671 research outputs found
A note on the metallization of compressed liquid hydrogen
We examine the molecular-atomic transition in liquid hydrogen as it relates
to metallization. Pair potentials are obtained from first principles molecular
dynamics and compared with potentials derived from quadratic response. The
results provide insight into the nature of covalent bonding under extreme
conditions. Based on this analysis, we construct a schematic
dissociation-metallization phase diagram and suggest experimental approaches
that should significantly reduce the pressures necessary for the realization of
the elusive metallic phase of hydrogen.Comment: 11 pages, 4 figure
Diffusion Monte Carlo study of the equation of state of solid ortho-D
We present results of Diffusion Monte Carlo calculations for a system of
solid ortho-D_2 at different densities, for pressure ranging from 0 up to
350MPa. We compare the equation of state obtained using two of the most used
effective intermolecular potentials, i.e. the Silvera--Goldman and the Buck
potentials, with experimental data, in order to assess the validity of the
model interactions. The Silvera-Goldman potential has been found to provide a
satisfactory agreement with experimental results, showing that, as opposed to
what recently found for p-H_2, three--body forces can be efficiently accounted
for by an effective two--body term.Comment: 11 pages, 4 figure
Structure and phase boundaries of compressed liquid hydrogen
We have mapped the molecular-atomic transition in liquid hydrogen using first
principles molecular dynamics. We predict that a molecular phase with
short-range orientational order exists at pressures above 100 GPa. The presence
of this ordering and the structure emerging near the dissociation transition
provide an explanation for the sharpness of the molecular-atomic crossover and
the concurrent pressure drop at high pressures. Our findings have non-trivial
implications for simulations of hydrogen; previous equation of state data for
the molecular liquid may require revision. Arguments for the possibility of a
order liquid-liquid transition are discussed
Diagrammatic quantum field formalism for localized electrons
We introduce a diagrammatic quantum field formalism for the evaluation of
normalized expectation values of operators, and suitable for systems with
localized electrons. It is used to develop a convergent series expansion for
the energy in powers of overlap integrals of single-particle orbitals. This
method gives intuitive and practical rules for writing down the expansion to
arbitrary order of overlap, and can be applied to any spin configuration and to
any dimension. Its applicability for systems with well localized electrons has
been illustrated with examples, including the two-dimensional Wigner crystal
and spin-singlets in the low-density electron gas.Comment: 13 pages, 0 figure
Self-Motions of General 3-RPR Planar Parallel Robots
This paper studies the kinematic geometry of general 3-RPR planar parallel
robots with actuated base joints. These robots, while largely overlooked, have
simple direct kinematics and large singularity-free workspace. Furthermore,
their kinematic geometry is the same as that of a newly developed parallel
robot with SCARA-type motions. Starting from the direct and inverse kinematic
model, the expressions for the singularity loci of 3-RPR planar parallel robots
are determined. Then, the global behaviour at all singularities is
geometrically described by studying the degeneracy of the direct kinematic
model. Special cases of self-motions are then examined and the degree of
freedom gained in such special configurations is kinematically interpreted.
Finally, a practical example is discussed and experimental validations
performed on an actual robot prototype are presented
Comet C/2004 Q2 (MACHHOLZ): Parent Volatiles, a Search for Deuterated Methane, and Constraint on the CH4 Spin Temperature
High-dispersion (l/dl ~ 25,000) infrared spectra of Comet C/2004 Q2
(Machholz) were acquired on Nov. 28-29, 2004, and Jan. 19, 2005 (UT dates) with
NIRSPEC at the Keck-2 telescope on Mauna Kea. We detected H2O, CH4, C2H2, C2H6,
CO, H2CO, CH3OH, HCN, and NH3 and we conducted a sensitive search for CH3D. We
report rotational temperatures, production rates, and mixing ratios (with
respect to H2O) at heliocentric distances of 1.49 AU (Nov. 2004) and 1.21 AU
(Jan. 2005). We highlight three principal results: (1) The mixing ratios of
parent volatiles measured at 1.49 AU and 1.21 AU agree within confidence
limits, consistent with homogeneous composition in the mean volatile release
from the nucleus of C/2004 Q2. Notably, the relative abundance of C2H6/C2H2 is
substantially higher than those measured in other comets, while the mixing
ratios C2H6/H2O, CH3OH/H2O, and HCN/H2O are similar to those observed in
comets, referred to as "organics-normal". (2) The spin temperature of CH4 is >
35-38 K, an estimate consistent with the more robust spin temperature found for
H2O. (3) We obtained a 3s upper limit of CH3D/CH4 < 0.020 (D/H < 0.005). This
limit suggests that methane released from the nucleus of C/2004 Q2 is not
dominated by a component formed in extremely cold (near 10 K) environments.
Formation pathways of both interstellar and nebular origin consistent with the
measured D/H in methane are discussed. Evaluating the relative contributions of
these pathways requires further modeling of chemistry including both gas-phase
and gas-grain processes in the natal interstellar cloud and in the
protoplanetary disk.Comment: Accepted by The Astrophysical Journa
Hydrogen-Helium Mixtures in the Interiors of Giant Planets
Equilibrium properties of hydrogen-helium mixtures under conditions similar
to the interior of giant gas planets are studied by means of first principle
density functional molecular dynamics simulations. We investigate the molecular
and atomic fluid phase of hydrogen with and without the presence of helium for
densities between gcm and gcm and
temperatures from K to . Helium has a crucial influence on
the ionic and electronic structure of the liquid. Hydrogen molecule bonds are
shortened as well as strengthened which leads to more stable hydrogen molecules
compared to pure hydrogen for the same thermodynamic conditions. The {\it ab
initio} treatment of the mixture enables us to investigate the validity of the
widely used linear mixing approximation. We find deviations of up to 8% in
energy and volume from linear mixing at constant pressure in the region of
molecular dissociation.Comment: 13 pages, 18 figures, submitted to PR
The unusual volatile composition of the Halley-type comet 8P/Tuttle: Addressing the existence of an Inner Oort Cloud
We measured organic volatiles (CH4, CH3OH, C2H6, H2CO), CO, and water in
comet 8P/Tuttle, a comet from the Oort cloud reservoir now in a short-period
Halley-type orbit. We compare its composition with two other comets in
Halley-type orbits, and with comets of the "organics-normal" and
"organics-depleted" classes. Chemical gradients are expected in the
comet-forming region of the proto-planetary disk, and an individual comet
should reflect its specific heritage. If Halley-type comets came from the inner
Oort cloud as proposed, we see no common characteristics that could distinguish
such comets from those that were stored in the outer Oort cloud.Comment: 14 pages, including 1 figure and 2 Table
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