence of stellar depletion of 6Li, yields significantly stronger
constraints on relic particle densities than the usual consideration of
overproduction of 3He. Production of 6Li during such an era of non-thermal
nucleosynthesis may also be regarded as a possible explanation for the
relatively high 6Li/H ratios observed in metal-poor halo stars.Comment: final version, Physical Review Letters, additional figure giving
limits on relic decaying particle
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