4,861 research outputs found
A level-set method for the evolution of cells and tissue during curvature-controlled growth
Most biological tissues grow by the synthesis of new material close to the
tissue's interface, where spatial interactions can exert strong geometric
influences on the local rate of growth. These geometric influences may be
mechanistic, or cell behavioural in nature. The control of geometry on tissue
growth has been evidenced in many in-vivo and in-vitro experiments, including
bone remodelling, wound healing, and tissue engineering scaffolds. In this
paper, we propose a generalisation of a mathematical model that captures the
mechanistic influence of curvature on the joint evolution of cell density and
tissue shape during tissue growth. This generalisation allows us to simulate
abrupt topological changes such as tissue fragmentation and tissue fusion, as
well as three dimensional cases, through a level-set-based method. The
level-set method developed introduces another Eulerian field than the level-set
function. This additional field represents the surface density of tissue
synthesising cells, anticipated at future locations of the interface. Numerical
tests performed with this level-set-based method show that numerical
conservation of cells is a good indicator of simulation accuracy, particularly
when cusps develop in the tissue's interface. We apply this new model to
several situations of curvature-controlled tissue evolutions that include
fragmentation and fusion.Comment: 15 pages, 10 figures, 3 supplementary figure
Non-linear Evolution of Matter Power Spectrum in Modified Theory of Gravity
We present a formalism to calculate the non-linear matter power spectrum in
modified gravity models that explain the late-time acceleration of the Universe
without dark energy. Any successful modified gravity models should contain a
mechanism to recover General Relativity (GR) on small scales in order to avoid
the stringent constrains on deviations from GR at solar system scales. Based on
our formalism, the quasi non-linear power spectrum in the
Dvali-Gabadadze-Porratti (DGP) braneworld models and gravity models are
derived by taking into account the mechanism to recover GR properly. We also
extrapolate our predictions to fully non-linear scales using the Parametrized
Post Friedmann (PPF) framework. In gravity models, the predicted
non-linear power spectrum is shown to reproduce N-body results. We find that
the mechanism to recover GR suppresses the difference between the modified
gravity models and dark energy models with the same expansion history, but the
difference remains large at weakly non-linear regime in these models. Our
formalism is applicable to a wide variety of modified gravity models and it is
ready to use once consistent models for modified gravity are developed.Comment: 25 pages, 8 figures, comparison to N-body simulations in DGP added,
published in PR
Dynamic Matter-Wave Pulse Shaping
In this paper we discuss possibilities to manipulate a matter-wave with
time-dependent potentials. Assuming a specific setup on an atom chip, we
explore how one can focus, accelerate, reflect, and stop an atomic wave packet,
with, for example, electric fields from an array of electrodes. We also utilize
this method to initiate coherent splitting. Special emphasis is put on the
robustness of the control schemes. We begin with the wave packet of a single
atom, and extend this to a BEC, in the Gross-Pitaevskii picture. In analogy to
laser pulse shaping with its wide variety of applications, we expect this work
to form the base for additional time-dependent potentials eventually leading to
matter-wave pulse shaping with numerous application
Graphitic-BN Based Metal-free Molecular Magnets From A First Principle Study
We perform a first principle calculation on the electronic properties of
carbon doped graphitic boron nitride graphitic BN. It was found that carbon
substitution for either boron or nitrogen atom in graphitic BN can induce
spontaneous magnetization. Calculations based on density functional theory with
the local spin density approximation on the electronic band structure revealed
a spin polarized, dispersionless band near the Fermi energy. Spin density
contours showed that the magnetization density originates from the carbon atom.
