230 research outputs found
Path Integral Monte Carlo Simulation of the Low-Density Hydrogen Plasma
Restricted path integral Monte Carlo simulations are used to calculate the
equilibrium properties of hydrogen in the density and temperature range of
and . We test the accuracy of the pair density matrix and
analyze the dependence on the system size, on the time step of the path
integral and on the type of nodal surface. We calculate the equation of state
and compare with other models for hydrogen valid in this regime. Further, we
characterize the state of hydrogen and describe the changes from a plasma to an
atomic and molecular liquid by analyzing the pair correlation functions and
estimating the number of atoms and molecules present.Comment: 12 pages, 21 figures, submitted for Phys. Rev.
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
Hydrogen-Helium Mixtures at High Pressure
The properties of hydrogen-helium mixtures at high pressure are crucial to
address important questions about the interior of Giant planets e.g. whether
Jupiter has a rocky core and did it emerge via core accretion? Using path
integral Monte Carlo simulations, we study the properties of these mixtures as
a function of temperature, density and composition. The equation of state is
calculated and compared to chemical models. We probe the accuracy of the ideal
mixing approximation commonly used in such models. Finally, we discuss the
structure of the liquid in terms of pair correlation functions.Comment: Proceedings article of the 5th Conference on Cryocrystals and Quantum
Crystals in Wroclaw, Poland, submitted to J. Low. Temp. Phys. (2004
Variational Density Matrix Method for Warm Condensed Matter and Application to Dense Hydrogen
A new variational principle for optimizing thermal density matrices is
introduced. As a first application, the variational many body density matrix is
written as a determinant of one body density matrices, which are approximated
by Gaussians with the mean, width and amplitude as variational parameters. The
method is illustrated for the particle in an external field problem, the
hydrogen molecule and dense hydrogen where the molecular, the dissociated and
the plasma regime are described. Structural and thermodynamic properties
(energy, equation of state and shock Hugoniot) are presented.Comment: 26 pages, 13 figures. submitted to Phys. Rev. E, October 199
Frontiers of the physics of dense plasmas and planetary interiors: experiments, theory, applications
Recent developments of dynamic x-ray characterization experiments of dense
matter are reviewed, with particular emphasis on conditions relevant to
interiors of terrestrial and gas giant planets. These studies include
characterization of compressed states of matter in light elements by x-ray
scattering and imaging of shocked iron by radiography. Several applications of
this work are examined. These include the structure of massive "Super Earth"
terrestrial planets around other stars, the 40 known extrasolar gas giants with
measured masses and radii, and Jupiter itself, which serves as the benchmark
for giant planets.Comment: Accepted to Physics of Plasmas special issue. Review from
HEDP/HEDLA-08, April 12-15, 200
Path Integral Monte Carlo and Density Functional Molecular Dynamics Simulations of Hot, Dense Helium
Two first-principles simulation techniques, path integral Monte Carlo (PIMC)
and density functional molecular dynamics (DFT-MD), are applied to study hot,
dense helium in the density-temperature range of 0.387 - 5.35 g/cc and 500 K -
1.28x10^8 K. One coherent equation of state (EOS) is derived by combining
DFT-MD data at lower temperatures with PIMC results at higher temperatures.
Good agreement between both techniques is found in an intermediate temperature
range. For the highest temperatures, the PIMC results converge to the
Debye-Hueckel limiting law. In order derive the entropy, a thermodynamically
consistent free energy fit is introduced that reproduces the internal energies
and pressure derived from the first-principles simulations. The equation of
state is presented in form of a table as well as a fit and is compared with
chemical models. In addition, the structure of the fluid is analyzed using pair
correlation functions. Shock Hugoniot curves are compared with recent laser
shock wave experiments.Comment: 16 pages, 15 figure
Mass-Radius Relationships for Solid Exoplanets
We use new interior models of cold planets to investigate the mass-radius
relationships of solid exoplanets, considering planets made primarily of iron,
silicates, water, and carbon compounds. We find that the mass-radius
relationships for cold terrestrial-mass planets of all compositions we
considered follow a generic functional form that is not a simple power law:
for up to , where and are scaled mass and radius
values. This functional form arises because the common building blocks of solid
planets all have equations of state that are well approximated by a modified
polytrope of the form .
We find that highly detailed planet interior models, including temperature
structure and phase changes, are not necessary to derive solid exoplanet bulk
composition from mass and radius measurements. For solid exoplanets with no
substantial atmosphere we have also found that: with 5% fractional uncertainty
in planet mass and radius it is possible to distinguish among planets composed
predominantly of iron or silicates or water ice but not more detailed
compositions; with ~5% uncertainty water ice planets with
water by mass may be identified; the minimum plausible planet size for a given
mass is that of a pure iron planet; and carbon planet mass-radius relationships
overlap with those of silicate and water planets due to similar zero-pressure
densities and equations of state. We propose a definition of "super Earths''
based on the clear distinction in radii between planets with significant gas
envelopes and those without.Comment: ApJ, in press, 33 pages including 16 figure
Modelling of microstructure evolution in advanced high strength steels
There is currently a significant development of new families of steels, i.e. advanced high strength steels,in response to the demands of the automotive and construction industries for materials with improved propertycharacteristics. The austenite-ferrite transformation is the key metallurgical tool to tailor the propertiesof steels. The design of processing paths that will lead to the desired microstructures is increasingly been aidedby computer simulations. The present paper illustrates state-of-the-art microstructure modelling approachesfor low carbon steels considering three important processing aspects: (i) run-out table cooling of hot-rolledsteels, (ii) intercritical annealing of cold-rolled sheets, (iii) girth welding of linepipe steels.Phenomenological models based on the Johnson-Mehl-Avrami-Kolmogorov (JMAK) approach incorporatingadditivity are now available to describe phase transformations during run-out table cooling of microalloyedsteels. Strengths and limitations of this approach will be discussed. Process models for intercritical annealingrequire an accurate description of the austenite formation kinetics where morphological complexities can becaptured using the phase field approach. During girth welding the control of the microstructure in the heataffected zone (HAZ) is of paramount importance. The HAZ experiences rapid thermal cycles and steeptemperature gradients. Phase field modelling is an excellent tool to describe the role of these spatialconstraints as will be illustrated for austenite grain growth
Path integral Monte Carlo simulation of charged particles in traps
This chapter is devoted to the computation of equilibrium (thermodynamic)
properties of quantum systems. In particular, we will be interested in the
situation where the interaction between particles is so strong that it cannot
be treated as a small perturbation. For weakly coupled systems many efficient
theoretical and computational techniques do exist. However, for strongly
interacting systems such as nonideal gases or plasmas, strongly correlated
electrons and so on, perturbation methods fail and alternative approaches are
needed. Among them, an extremely successful one is the Monte Carlo (MC) method
which we are going to consider in this chapter.Comment: 18 pages, based on talks on Hareaus school on computational methods,
Greifswald, September 200
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