940 research outputs found
VPLanet: The Virtual Planet Simulator
We describe a software package called VPLanet that simulates fundamental
aspects of planetary system evolution over Gyr timescales, with a focus on
investigating habitable worlds. In this initial release, eleven physics modules
are included that model internal, atmospheric, rotational, orbital, stellar,
and galactic processes. Many of these modules can be coupled simultaneously to
simulate the evolution of terrestrial planets, gaseous planets, and stars. The
code is validated by reproducing a selection of observations and past results.
VPLanet is written in C and designed so that the user can choose the physics
modules to apply to an individual object at runtime without recompiling, i.e.,
a single executable can simulate the diverse phenomena that are relevant to a
wide range of planetary and stellar systems. This feature is enabled by
matrices and vectors of function pointers that are dynamically allocated and
populated based on user input. The speed and modularity of VPLanet enables
large parameter sweeps and the versatility to add/remove physical phenomena to
assess their importance. VPLanet is publicly available from a repository that
contains extensive documentation, numerous examples, Python scripts for
plotting and data management, and infrastructure for community input and future
development.Comment: 75 pages, 34 figures, 10 tables, accepted to the Proceedings of the
Astronomical Society of the Pacific. Source code, documentation, and examples
available at https://github.com/VirtualPlanetaryLaboratory/vplane
Diffusion under temperature gradient: A phase-field model study
A diffuse interface model was devised and employed to investigate the effect of thermotransport (a.k.a., thermomigration) process in single-phase and two-phase alloys of a binary system. Simulation results show that an applied temperature gradient can cause significant redistribution of constituent elements and phases in the alloy. The magnitude and the direction of the redistribution depend on the initial composition, the atomic mobility and the heat of transport of the respective elements. In two-phase alloys, the thermomigration effect can cause the formation of single-element rich phases at the cold and hot ends of the alloy (i.e., demixing)
Instability of insulating states in optical lattices due to collective phonon excitations
The role of collective phonon excitations on the properties of cold atoms in
optical lattices is investigated. These phonon excitations are collective
excitations, whose appearance is caused by intersite atomic interactions
correlating the atoms, and they do not arise without such interactions. These
collective excitations should not be confused with lattice vibrations produced
by an external force. No such a force is assumed. But the considered phonons
are purely self-organized collective excitations, characterizing atomic
oscillations around lattice sites, due to intersite atomic interactions. It is
shown that these excitations can essentially influence the possibility of atoms
to be localized. The states that would be insulating in the absence of phonon
excitations can become delocalized when these excitations are taken into
account. This concerns long-range as well as local atomic interactions. To
characterize the region of stability, the Lindemann criterion is used.Comment: Latex file, 27 pages, 1 figur
A study of 15N14N isotopic exchange over cobalt molybdenum nitrides
The 14N/15N isotopic exchange pathways over Co3Mo3N, a material of interest as an ammonia synthesis catalyst and for the development of nitrogen transfer reactions, have been investigated. Both the homomolecular and heterolytic exchange processes have been studied, and it has been shown that lattice nitrogen species are exchangeable. The exchange behavior was found to be a strong function of pretreatment with ca. 25% of lattice N atoms being exchanged after 40 min at 600 °C after N2 pretreatment at 700 °C compared to only 6% following similar Ar pretreatment. This observation, for which the potential contribution of adsorbed N species can be discounted, is significant in terms of the application of this material. In the case of the Co6Mo6N phase, regeneration to Co3Mo3N under 15N2 at 600 °C occurs concurrently with 14N15N formation. These observations demonstrate the reactivity of nitrogen in the Co–Mo–N system to be a strong function of pretreatment and worthy of further consideration
Observation of a new excitation in bcc solid 4He by inelastic neutron scattering
We report neutron scattering measurements of the phonons in bcc solid 4He. In
general, only 3 accoustic phonon branches should exist in a monoatomic cubic
crystal. In addition to these phonon branches, we found a new ''optic-like''
mode along the [110] direction. One possible interpretation of this new mode is
in terms of localized excitations unique to a quantum solid.Comment: Text and 4 figures, to appear in Phys. Rev. Let
Epitaxial growth in dislocation-free strained alloy films: Morphological and compositional instabilities
The mechanisms of stability or instability in the strained alloy film growth
are of intense current interest to both theorists and experimentalists. We
consider dislocation-free, coherent, growing alloy films which could exhibit a
morphological instability without nucleation. We investigate such strained
films by developing a nonequilibrium, continuum model and by performing a
linear stability analysis. The couplings of film-substrate misfit strain,
compositional stress, deposition rate, and growth temperature determine the
stability of film morphology as well as the surface spinodal decomposition. We
consider some realistic factors of epitaxial growth, in particular the
composition dependence of elastic moduli and the coupling between top surface
and underlying bulk of the film. The interplay of these factors leads to new
stability results. In addition to the stability diagrams both above and below
the coherent spinodal temperature, we also calculate the kinetic critical
thickness for the onset of instability as well as its scaling behavior with
respect to misfit strain and deposition rate. We apply our results to some real
growth systems and discuss the implications related to some recent experimental
observations.Comment: 26 pages, 13 eps figure
Critical dimensions for random walks on random-walk chains
The probability distribution of random walks on linear structures generated
by random walks in -dimensional space, , is analytically studied
for the case . It is shown to obey the scaling form
, where is
the density of the chain. Expanding in powers of , we find that
there exists an infinite hierarchy of critical dimensions, ,
each one characterized by a logarithmic correction in . Namely, for
, ; for ,
; for , ; for , ; for , , {\it etc.\/} In particular, for
, this implies that the temporal dependence of the probability density of
being close to the origin .Comment: LATeX, 10 pages, no figures submitted for publication in PR
Resonant steps and spatiotemporal dynamics in the damped dc-driven Frenkel-Kontorova chain
Kink dynamics of the damped Frenkel-Kontorova (discrete sine-Gordon) chain
driven by a constant external force are investigated. Resonant steplike
transitions of the average velocity occur due to the competitions between the
moving kinks and their radiated phasonlike modes. A mean-field consideration is
introduced to give a precise prediction of the resonant steps. Slip-stick
motion and spatiotemporal dynamics on those resonant steps are discussed. Our
results can be applied to studies of the fluxon dynamics of 1D
Josephson-junction arrays and ladders, dislocations, tribology and other
fields.Comment: 20 Plain Latex pages, 10 Eps figures, to appear in Phys. Rev.
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