The magnetization can be attributed to the carbon 2p electron. Charge density
distribution shows that the carbon atom forms covalent bonds with its three
nearest neighbourhood. The spontaneous magnetization survives the curvature
effect in BN nanotubes, suggesting the possibility of molecular magnets made
from BN. Compared to other theoretical models of light-element or metal-free
magnetic materials, the carbon-doped BN are more experimentally accessible and
can be potentially useful.Comment: 8 pages, 4 figure
Repulsive Fermions in Optical Lattices: Phase separation versus Coexistence of Antiferromagnetism and d-Superfluidity
We investigate a system of fermions on a two-dimensional optical square
lattice in the strongly repulsive coupling regime. In this case, the
interactions can be controlled by laser intensity as well as by Feshbach
resonance. We compare the energetics of states with resonating valence bond
d-wave superfluidity, antiferromagnetic long range order and a homogeneous
state with coexistence of superfluidity and antiferromagnetism. We show that
the energy density of a hole has a minimum at doping that
signals phase separation between the antiferromagnetic and d-wave paired
superfluid phases. The energy of the phase-separated ground state is however
found to be very close to that of a homogeneous state with coexisting
antiferromagnetic and superfluid orders. We explore the dependence of the
energy on the interaction strength and on the three-site hopping terms and
compare with the nearest neighbor hopping {\it t-J} model
XMM-Newton View of PKS 2155-304: Characterizing the X-ray Variability Properties with EPIC-PN
Starting from XMM-Newton EPIC-PN data, we present the X-ray variability
characteristics of PKS 2155-304 using a simple analysis of the excess variance,
\xs, and of the fractional rms variability amplitude, fvar. The scatter in \xs\
and \fvar, calculated using 500 s long segments of the light curves, is smaller
than the scatter expected for red noise variability. This alone does not imply
that the underlying process responsible for the variability of the source is
stationary, since the real changes of the individual variance estimates are
possibly smaller than the large scatters expected for a red noise process. In
fact the averaged \xs and \fvar, reducing the fluctuations of the individual
variances, chang e with time, indicating non-stationary variability. Moreover,
both the averaged \sqxs (absolute rms variability amplitude) and \fvar show
linear correlation with source flux but in an opposite sense: \sqxs correlates
with flux, but \fvar anti-correlates with flux. These correlations suggest that
the variability process of the source is strongly non-stationary as random
scatters of variances should not yield any correlation. \fvar spectra were
constructed to compare variability amplitudes in different energy bands. We
found that the fractional rms variability amplitude of the source, when
significant variability is observed, increases logarithmically with the photon
energy, indicating significant spectral variability. The point-to-point
variability amplitude may also track this trend, suggesting that the slopes of
the power spectral density of the source are energy-independent. Using the
normalized excess variance the black hole mass of \pks was estimated to be
about . This is compared and contrasted with the
estimates derived from measurements of the host galaxies.Comment: Accepted for publication in The Astrophysical Journa
Fluctuations Do Matter: Large Noise-Enhanced Halos in Charged-Particle Beams
The formation of beam halos has customarily been described in terms of a
particle-core model in which the space-charge field of the oscillating core
drives particles to large amplitudes. This model involves parametric resonance
and predicts a hard upper bound to the orbital amplitude of the halo particles.
We show that the presence of colored noise due to space-charge fluctuations
and/or machine imperfections can eject particles to much larger amplitudes than
would be inferred from parametric resonance alone.Comment: 13 pages total, including 5 figure
One-loop fermionic corrections to the instanton transition in two dimensional chiral Higgs model
The one-loop fermionic contribution to the probability of an instanton
transition with fermion number violation is calculated in the chiral Abelian
Higgs model in 1+1 dimensions, where the fermions have a Yukawa coupling to the
scalar field. The dependence of the determinant on fermionic, scalar and vector
mass is determined. We show in detail how to renormalize the fermionic
determinant in partial wave analysis, which is convenient for computations.Comment: 36 pages, 5 figure
Quantum Monte Carlo simulations of a particle in a random potential
In this paper we carry out Quantum Monte Carlo simulations of a quantum
particle in a one-dimensional random potential (plus a fixed harmonic
potential) at a finite temperature. This is the simplest model of an interface
in a disordered medium and may also pertain to an electron in a dirty metal. We
compare with previous analytical results, and also derive an expression for the
sample to sample fluctuations of the mean square displacement from the origin
which is a measure of the glassiness of the system. This quantity as well as
the mean square displacement of the particle are measured in the simulation.
The similarity to the quantum spin glass in a transverse field is noted. The
effect of quantum fluctuations on the glassy behavior is discussed.Comment: 23 pages, 7 figures included as eps files, uses RevTeX. Accepted for
publication in J. of Physics A: Mathematical and Genera
Conserving GW scheme for nonequilibrium quantum transport in molecular contacts
We give a detailed presentation of our recent scheme to include correlation
effects in molecular transport calculations using the GW approximation within
the non-equilibrium Keldysh formalism. We restrict the GW self-energy to the
central region, and describe the leads by density functional theory (DFT). A
minimal basis of maximally localized Wannier functions is applied both in the
central GW region and the leads. The importance of using a conserving, i.e.
fully self-consistent, GW self-energy is demonstrated both analytically and by
numerical examples. We introduce an effective spin-dependent interaction which
automatically reduces self-interaction errors to all orders in the interaction.
The scheme is applied to the Anderson model in- and out of equilibrium. In
equilibrium at zero temperature we find that GW describes the Kondo resonance
fairly well for intermediate interaction strengths. Out of equilibrium we
demonstrate that the one-shot G0W0 approximation can produce severe errors, in
particular at high bias. Finally, we consider a benzene molecule between
featureless leads. It is found that the molecule's HOMO-LUMO gap as calculated
in GW is significantly reduced as the coupling to the leads is increased,
reflecting the more efficient screening in the strongly coupled junction. For
the IV characteristics of the junction we find that HF and G0W0[G_HF] yield
results closer to GW than does DFT and G0W0[G_DFT]. This is explained in terms
of self-interaction effects and life-time reduction due to electron-electron
interactions.Comment: 23 pages, 16 figure
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