507 research outputs found
Thermophysical properties of warm dense hydrogen
We study the thermophysical properties of warm dense hydrogen using quantum
molecular dynamics simulations. New results are presented for the pair
distribution functions, the equation of state, the Hugoniot curve, and the
reflectivity. We compare with available experimental data and predictions of
the chemical picture. Especially, we discuss the nonmetal-to-metal transition
which occurs at about 40 GPa in the dense fluid
Electrical conductivity of SiO2 at extreme conditions and planetary dynamos
Ab intio molecular dynamics simulations show that the electrical conductivity of liquid SiO2 is semimetallic at the conditions of the deep molten mantle of early Earth and super-Earths, raising the possibility of silicate dynamos in these bodies. Whereas the electrical conductivity increases uniformly with increasing temperature, it depends nonmonotonically on compression. At very high pressure, the electrical conductivity decreases on compression, opposite to the behavior of many materials. We show that this behavior is caused by a novel compression mechanism: the development of broken charge ordering, and its influence on the electronic band gap
Probing the interiors of the ice giants: Shock compression of water to 700 GPa and 3.8 g/ccm
Recently there has been tremendous increase in the number of identified
extra-solar planetary systems. Our understanding of their formation is tied to
exoplanet internal structure models, which rely upon equations of state of
light elements and compounds like water. Here we present shock compression data
for water with unprecedented accuracy that shows water equations of state
commonly used in planetary modeling significantly overestimate the
compressibility at conditions relevant to planetary interiors. Furthermore, we
show its behavior at these conditions, including reflectivity and isentropic
response, is well described by a recent first-principles based equation of
state. These findings advocate this water model be used as the standard for
modeling Neptune, Uranus, and "hot Neptune" exoplanets, and should improve our
understanding of these types of planets.Comment: Accepted to Phys. Rev. Lett.; supplementary material attached
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Quantum molecular dynamics simulations for the nonmetal-to-metal transition in fluid helium
We have performed quantum molecular dynamics simulations for dense helium to
study the nonmetal-to-metal transition at high pressures. We present new
results for the equation of state and the Hugoniot curve in the warm dense
matter region. The optical conductivity is calculated via the Kubo-Greenwood
formula from which the dc conductivity is derived. The nonmetal-to-metal
transition is identified at about 1 g/ccm. We compare with experimental results
as well as with other theoretical approaches, especially with predictions of
chemical models.Comment: 4 pages, 5 figure
XUV Opacity of Aluminum between the Cold-Solid to Warm-Plasma Transition
We present calculations of the free-free XUV opacity of warm, solid-density
aluminum at photon energies between the plasma frequency at 15 eV and the
L-edge at 73 eV, using both density functional theory combined with molecular
dynamics and a semi-analytical model in the RPA framework with the inclusion of
local field corrections. As the temperature is increased from room temperature
to 10 eV, with the ion and electron temperatures equal, we calculate an
increase in the opacity in the range over which the degree of ionization is
constant. The effect is less pronounced if only the electron temperature is
allowed to increase. The physical significance of these increases is discussed
in terms of intense XUV-laser matter interactions on both femtosecond and
picosecond time-scales.Comment: 4 pages, 3 figure
Empowerment and the Transition to Housing for Homeless Mentally Ill People: An Anthropological Perspective
Often lacking in scholarly and policy-oriented discussions of homelessness are contextualized understandings of the problems faced, and the values held, by homeless mentally ill people. This article, using an anthropological perspective, examines issues that arise for homeless mentally ill individuals in making the transition from shelter living to permanent residences. The transition occurs as part of a housing initiative driven by the philosophy of consumer empowerment. Project participants are placed in independent apartments or evolving consumer households (ECH) — shared, staffed residences designed to transform themselves into consumer-directed living situations over time. The effects of an empowerment paradigm on the organization of space, the nature of social relations, and the management of economic resources in the ECHs are discussed to show that consumers and staff sometimes have contrasting views of what empowerment entails. It is suggested that anthropological research can help to illuminate the issues at stake in determining policy for homeless people with major mental illness
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