11,913 research outputs found
Molecular Dynamics Simulation of Vascular Network Formation
Endothelial cells are responsible for the formation of the capillary blood
vessel network. We describe a system of endothelial cells by means of
two-dimensional molecular dynamics simulations of point-like particles. Cells'
motion is governed by the gradient of the concentration of a chemical substance
that they produce (chemotaxis). The typical time of degradation of the chemical
substance introduces a characteristic length in the system. We show that
point-like model cells form network resembling structures tuned by this
characteristic length, before collapsing altogether. Successively, we improve
the non-realistic point-like model cells by introducing an isotropic strong
repulsive force between them and a velocity dependent force mimicking the
observed peculiarity of endothelial cells to preserve the direction of their
motion (persistence). This more realistic model does not show a clear network
formation. We ascribe this partial fault in reproducing the experiments to the
static geometry of our model cells that, in reality, change their shapes by
elongating toward neighboring cells.Comment: 10 pages, 3 figures, 2 of which composite with 8 pictures each.
Accepted on J.Stat.Mech. (2009). Appeared at the poster session of
StatPhys23, Genoa, Italy, July 13 (2007
Phase diagram of softly repulsive systems: The Gaussian and inverse-power-law potentials
We redraw, using state-of-the-art methods for free-energy calculations, the
phase diagrams of two reference models for the liquid state: the Gaussian and
inverse-power-law repulsive potentials. Notwithstanding the different behavior
of the two potentials for vanishing interparticle distances, their
thermodynamic properties are similar in a range of densities and temperatures,
being ruled by the competition between the body-centered-cubic (BCC) and
face-centered-cubic (FCC) crystalline structures and the fluid phase. We
confirm the existence of a reentrant BCC phase in the phase diagram of the
Gaussian-core model, just above the triple point. We also trace the BCC-FCC
coexistence line of the inverse-power-law model as a function of the power
exponent and relate the common features in the phase diagrams of such
systems to the softness degree of the interaction.Comment: 22 pages, 8 figure
Dark Matter Axions Revisited
We study for what specific values of the theoretical parameters the axion can
form the totality of cold dark matter. We examine the allowed axion parameter
region in the light of recent data collected by the WMAP5 mission plus baryon
acoustic oscillations and supernovae, and assume an inflationary scenario and
standard cosmology. If the Peccei-Quinn symmetry is restored after inflation,
we recover the usual relation between axion mass and density, so that an axion
mass makes the axion 100% of the cold dark matter. If
the Peccei-Quinn symmetry is broken during inflation, the axion can instead be
100% of the cold dark matter for provided a specific value
of the initial misalignment angle is chosen in correspondence to a
given value of its mass . Large values of the Peccei-Quinn symmetry
breaking scale correspond to small, perhaps uncomfortably small, values of the
initial misalignment angle .Comment: 14 pages, 3 figure
Quantum fidelity and quantum phase transitions in matrix product states
Matrix product states, a key ingredient of numerical algorithms widely
employed in the simulation of quantum spin chains, provide an intriguing tool
for quantum phase transition engineering. At critical values of the control
parameters on which their constituent matrices depend, singularities in the
expectation values of certain observables can appear, in spite of the
analyticity of the ground state energy. For this class of generalized quantum
phase transitions we test the validity of the recently introduced fidelity
approach, where the overlap modulus of ground states corresponding to slightly
different parameters is considered. We discuss several examples, successfully
identifying all the present transitions. We also study the finite size scaling
of fidelity derivatives, pointing out its relevance in extracting critical
exponents.Comment: 7 pages, 3 figure
Coarsening scenarios in unstable crystal growth
Crystal surfaces may undergo thermodynamical as well kinetic,
out-of-equilibrium instabilities. We consider the case of mound and pyramid
formation, a common phenomenon in crystal growth and a long-standing problem in
the field of pattern formation and coarsening dynamics. We are finally able to
attack the problem analytically and get rigorous results. Three dynamical
scenarios are possible: perpetual coarsening, interrupted coarsening, and no
coarsening. In the perpetual coarsening scenario, mound size increases in time
as L=t^n, where the coasening exponent is n=1/3 when faceting occurs, otherwise
n=1/4.Comment: Changes in the final part. Accepted for publication in Phys. Rev.
Let
Classical limit of the quantum Zeno effect
The evolution of a quantum system subjected to infinitely many measurements
in a finite time interval is confined in a proper subspace of the Hilbert
space. This phenomenon is called "quantum Zeno effect": a particle under
intensive observation does not evolve. This effect is at variance with the
classical evolution, which obviously is not affected by any observations. By a
semiclassical analysis we will show that the quantum Zeno effect vanishes at
all orders, when the Planck constant tends to zero, and thus it is a purely
quantum phenomenon without classical analog, at the same level of tunneling.Comment: 10 pages, 2 figure
Virtual Quantum Subsystems
The physical resources available to access and manipulate the degrees of
freedom of a quantum system define the set of operationally relevant
observables. The algebraic structure of selects a preferred tensor
product structure i.e., a partition into subsystems. The notion of compoundness
for quantum system is accordingly relativized. Universal control over virtual
subsystems can be achieved by using quantum noncommutative holonomiesComment: Presentation improved, to appear in PRL. 4 Pages, RevTe
Power-Law Time Distribution of Large Earthquakes
We study the statistical properties of time distribution of seimicity in
California by means of a new method of analysis, the Diffusion Entropy. We find
that the distribution of time intervals between a large earthquake (the main
shock of a given seismic sequence) and the next one does not obey Poisson
statistics, as assumed by the current models. We prove that this distribution
is an inverse power law with an exponent . We propose the
Long-Range model, reproducing the main properties of the diffusion entropy and
describing the seismic triggering mechanisms induced by large earthquakes.Comment: 4 pages, 3 figures. Revised version accepted for publication. Typos
corrected, more detailed discussion on the method used, refs added. Phys.
Rev. Lett. (2003) in pres
Graph hypersurfaces and a dichotomy in the Grothendieck ring
The subring of the Grothendieck ring of varieties generated by the graph
hypersurfaces of quantum field theory maps to the monoid ring of stable
birational equivalence classes of varieties. We show that the image of this map
is the copy of Z generated by the class of a point. Thus, the span of the graph
hypersurfaces in the Grothendieck ring is nearly killed by setting the
Lefschetz motive L to zero, while it is known that graph hypersurfaces generate
the Grothendieck ring over a localization of Z[L] in which L becomes
invertible. In particular, this shows that the graph hypersurfaces do not
generate the Grothendieck ring prior to localization. The same result yields
some information on the mixed Hodge structures of graph hypersurfaces, in the
form of a constraint on the terms in their Deligne-Hodge polynomials.Comment: 8 pages, LaTe
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