55 research outputs found
Ortho–para conversion in the solid hydrogens at high pressures
At low pressures the ortho–para conversion in H₂ and D₂ is a slow process governed by the magnetic dipole interaction of nuclear magnetic moments, phonons being the main energy sink. As the pressure is raised to a few GPa and the Debye temperature increases substantially, the conversion energy finds itself in an area where phonon states are depleted and conversion slows down. The recent Raman and NMR experiments showed that the conversion rate in H₂ after an initial slowdown predicted by theory increases immensely. As for solid D₂, conversion rates have apparently not yet been directly measured under pressure. In order to explain the anomaly observed in H₂, we have suggested a new conversion mechanism, in which the basic conversion-producing interaction only initiates conversion whereas the energy is removed by rotational excitations via the stronger electric quadrupole-quadrupole interaction. Estimated conversion rates are in good qualitative agreement with available experimental observations. Here we extend the theory to solid D₂ taking into account the differences between H₂ and D₂ in the molecular and solid-state parameters. The new libron-mediated channel is predicted to result for D₂ in conversion rates under pressure that are by an order of magnitude larger than at P = 0
Molecular rotation in p-H₂ and o-D₂ in phase I under pressure
The orientational order parameter, rotational ground-state energy, and lattice distortion parameter (the deviation of the c/a ratio from the ideal hcp value 1.633) in hcp lattice of phase I of p-H₂ and o-D₂ are calculated using a semi-empirical approach. It is shown that the lattice distortion in these J-even species is small compared with that found in n-H₂, and n-D₂. The difference presumably is caused by the J-odd species
Developments in synthesis, characterization, and application of large, high-quality CVD single crystal diamond
Single crystal diamond synthesis by microwave plasma chemical vapor deposition at rapid growth rate has considerably advanced in the past few years. Developments have been made in growth, optical quality, and mechanical properties. Of the various types of single crystal diamond that can be produced using these techniques, high quality single crystal CVD diamond can be routinely produced, and this material is playing an increasing role in research on materials under extreme conditions. This article highlights recent developments in single crystal CVD diamond synthesis and characterization, as well as various applications in high-pressure materials research.За останні кілька роки синтез монокристалів алмазу мікрохвильовим плазмовим хімічним осадженням з газової фази з високою швидкістю значно просунувся. Прогрес було досягнуто у рості, оптичних і механічних властивостях. З різних типів монокристалічного алмазу, що можна отримати з використанням цього методу, регулярно виробляють високоякісні монокристалічні CVD-алмази, і цей матеріал відіграє все більш важливу роль при дослідженні матеріалів в екстремальних умовах. У даній роботі висвітлено останні досягнення в синтезі монокристалічного CVD-алмазу і дослідженні його характеристик, а також його різне застосування у дослідженні матеріалів при високому тиску.За последние несколько лет синтез монокристаллов алмаза микроволновым плазменным химическим осаждением из газовой фазы с высокой скоростью значительно продвинулся. Прогресс был достигнут в росте, оптических и механических свойствах. Из различных типов монокристаллического алмаза, которые могут быть получены с использованием этого метода, регулярно производят высококачественные монокристаллы CVD-алмаза, и этот материал играет все более важную роль в исследовании материалов в экстремальных условиях. В этой работе осветлены последние разработки в синтезе монокристаллов CVD-алмаза и исследовании его характерист
Signature of small rings in the Raman spectra of normal and compressed amorphous silica: A combined classical and ab initio study
We calculate the parallel (VV) and perpendicular (VH) polarized Raman spectra
of amorphous silica. Model SiO2 glasses, uncompressed and compressed, were
generated by a combination of classical and ab initio molecular-dynamics
simulations and their dynamical matrices were computed within the framework of
the density functional theory. The Raman scattering intensities were determined
using the bond-polarizability model and a good agreement with experimental
spectra was found. We confirm that the modes associated to the fourfold and
threefold rings produce most of the Raman intensity of the D1 and D2 peaks,
respectively, in the VV Raman spectra. Modifications of the Raman spectra upon
compression are found to be in agreement with experimental data. We show that
the modes associated to the fourfold rings still exist upon compression but do
not produce a strong Raman intensity, whereas the ones associated to the
threefold rings do. This result strongly suggests that the area under the D1
and D2 peaks is not directly proportional to the concentration of small rings
in amorphous SiO2.Comment: 21 pages, 8 figures. Phys. Rev. B, in pres
Variational Monte Carlo study of the ground state properties and vacancy formation energy of solid para-H2 using a shadow wave function
A Shadow Wave Function (SWF) is employed along with Variational Monte Carlo
techniques to describe the ground state properties of solid molecular
para-hydrogen. The study has been extended to densities below the equilibrium
value, to obtain a parameterization of the SWF useful for the description of
inhomogeneous phases. We also present an estimate of the vacancy formation
energy as a function of the density, and discuss the importance of relaxation
effects near the vacant site
Mechanical versus thermodynamical melting in pressure-induced amorphization: the role of defects
We study numerically an atomistic model which is shown to exhibit a one--step
crystal--to--amorphous transition upon decompression. The amorphous phase
cannot be distinguished from the one obtained by quenching from the melt. For a
perfectly crystalline starting sample, the transition occurs at a pressure at
which a shear phonon mode destabilizes, and triggers a cascade process leading
to the amorphous state. When defects are present, the nucleation barrier is
greatly reduced and the transformation occurs very close to the extrapolation
of the melting line to low temperatures. In this last case, the transition is
not anticipated by the softening of any phonon mode. Our observations reconcile
different claims in the literature about the underlying mechanism of pressure
amorphization.Comment: 7 pages, 7 figure
Reversible Pressure-Induced Amorphization in Solid C70 : Raman and Photoluminescence Study
We have studied single crystals of by Raman scattering and
photoluminescence in the pressure range from 0 to 31.1 GPa. The Raman spectrum
at 31.1 GPa shows only a broad band similar to that of the amorphous carbon
without any trace of the Raman lines of . After releasing the pressure
from 31.1 GPa, the Raman and the photoluminescence spectra of the recovered
sample are that of the starting crystal. These results indicate that
the molecules are stable upto 31.1 GPa and the amorphous carbon high
pressure phase is reversible, in sharp contrast to the results on solid
. A qualitative explaination is suggested in terms of inter- versus
intra-molecular interactions.Comment: To appear in Phys. Rev. Lett., 12 pages, RevTeX (preprint format), 3
figures available upon reques
Consistent Anisotropic Repulsions for Simple Molecules
We extract atom-atom potentials from the effective spherical potentials that
suc cessfully model Hugoniot experiments on molecular fluids, e.g., and
. In the case of the resulting potentials compare very well with the
atom-atom potentials used in studies of solid-state propertie s, while for
they are considerably softer at short distances. Ground state (T=0K) and
room temperatu re calculations performed with the new potential resolve
the previous discrepancy between experimental and theoretical results.Comment: RevTeX, 5 figure
Molecular Dynamics for Fermions
The time-dependent variational principle for many-body trial states is used
to discuss the relation between the approaches of different molecular dynamics
models to describe indistinguishable fermions. Early attempts to include
effects of the Pauli principle by means of nonlocal potentials as well as more
recent models which work with antisymmetrized many-body states are reviewed
under these premises.
Keywords: Many-body theory; Fermion system; Molecular dynamics; Wave-packet
dynamics; Time-dependent variational principle; Statistical properties;
Canonical ensemble; Ergodicity; Time averagingComment: 97 pages, 13 postscript figures. To be published in July 2000 issue
of Reviews of Modern Physics. More information at http://www-aix.gsi.de/~fmd
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