6,072 research outputs found
Bilayer Membrane in Confined Geometry: Interlayer Slide and Steric Repulsion
We derived free energy functional of a bilayer lipid membrane from the first
principles of elasticity theory. The model explicitly includes
position-dependent mutual slide of monolayers and bending deformation. Our free
energy functional of liquid-crystalline membrane allows for incompressibility
of the membrane and vanishing of the in-plane shear modulus and obeys
reflectional and rotational symmetries of the flat bilayer. Interlayer slide at
the mid-plane of the membrane results in local difference of surface densities
of the monolayers. The slide amplitude directly enters free energy via the
strain tensor. For small bending deformations the ratio between bending modulus
and area compression coefficient, Kb/KA, is proportional to the square of
monolayer thickness, h. Using the functional we performed self-consistent
calculation of steric potential acting on bilayer between parallel confining
walls separated by distance 2d. We found that temperature-dependent curvature
at the minimum of confining potential is enhanced four times for a bilayer with
slide as compared with a unit bilayer. We also calculate viscous modes of
bilayer membrane between confining walls. Pure bending of the membrane is
investigated, which is decoupled from area dilation at small amplitudes. Three
sources of viscous dissipation are considered: water and membrane viscosities
and interlayer drag. Dispersion has two branches. Confinement between the walls
modifies the bending mode with respect to membrane in bulk solution.
Simultaneously, inter-layer slipping mode, damped by viscous drag, remains
unchanged by confinement.Comment: 23 pages,3 figures, pd
Dynamical Evolution of Galaxies in Clusters
Tidal forces acting on galaxies in clusters lead to a strong dynamical
evolution. In order to quantify the amount of evolution, I run self-consistent
N-body simulations of disk galaxies for a variety of models in the
hierarchically forming clusters. The tidal field along the galactic orbits is
extracted from the simulations of cluster formation in the Omega_0=1;
Omega_0=0.4; and Omega_0=0.4, Omega_Lambda=0.6 cosmological scenarios. For
large spiral galaxies with the rotation speed of 250 km/s, tidal interactions
truncate massive dark matter halos at 30 +- 6 kpc, and thicken stellar disks by
a factor 2 to 3, increasing Toomre's parameter to Q > 2 and halting star
formation. Low density galaxies, such as the dwarf spheroidals with the
circular velocity of 20 km/s and the extended low surface brightness galaxies
with the scale length of 10-15 kpc, are completely disrupted by tidal shocks.
Their debris contribute to the diffuse intracluster light. The tidal effects
are significant not only in the core but throughout the cluster and can be
parametrized by the critical tidal density. The tidally-induced evolution
results in the transformation of the infalling spirals into S0 galaxies and in
the depletion of the LSB population. In the low Omega_0 cosmological models,
clusters form earlier and produce stronger evolution of galaxies.Comment: accepted to Ap
Ab initio simulations of liquid systems: Concentration dependence of the electric conductivity of NaSn alloys
Liquid NaSn alloys in five different compositions (20, 40, 50, 57 and 80%
sodium) are studied using density functional calculations combined with
molecular dynamics(Car-Parrinello method). The frequency-dependent electric
conductivities for the systems are calculated by means of the Kubo-Greenwood
formula.
The extrapolated DC conductivities are in good agreement with the
experimental data and reproduce the strong variation with the concentration.
The maximum of conductivity is obtained, in agreement with experiment, near the
equimolar composition.
The strong variation of conductivity, ranging from almost semiconducting up
to metallic behaviour, can be understood by an analysis of the
densities-of-states.Comment: LaTex 6 pages and 2 figures, to appear in J.Phys. Cond. Ma
Photoconductance of a submicron oxidized line in surface conductive single crystalline diamond
We report on sub-bandgap optoelectronic phenomena of hydrogen-terminated
diamond patterned with a submicron oxidized line. The line acts as an energy
barrier for the two-dimensional hole gas located below the hydrogenated diamond
surface. A photoconductive gain of the hole conductivity across the barrier is
measured for sub-bandgap illumination. The findings are consistent with
photogenerated electrons being trapped in defect levels within the barrier. We
discuss the spatial and energetic characteristics of the optoelectronic
phenomena, as well as possible photocurrent effects
A general variational principle for spherically symmetric perturbations in diffeomorphism covariant theories
We present a general method for the analysis of the stability of static,
spherically symmetric solutions to spherically symmetric perturbations in an
arbitrary diffeomorphism covariant Lagrangian field theory. Our method involves
fixing the gauge and solving the linearized gravitational field equations to
eliminate the metric perturbation variable in terms of the matter variables. In
a wide class of cases--which include f(R) gravity, the Einstein-aether theory
of Jacobson and Mattingly, and Bekenstein's TeVeS theory--the remaining
perturbation equations for the matter fields are second order in time. We show
how the symplectic current arising from the original Lagrangian gives rise to a
symmetric bilinear form on the variables of the reduced theory. If this
bilinear form is positive definite, it provides an inner product that puts the
equations of motion of the reduced theory into a self-adjoint form. A
variational principle can then be written down immediately, from which
stability can be tested readily. We illustrate our method in the case of
Einstein's equation with perfect fluid matter, thereby re-deriving, in a
systematic manner, Chandrasekhar's variational principle for radial
oscillations of spherically symmetric stars. In a subsequent paper, we will
apply our analysis to f(R) gravity, the Einstein-aether theory, and
Bekenstein's TeVeS theory.Comment: 13 pages; submitted to Phys. Rev. D. v2: changed formatting, added
conclusion, corrected sign convention
Stability of spherically symmetric solutions in modified theories of gravity
In recent years, a number of alternative theories of gravity have been
proposed as possible resolutions of certain cosmological problems or as toy
models for possible but heretofore unobserved effects. However, the
implications of such theories for the stability of structures such as stars
have not been fully investigated. We use our "generalized variational
principle", described in a previous work, to analyze the stability of static
spherically symmetric solutions to spherically symmetric perturbations in three
such alternative theories: Carroll et al.'s f(R) gravity, Jacobson &
Mattingly's "Einstein-aether theory", and Bekenstein's TeVeS. We find that in
the presence of matter, f(R) gravity is highly unstable; that the stability
conditions for spherically symmetric curved vacuum Einstein-aether backgrounds
are the same as those for linearized stability about flat spacetime, with one
exceptional case; and that the "kinetic terms" of vacuum TeVeS are indefinite
in a curved background, leading to an instability.Comment: ReVTex; 20 pages, 3 figures. v2: references added, submitted to PRD;
v3: expanded discussion of TeVeS; v4: minor typos corrected (version to
appear in PRD
The Valuation of Mortgage Loan Commitments Using Option Pricing Estimates
This paper values mortgage loan commitments in the context of the option pricing theory developed by Black and Scholes, and Merton. A valuation model is derived and empirical results are presented.
Age spread in Galactic star forming region W3 Main
We present near-infrared JHKs imaging as well as K-band multi-object
spectroscopy of the massive stellar content of W3 Main using LUCI at the LBT.
We confirm 13 OB stars by their absorption line spectra in W3 Main and spectral
types between O5V and B4V have been found. Three massive Young Stellar Objects
are identified by their emission line spectra and near-infrared excess. From
our spectrophotometric analysis of the massive stars and the nature of their
surrounding HII regions we derive the evolutionary sequence of W3 Main and we
find an age spread of 2-3 Myr.Comment: 4 pages, 2 figures, To appear in conference proceedings of "370 years
of Astronomy in Utrecht
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