3,394 research outputs found
Role of configurational entropy in the thermodynamics of clusters of point defects in crystalline solids
The internal configurational entropy of point defect clusters in crystalline silicon is studied in detail by analyzing their potential energy landscapes. Both on-lattice and off-lattice calculation approaches are employed to demonstrate the importance of off-lattice configurational states that arise due to a large number of inherent structures (local minima) in the energy landscape generated by the interatomic potential function. The resulting cluster configurational entropy of formation is shown to exhibit behavior that is qualitatively similar to that observed in supercooled liquids and amorphous solids and substantially alters the thermodynamic properties of point defect clusters in crystals at high temperature. This behavior is shown to be independent of interatomic potential and cluster type, and suggests that defects in crystals at high temperature should be generally described by a quasicontinuous collection of nondegenerate states rather than as a single ground state structure. The modified thermodynamic properties of vacancy clusters at high temperature are found to explain a longstanding discrepancy between simulation predictions and experimental measurements of vacancy aggregation dynamics in silicon
New Pseudo-Phase Structure for -Pu
In this paper we propose a new pseudo-phase crystal structure, based on an
orthorhombic distortion of the diamond structure, for the ground-state
-phase of plutonium. Electronic-structure calculations in the
generalized-gradient approximation give approximately the same total energy for
the two structures. Interestingly, our new pseudo-phase structure is the same
as the Pu -phase structure except with very different b/a and c/a
ratios. We show how the contraction relative to the phase, principally
in the direction, leads to an -like structure in the [0,1,1] plane.
This is an important link between two complex structures of plutonium and opens
new possibilities for exploring the very rich phase diagram of Pu through
theoretical calculations
Hydrodynamic Coupling of Two Brownian Spheres to a Planar Surface
We describe direct imaging measurements of the collective and relative
diffusion of two colloidal spheres near a flat plate. The bounding surface
modifies the spheres' dynamics, even at separations of tens of radii. This
behavior is captured by a stokeslet analysis of fluid flow driven by the
spheres' and wall's no-slip boundary conditions. In particular, this analysis
reveals surprising asymmetry in the normal modes for pair diffusion near a flat
surface.Comment: 4 pages, 4 figure
Like-charge attraction through hydrodynamic interaction
We demonstrate that the attractive interaction measured between like-charged
colloidal spheres near a wall can be accounted for by a nonequilibrium
hydrodynamic effect. We present both analytical results and Brownian dynamics
simulations which quantitatively capture the one-wall experiments of Larsen and
Grier (Nature 385, p. 230, 1997).Comment: 10 pages, 4 figure
S-band Polarization All Sky Survey (S-PASS): survey description and maps
We present the S-Band Polarization All Sky Survey (S-PASS), a survey of
polarized radio emission over the southern sky at Dec~ taken with
the Parkes radio telescope at 2.3~GHz. The main aim was to observe at a
frequency high enough to avoid strong depolarization at intermediate Galactic
latitudes (still present at 1.4 GHz) to study Galactic magnetism, but low
enough to retain ample Signal-to-Noise ratio (S/N) at high latitudes for
extragalactic and cosmological science. We developed a new scanning strategy
based on long azimuth scans, and a corresponding map-making procedure to make
recovery of the overall mean signal of Stokes and possible, a
long-standing problem with polarization observations. We describe the scanning
strategy, map-making procedure, and validation tests. The overall mean signal
is recovered with a precision better than 0.5\%. The maps have a mean
sensitivity of 0.81 mK on beam--size scales and show clear polarized signals,
typically to within a few degrees of the Galactic plane, with ample S/N
everywhere (the typical signal in low emission regions is 13 mK, and 98.6\% of
the pixels have S/N ). The largest depolarization areas are in the inner
Galaxy, associated with the Sagittarius Arm. We have also computed a Rotation
Measure map combining S-PASS with archival data from the WMAP and Planck
experiments. A Stokes map has been generated, with a sensitivity limited to
the confusion level of 9 mK.Comment: Accepted for publication on MNRAS. Maps are available for download at
the website indicated in the manuscrip
Radio and gamma-ray constraints on dark matter annihilation in the Galactic center
We determine upper limits on the dark matter (DM) self-annihilation cross
section for scenarios in which annihilation leads to the production of
electron--positron pairs. In the Galactic centre (GC), relativistic electrons
and positrons produce a radio flux via synchroton emission, and a gamma ray
flux via bremsstrahlung and inverse Compton scattering. On the basis of
archival, interferometric and single-dish radio data, we have determined the
radio spectrum of an elliptical region around the Galactic centre of extent 3
degrees semi-major axis (along the Galactic plane) and 1 degree semi-minor axis
and a second, rectangular region, also centered on the GC, of extent 1.6
degrees x 0.6 degrees. The radio spectra of both regions are non-thermal over
the range of frequencies for which we have data: 74 MHz -- 10 GHz. We also
consider gamma-ray data covering the same region from the EGRET instrument
(about GeV) and from HESS (around TeV). We show how the combination of these
data can be used to place robust constraints on DM annihilation scenarios, in a
way which is relatively insensitive to assumptions about the magnetic field
amplitude in this region. Our results are approximately an order of magnitude
more constraining than existing Galactic centre radio and gamma ray limits. For
a DM mass of m_\chi =10 GeV, and an NFW profile, we find that the
velocity-averaged cross-section must be less than a few times 10^-25 cm^3 s^-1.Comment: 14 pages, 9 figures. Version accepted for publication in PRD.
Reference section updated/extended
Microrheology, stress fluctuations and active behavior of living cells
We report the first measurements of the intrinsic strain fluctuations of
living cells using a recently-developed tracer correlation technique along with
a theoretical framework for interpreting such data in heterogeneous media with
non-thermal driving. The fluctuations' spatial and temporal correlations
indicate that the cytoskeleton can be treated as a course-grained continuum
with power-law rheology, driven by a spatially random stress tensor field.
Combined with recent cell rheology results, our data imply that intracellular
stress fluctuations have a nearly power spectrum, as expected for
a continuum with a slowly evolving internal prestress.Comment: 4 pages, 2 figures, to appear in Phys. Rev. Let
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