10,497 research outputs found
Crystal nucleation and cluster-growth kinetics in a model glass under shear
Crystal nucleation and growth processes induced by an externally applied
shear strain in a model metallic glass are studied by means of nonequilibrium
molecular dynamics simulations, in a range of temperatures. We observe that the
nucleation-growth process takes place after a transient, induction regime. The
critical cluster size and the lag-time associated with this induction period
are determined from a mean first-passage time analysis. The laws that describe
the cluster growth process are studied as a function of temperature and strain
rate. A theoretical model for crystallization kinetics that includes the time
dependence for nucleation and cluster growth is developed within the framework
of the Kolmogorov-Johnson-Mehl-Avrami scenario and is compared with the
molecular dynamics data. Scalings for the cluster growth laws and for the
crystallization kinetics are also proposed and tested. The observed nucleation
rates are found to display a nonmonotonic strain rate dependency
A note on Weyl transformations in two-dimensional dilaton gravity
We discuss Weyl (conformal) transformations in two-dimensional matterless
dilaton gravity. We argue that both classical and quantum dilaton gravity
theories are invariant under Weyl transformations.Comment: 8 pages, accepted for publication in Mod. Phys. Lett.
Hole Pairs in the Two-Dimensional Hubbard Model
The interactions between holes in the Hubbard model, in the low density,
intermediate to strong coupling limit, are investigated. Dressed spin polarons
in neighboring sites have an increased kinetic energy and an enhanced hopping
rate. Both effects are of the order of the hopping integral and lead to an
effective attraction at intermediate couplings. Our results are derived by
systematically improving mean field calculations. The method can also be used
to derive known properties of isolated spin polarons.Comment: 4 page
Universality of Cluster Dynamics
We have studied the kinetics of cluster formation for dynamical systems of
dimensions up to interacting through elastic collisions or coalescence.
These systems could serve as possible models for gas kinetics, polymerization
and self-assembly. In the case of elastic collisions, we found that the cluster
size probability distribution undergoes a phase transition at a critical time
which can be predicted from the average time between collisions. This enables
forecasting of rare events based on limited statistical sampling of the
collision dynamics over short time windows. The analysis was extended to
L-normed spaces () to allow for some amount of
interpenetration or volume exclusion. The results for the elastic collisions
are consistent with previously published low-dimensional results in that a
power law is observed for the empirical cluster size distribution at the
critical time. We found that the same power law also exists for all dimensions
, 2D L norms, and even for coalescing collisions in 2D. This
broad universality in behavior may be indicative of a more fundamental process
governing the growth of clusters
Radio emission from satellite-Jupiter interactions (especially Ganymede)
Analyzing a database of 26 years of observations of Jupiter from the
Nan\c{c}ay Decameter Array, we study the occurrence of Io-independent emissions
as a function of the orbital phase of the other Galilean satellites and
Amalthea. We identify unambiguously the emissions induced by Ganymede and
characterize their intervals of occurrence in CML and Ganymede phase and
longitude. We also find hints of emissions induced by Europa and, surprisingly,
by Amalthea. The signature of Callisto-induced emissions is more tenuous.Comment: 14 pages, 7 figures, in "Planetary Radio Emissions VIII", G. Fischer,
G. Mann, M. Panchenko and P. Zarka eds., Austrian Acad. Sci. Press, Vienna,
in press, 201
Ballistic magnon transport and phonon scattering in the antiferromagnet NdCuO
The thermal conductivity of the antiferromagnet NdCuO was measured
down to 50 mK. Using the spin-flop transition to switch on and off the acoustic
Nd magnons, we can reliably separate the magnon and phonon contributions to
heat transport. We find that magnons travel ballistically below 0.5 K, with a
thermal conductivity growing as , from which we extract their velocity. We
show that the rate of scattering of acoustic magnons by phonons grows as ,
and the scattering of phonons by magnons peaks at twice the average Nd magnon
frequency.Comment: 4 pages, 3 figures, one figure modifie
Spherically symmetric scalar field collapse in any dimension
We describe a formalism and numerical approach for studying spherically
symmetric scalar field collapse for arbitrary spacetime dimension d and
cosmological constant Lambda. The presciption uses a double null formalism, and
is based on field redefinitions first used to simplify the field equations in
generic two-dimensional dilaton gravity. The formalism is used to construct
code in which d and Lambda are input parameters. The code reproduces known
results in d = 4 and d = 6 with Lambda = 0. We present new results for d = 5
with zero and negative Lambda.Comment: 16 pages, 6 figures, typos corrected, presentational changes, PRD in
pres
Kondo effect in transport through molecules adsorbed on metal surfaces: from Fano dips to Kondo peaks
The Kondo effect observed in recent STM experiments on transport through CoPc
and TBrPP-Co molecules adsorbed on Au(111) and Cu(111) surfaces, respectively,
is discussed within the framework of a simple model (Phys. Rev. Lett. {\bf 97},
076806 (2006)). It is shown that, in the Kondo regime and by varying the
adequate model parameters, it is possible to produce a crossover from a
conductance Kondo peak (CoPc) to a conductance Fano dip (TBrPP-Co). In the case
of TBrPP-Co/Cu(111) we show that the model reproduces the changes in the shape
of the Fano dip, the raising of the Kondo temperature and shifting to higher
energies of the dip minimum when the number of nearest neighbors molecules is
lowered. These features are in line with experimental observations indicating
that our simple model contains the essential physics underlying the transport
properties of such complex molecules.Comment: 4 pages, 3 figures, submitted to PR
Real-Time Maps of Fluid Flow Fields in Porous Biomaterials
Mechanical forces such as fluid shear have been shown to enhance cell growth
and differentiation, but knowledge of their mechanistic effect on cells is
limited because the local flow patterns and associated metrics are not
precisely known. Here we present real-time, noninvasive measures of local
hydrodynamics in 3D biomaterials based on nuclear magnetic resonance. Microflow
maps were further used to derive pressure, shear and fluid permeability fields.
Finally, remodeling of collagen gels in response to precise fluid flow
parameters was correlated with structural changes. It is anticipated that
accurate flow maps within 3D matrices will be a critical step towards
understanding cell behavior in response to controlled flow dynamics.Comment: 23 pages, 4 figure
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