12,230 research outputs found
Reliable First-Principles Alloy Thermodynamics via Truncated Cluster Expansions
In alloys cluster expansions (CE) are increasingly used to combine
first-principles electronic-structure and Monte Carlo methods to predict
thermodynamic properties. As a basis-set expansion in terms of lattice
geometrical clusters and effective cluster interactions, the CE is exact if
infinite, but is tractable only if truncated. Yet until now a truncation
procedure was not well-defined and did not guarantee a reliable truncated CE.
We present an optimal truncation procedure for CE basis sets that provides
reliable thermodynamics. We then exemplify its importance in NiV, where the
CE has failed unpredictably, and now show agreement to a range of measured
values, predict new low-energy structures, and explain the cause of previous
failures.Comment: 4 pages, 2 figure
Observability of counterpropagating modes at fractional-quantum-Hall edges
When the bulk filling factor is equal to 1 - 1/m with m odd, at least one
counterpropagating chiral collective mode occurs simultaneously with
magnetoplasmons at the edge of fractional-quantum-Hall samples. Initial
experimental searches for an additional mode were unsuccessful. In this paper,
we address conditions under which its observation should be expected in
experiments where the electronic system is excited and probed by capacitive
coupling. We derive realistic expressions for the velocity of the slow
counterpropagating mode, starting from a microscopic calculation which is
simplified by a Landau-Silin-like separation between long-range Hartree and
residual interactions. The microscopic calculation determines the stiffness of
the edge to long-wavelength neutral excitations, which fixes the slow-mode
velocity, and the effective width of the edge region, which influences the
magnetoplasmon dispersion.Comment: 18 pages, RevTex, 6 figures, final version to be published in
Physical Review
Representability problems for coarse-grained water potentials
The use of an effective intermolecular potential often involves a compromise
between more accurate, complex functional forms and more tractable simple
representations. To study this choice in detail, we systematically derive
coarse-grained isotropic pair potentials that accurately reproduce the
oxygen-oxygen radial distribution function of the TIP4P-Ew water model at state
points over density ranges from 0.88-1.30g/cc and temperature ranges from
235K-310K. Although by construction these effective potentials correctly
represent the isothermal compressibility of TIP4P-Ew water, they do not
accurately resolve other thermodynamic properties such as the virial pressure,
the internal energy or thermodynamic anomalies. Because at a given state point
the pair potential that reproduces the pair structure is unique, we have
therefore explicitly demonstrated that it is impossible to simultaneously
represent the pair-structure and several key equilibrium thermodynamic
properties of water with state-point dependent radially symmetric pair
potentials. We argue that such representability problems are related to, but
different from, more widely acknowledged transferability problems, and discuss
in detail the implications this has for the modeling of water and other liquids
by coarse-grained potentials. Nevertheless, regardless of thermodynamic
inconsistencies, the state-point dependent effective potentials for water do
generate structural and dynamical anomalies.Comment: 22 page
Stress in nurses : stress-related affect and its determinants examined over the nursing day
Peer reviewedPostprin
Bondi flow from a slowly rotating hot atmosphere
A supermassive black hole in the nucleus of an elliptical galaxy at the
centre of a cool-core group or cluster of galaxies is immersed in hot gas.
Bondi accretion should occur at a rate determined by the properties of the gas
at the Bondi radius and the mass of the black hole. X-ray observations of
massive nearby elliptical galaxies, including M87 in the Virgo cluster,
indicate a Bondi accretion rate Mdot which roughly matches the total kinetic
power of the jets, suggesting that there is a tight coupling between the jet
power and the mass accretion rate. While the Bondi model considers non-rotating
gas, it is likely that the external gas has some angular momentum, which
previous studies have shown could decrease the accretion rate drastically. We
investigate here the possibility that viscosity acts at all radii to transport
angular momentum outward so that the accretion inflow proceeds rapidly and
steadily. The situation corresponds to a giant Advection Dominated Accretion
Flow (ADAF) which extends from beyond the Bondi radius down to the black hole.
We find solutions of the ADAF equations in which the gas accretes at just a
factor of a few less than Mdot. These solutions assume that the atmosphere
beyond the Bondi radius rotates with a sub-Keplerian velocity and that the
viscosity parameter is large, alpha~0.1. The infall time of the ADAF solutions
is no more than a few times the free-fall time. Thus the accretion rate at the
black hole is closely coupled to the surrounding gas, enabling tight feedback
to occur. We show that jet powers of a few per cent of Mdot c^2 are expected if
either a fraction of the accretion power is channeled into the jet or the black
hole spin energy is tapped by a strong magnetic field pressed against the black
hole by the pressure of the accretion flow.(Truncated)Comment: 10 pages, 6 figures, MNRAS, in pres
Complete genome sequence of the encephalomyelitic Burkholderia pseudomallei strain MSHR305
We describe the complete genome sequence of Burkholderia pseudomallei MSHR305, a clinical isolate taken from a fatal encephalomyelitis case, a rare form of melioidosis. This sequence will be used for comparisons to identify the genes that are involved in neurological cases
Complete genome sequence of the encephalomyelitic Burkholderia pseudomallei strain MSHR305
We describe the complete genome sequence of Burkholderia pseudomallei MSHR305, a clinical isolate taken from a fatal encephalomyelitis case, a rare form of melioidosis. This sequence will be used for comparisons to identify the genes that are involved in neurological cases
Coulomb correlation effects in semiconductor quantum dots: The role of dimensionality
We study the energy spectra of small three-dimensional (3D) and
two-dimensional (2D) semiconductor quantum dots through different theoretical
approaches (single-site Hubbard and Hartree-Fock hamiltonians); in the smallest
dots we also compare with exact results. We find that purely 2D models often
lead to an inadequate description of the Coulomb interaction existing in
realistic structures, as a consequence of the overestimated carrier
localization. We show that the dimensionality of the dots has a crucial impact
on (i) the accuracy of the predicted addition spectra; (ii) the range of
validity of approximate theoretical schemes. When applied to realistic 3D
geometries, the latter are found to be much more accurate than in the
corresponding 2D cases for a large class of quantum dots; the single-site
Hubbard hamiltonian is shown to provide a very effective and accurate scheme to
describe quantum dot spectra, leading to good agreement with experiments.Comment: LaTeX 2.09, RevTeX, 25 pages, 9 Encapsulated Postscript figures. To
be published in Physical Review
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