8,128 research outputs found
Weighted-density approximation for general nonuniform fluid mixtures
In order to construct a general density-functional theory for nonuniform
fluid mixtures, we propose an extension to multicomponent systems of the
weighted-density approximation (WDA) of Curtin and Ashcroft [Phys. Rev. A 32,
2909 (1985)]. This extension corrects a deficiency in a similar extension
proposed earlier by Denton and Ashcroft [Phys. Rev. A 42, 7312 (1990)], in that
that functional cannot be applied to the multi-component nonuniform fluid
systems with spatially varying composition, such as solid-fluid interfaces. As
a test of the accuracy of our new functional, we apply it to the calculation of
the freezing phase diagram of a binary hard-sphere fluid, and compare the
results to simulation and the Denton-Ashcroft extension.Comment: 4 pages, 4 figures, to appear in Phys. Rev. E as Brief Repor
Injection-Locked Dye Laser Pumped By A Xenon-lon Laser
Injection locking of a dye laser is reported for a 4-minor ring-cavity dye User pumped by a xenon-ion laser. Both a He-Ne laser and tunable CW dye laser were used as the injection sources. Copyright © 1980 by The Institute of Electrical and Electronics Engineers, Inc
Adjusting the melting point of a model system via Gibbs-Duhem integration: application to a model of Aluminum
Model interaction potentials for real materials are generally optimized with
respect to only those experimental properties that are easily evaluated as
mechanical averages (e.g., elastic constants (at T=0 K), static lattice
energies and liquid structure). For such potentials, agreement with experiment
for the non-mechanical properties, such as the melting point, is not guaranteed
and such values can deviate significantly from experiment. We present a method
for re-parameterizing any model interaction potential of a real material to
adjust its melting temperature to a value that is closer to its experimental
melting temperature. This is done without significantly affecting the
mechanical properties for which the potential was modeled. This method is an
application of Gibbs-Duhem integration [D. Kofke, Mol. Phys.78, 1331 (1993)].
As a test we apply the method to an embedded atom model of aluminum [J. Mei and
J.W. Davenport, Phys. Rev. B 46, 21 (1992)] for which the melting temperature
for the thermodynamic limit is 826.4 +/- 1.3K - somewhat below the experimental
value of 933K. After re-parameterization, the melting temperature of the
modified potential is found to be 931.5K +/- 1.5K.Comment: 9 pages, 5 figures, 4 table
Particle Dark Energy
We explore the physics of a gas of particles interacting with a condensate
that spontaneously breaks Lorentz invariance. The equation of state of this gas
varies from 1/3 to less than -1 and can lead to the observed cosmic
acceleration. The particles are always stable. In our particular class of
models these particles are fermions with a chiral coupling to the condensate.
They may behave as relativistic matter at early times, produce a brief period
where they dominate the expansion with w<0 today, and behave as matter at late
time. There are no small parameters in our models, which generically lead to
dark energy clustering and, depending on the choice of parameters, smoothing of
small scale power.Comment: 8 pages, 5 figures; minor update with added refs; version appearing
in Phys. Rev.
Interplay between distribution of live cells and growth dynamics of solid tumours
Experiments show that simple diffusion of nutrients and waste molecules is not sufficient to explain the typical multilayered structure of solid tumours, where an outer rim of proliferating cells surrounds a layer of quiescent but viable cells and a central necrotic region. These experiments challenge models of tumour growth based exclusively on diffusion. Here we propose a model of tumour growth that incorporates the volume dynamics and the distribution of cells within the viable cell rim. The model is suggested by in silico experiments and is validated using in vitro data. The results correlate with in vivo data as well, and the model can be used to support experimental and clinical oncology
Measurement of Temporal Correlations of the Overhauser Field in a Double Quantum Dot
In quantum dots made from materials with nonzero nuclear spins, hyperfine
coupling creates a fluctuating effective Zeeman field (Overhauser field) felt
by electrons, which can be a dominant source of spin qubit decoherence. We
characterize the spectral properties of the fluctuating Overhauser field in a
GaAs double quantum dot by measuring correlation functions and power spectra of
the rate of singlet-triplet mixing of two separated electrons. Away from zero
field, spectral weight is concentrated below 10 Hz, with 1/f^2 dependence on
frequency, f. This is consistent with a model of nuclear spin diffusion, and
indicates that decoherence can be largely suppressed by echo techniques.Comment: related papers available at http://marcuslab.harvard.ed
Direct calculation of the hard-sphere crystal/melt interfacial free energy
We present a direct calculation by molecular-dynamics computer simulation of
the crystal/melt interfacial free energy, , for a system of hard
spheres of diameter . The calculation is performed by thermodynamic
integration along a reversible path defined by cleaving, using specially
constructed movable hard-sphere walls, separate bulk crystal and fluid systems,
which are then merged to form an interface. We find the interfacial free energy
to be slightly anisotropic with = 0.62, 0.64 and
0.58 for the (100), (110) and (111) fcc crystal/fluid
interfaces, respectively. These values are consistent with earlier density
functional calculations and recent experiments measuring the crystal nucleation
rates from colloidal fluids of polystyrene spheres that have been interpreted
[Marr and Gast, Langmuir {\bf 10}, 1348 (1994)] to give an estimate of
for the hard-sphere system of , slightly lower
than the directly determined value reported here.Comment: 4 pages, 4 figures, submitted to Physical Review Letter
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