2,275 research outputs found
New Phases of Water Ice Predicted at Megabar Pressures
Based on density functional calculations we predict water ice to attain two
new crystal structures with Pbca and Cmcm symmetry at 7.6 and 15.5 Mbar,
respectively. The known high pressure ice phases VII, VIII, X, and Pbcm as well
as the Pbca phase are all insulating and composed of two interpenetrating
hydrogen bonded networks, but the Cmcm structure is metallic and consists of
corrugated sheets of H and O atoms. The H atoms are squeezed into octahedral
positions between next-nearest O atoms while they occupy tetrahedral positions
between nearest O atoms in the ice X, Pbcm, and Pbca phases.Comment: submitted to Physical Review Letters on Jan 25, 201
Tidal Response of Preliminary Jupiter Model
In anticipation of improved observational data for Jupiter's gravitational
field from the Juno spacecraft, we predict the static tidal response for a
variety of Jupiter interior models based on ab initio computer simulations of
hydrogen-helium mixtures. We calculate hydrostatic-equilibrium gravity terms
using the non-perturbative concentric Maclaurin Spheroid (CMS) method that
eliminates lengthy expansions used in the theory of figures. Our method
captures terms arising from the coupled tidal and rotational perturbations,
which we find to be important for a rapidly-rotating planet like Jupiter. Our
predicted static tidal Love number is 10\% larger than
previous estimates. The value is, as expected, highly correlated with the zonal
harmonic coefficient , and is thus nearly constant when plausible changes
are made to interior structure while holding fixed at the observed value.
We note that the predicted static might change due to Jupiter's dynamical
response to the Galilean moons, and find reasons to argue that the change may
be detectable, although we do not present here a theory of dynamical tides for
highly oblate Jovian planets. An accurate model of Jupiter's tidal response
will be essential for interpreting Juno observations and identifying tidal
signals from effects of other interior dynamics in Jupiter's gravitational
field.Comment: 10 Pages, 6 figures, 4 table
Master of Science
thesisThis thesis reviews the techniques used to model the circulation of the atmosphere and oceans. Models of the atmospheric and oceanic circulation can be used to evaluate future and past conditions of the climate of the Earth. They can also be used to evaluate what affect any changes to the parameters for the Earth's environment might have on the climate of the Earth. For models of the atmosphere, physical properties of the air and of the rotating Earth are used to develop equations for the state and motion of the air in the atmosphere. These equations represent the relationships between the temperature, density, and wind speeds of the air in the atmosphere. The final equations are a set of nonlinear partial differential equations which require a numerical solution. There are several methods used to accomplish a numerical solution to these equations. Models of the ocean can be developed in much the same way as models of the atmosphere. The differences include the fact that sea water is essentially incompress- ible. The models are much more complicated as the oceans are much more limited by the shape of the continents and so do not cover the entire Earth like the atmosphere does. To aid in the understanding of ocean circulation, simpler models of the main circulation of the oceans have been developed as "boxes" of sea water connected by flows of sea water between the boxes which are driven by differences in density between the boxes of sea water. The density of the sea water is a function of the temperature and salinity of the water in each part of the ocean. These box models show that there are multiple equilibrium states for the circulation of the ocean
Correlations in Hot Dense Helium
Hot dense helium is studied with first-principles computer simulations. By
combining path integral Monte Carlo and density functional molecular dynamics,
a large temperature and density interval ranging from 1000 to 1000000 K and 0.4
to 5.4 g/cc becomes accessible to first-principles simulations and the changes
in the structure of dense hot fluids can be investigated. The focus of this
article are pair correlation functions between nuclei, between electrons, and
between electrons and nuclei. The density and temperature dependence of these
correlation functions is analyzed in order to describe the structure of the
dense fluid helium at extreme conditions.Comment: accepted for publication in Journal of Physics
Valence Fluctuation in CeMo2Si2C
We report on the valence fluctuation of Ce in CeMoSiC as studied
by means of magnetic susceptibility , specific heat , electrical
resistivity and x-ray absorption spectroscopy. Powder x-ray
diffraction revealed that CeMoSiC crystallizes in
CeCrSiC-type layered tetragonal crystal structure (space group
\textit{P4/mmm}). The unit cell volume of CeMoSiC deviates from the
expected lanthanide contraction, indicating non-trivalent state of Ce ions in
this compound. The observed weak temperature dependence of the magnetic
susceptibility and its low value indicate that Ce ions are in valence
fluctuating state. The formal Ce valence in CeMoSiC
= 3.11 as determined from x-ray absorption spectroscopy
measurement is well bellow the value 3.4 in
tetravalent Ce compound CeO. The temperature dependence of specific heat
does not show any anomaly down to 1.8 K which rules out any magnetic ordering
in the system. The Sommerfeld coefficient obtained from the specific heat data
is = 23.4 mJ/mol\,K. The electrical resistivity follows the
behavior in the low temperature range below 35 K confirming a Fermi
liquid behavior. Accordingly both the Kadowaki Wood ratio and
the Sommerfeld Wilson ratio are in the range expected for
Fermi-liquid systems. In order to get some information on the electronic
states, we calculated the band structure within the density functional theory,
eventhough this approach is not able to treat 4f electrons accurately. The
non- electron states crossing the Fermi level have mostly Mo 4d character.
They provide the states with which the 4f sates are strongly hybridized,
leading to the intermediate valent state.Comment: 18 pages, 10 figures Submitted to Journal of Alloys and Compound
Are gravitational waves from giant magnetar flares observable?
Are giant flares in magnetars viable sources of gravitational radiation? Few
theoretical studies have been concerned with this problem, with the small
number using either highly idealized models or assuming a magnetic field orders
of magnitude beyond what is supported by observations. We perform nonlinear
general-relativistic magnetohydrodynamics simulations of large-scale
hydromagnetic instabilities in magnetar models. We utilise these models to find
gravitational wave emissions over a wide range of energies, from 10^40 to 10^47
erg. This allows us to derive a systematic relationship between the surface
field strength and the gravitational wave strain, which we find to be highly
nonlinear. In particular, for typical magnetar fields of a few times 10^15 G,
we conclude that a direct observation of f-modes excited by global magnetic
field reconfigurations is unlikely with present or near-future gravitational
wave observatories, though we also discuss the possibility that modes in a
low-frequency band up to 100 Hz could be sufficiently excited to be relevant
for observation.Comment: 4 pages, 3 figures. Further information can be found at
http://www.physik.uni-tuebingen.de/institute/astronomie-astrophysik/institut/theoretische-astrophysik/forschung.htm
Electrophoresis of colloidal dispersions in the low-salt regime
We study the electrophoretic mobility of spherical charged colloids in a
low-salt suspension as a function of the colloidal concentration. Using an
effective particle charge and a reduced screening parameter, we map the data
for systems with different particle charges and sizes, including numerical
simulation data with full electrostatics and hydrodynamics and experimental
data for latex dispersions, on a single master curve. We observe two different
volume fraction-dependent regimes for the electrophoretic mobility that can be
explained in terms of the static properties of the ionic double layer.Comment: Substantially revised versio
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