18,908 research outputs found
Monte Carlo simulations of liquid crystals near rough walls
The effect of surface roughness on the structure of liquid crystalline fluids near solid substrates is studied by Monte Carlo simulations. The liquid crystal is modeled as a fluid of soft ellipsoidal molecules and the substrate is modeled as a hard wall that excludes the centers of mass of the fluid molecules. Surface roughness is introduced by embedding a number of molecules with random positions and orientations within the wall. It is found that the density and order near the wall are reduced as the wall becomes rougher, i.e., the number of embedded molecules is increased). Anchoring coefficients are determined from fluctuations in the reciprocal space order tensor. It is found that the anchoring strength decreases with increasing surface roughness
Disorder Induced Stripes in d-Wave Superconductors
Stripe phases are observed experimentally in several copper-based high-Tc
superconductors near 1/8 hole doping. However, the specific characteristics may
vary depending on the degree of dopant disorder and the presence or absence of
a low- temperature tetragonal phase. On the basis of a Hartree-Fock decoupling
scheme for the t-J model we discuss the diverse behavior of stripe phases. In
particular the effect of inhomogeneities is investigated in two distinctly
different parameter regimes which are characterized by the strength of the
interaction. We observe that small concen- trations of impurities or vortices
pin the unidirectional density waves, and dopant disorder is capable to
stabilize a stripe phase in parameter regimes where homogeneous phases are
typically favored in clean systems. The momentum-space results exhibit
universal features for all coexisting density-wave solutions, nearly unchanged
even in strongly disordered systems. These coexisting solutions feature
generically a full energy gap and a particle-hole asymmetry in the density of
states.Comment: 28 pages, 8 figure
Entropic stochastic resonance: the constructive role of the unevenness
We demonstrate the existence of stochastic resonance (SR) in confined systems
arising from entropy variations associated to the presence of irregular
boundaries. When the motion of a Brownian particle is constrained to a region
with uneven boundaries, the presence of a periodic input may give rise to a
peak in the spectral amplification factor and therefore to the appearance of
the SR phenomenon. We have proved that the amplification factor depends on the
shape of the region through which the particle moves and that by adjusting its
characteristic geometric parameters one may optimize the response of the
system. The situation in which the appearance of such entropic stochastic
resonance (ESR) occurs is common for small-scale systems in which confinement
and noise play an prominent role. The novel mechanism found could thus
constitute an important tool for the characterization of these systems and can
put to use for controlling their basic properties.Comment: 8 pages, 8 figure
Raman Scattered He II 6545 Line in the Symbiotic Star V1016 Cygni
We present a spectrum of the symbiotic star V1016 Cyg observed with the 3.6 m
Canada-France-Hawaii Telescope, in order to illustrate a method to measure the
covering factor of the neutral scattering region around the giant component
with respect to the hot emission region around the white dwarf component. In
the spectrum, we find broad wings around H and a broad emission feature
around 6545 that is blended with the [N II] 6548 line.
These two features are proposed to be formed by Raman scattering by atomic
hydrogen, where the incident radiation is proposed to be UV continuum radiation
around Ly in the former case and He II 1025 emission line
arising from transitions for the latter feature. We remove the
H wings by a template Raman scattering wing profile and subtract the [N
II] 6548 line using the 3 times stronger [N II] 6583
feature in order to isolate the He II Raman scattered 6545 \AA line. We obtain
the flux ratio of the He II 6560 emission
line and the 6545 \AA feature for V1016 Cyg. Under the assumption that the He
II emission from this object is isotropic, this ratio is converted to the ratio
of the number of the incident photons and that
of the scattered photons. This implies that the scattering region with H I
column density covers 17 per cent of the
emission region. By combining the presumed binary period yrs of this
system we infer that a significant fraction of the slow stellar wind from the
Mira component is ionized and that the scattering region around the Mira
extends a few tens of AU, which is closely associated with the mass loss
process of the Mira component.Comment: 12 pages, 6 figures, accepted for publication in Ap
Biased diffusion in confined media: Test of the Fick-Jacobs approximation and validity criteria
We study biased, diffusive transport of Brownian particles through narrow,
spatially periodic structures in which the motion is constrained in lateral
directions. The problem is analyzed under the perspective of the Fick-Jacobs
equation which accounts for the effect of the lateral confinement by
introducing an entropic barrier in a one dimensional diffusion. The validity of
this approximation, being based on the assumption of an instantaneous
equilibration of the particle distribution in the cross-section of the
structure, is analyzed by comparing the different time scales that characterize
the problem. A validity criterion is established in terms of the shape of the
structure and of the applied force. It is analytically corroborated and
verified by numerical simulations that the critical value of the force up to
which this description holds true scales as the square of the periodicity of
the structure. The criterion can be visualized by means of a diagram
representing the regions where the Fick-Jacobs description becomes inaccurate
in terms of the scaled force versus the periodicity of the structure.Comment: 20 pages, 7 figure
Double Entropic Stochastic Resonance
We demonstrate the appearance of a purely entropic stochastic resonance (ESR)
occurring in a geometrically confined system, where the irregular boundaries
cause entropic barriers. The interplay between a periodic input signal, a
constant bias and intrinsic thermal noise leads to a resonant ESR-phenomenon in
which feeble signals become amplified. This new phenomenon is characterized by
the presence of two peaks in the spectral amplification at corresponding
optimal values of the noise strength. The main peak is associated with the
manifest stochastic resonance synchronization mechanism involving the
inter-well noise-activated dynamics while a second peak relates to a regime of
optimal sensitivity for intra-well dynamics. The nature of ESR, occurring when
the origin of the barrier is entropic rather than energetic, offers new
perspectives for novel investigations and potential applications. ESR by itself
presents yet another case where one constructively can harvest noise in driven
nonequilibrium systems.Comment: 6 pages, 7 figures ; Europhys. Lett., in press (2009
Ferrotoroidic Moment as a Quantum Geometric Phase
We present a geometric characterization of the ferrotoroidic moment in terms
of a set of Abelian Berry phases. We also introduce a fundamental complex
quantity which provides an alternative way to calculate the ferrotoroidic
moment and its moments, and is derived from a second order tensor. This
geometric framework defines a natural computational approach for density
functional and many-body theories
The generalized Heun equation in QFT in curved space-times
In this article we give a brief outline of the applications of the
generalized Heun equation (GHE) in the context of Quantum Field Theory in
curved space-times. In particular, we relate the separated radial part of a
massive Dirac equation in the Kerr-Newman metric and the static perturbations
for the non-extremal Reissner-Nordstr\"{o}m solution to a GHE.Comment: 7 pages, some small improvements in section
A variational framework for flow optimization using semi-norm constraints
When considering a general system of equations describing the space-time
evolution (flow) of one or several variables, the problem of the optimization
over a finite period of time of a measure of the state variable at the final
time is a problem of great interest in many fields. Methods already exist in
order to solve this kind of optimization problem, but sometimes fail when the
constraint bounding the state vector at the initial time is not a norm, meaning
that some part of the state vector remains unbounded and might cause the
optimization procedure to diverge. In order to regularize this problem, we
propose a general method which extends the existing optimization framework in a
self-consistent manner. We first derive this framework extension, and then
apply it to a problem of interest. Our demonstration problem considers the
transient stability properties of a one-dimensional (in space) averaged
turbulent model with a space- and time-dependent model "turbulent viscosity".
We believe this work has a lot of potential applications in the fluid
dynamics domain for problems in which we want to control the influence of
separate components of the state vector in the optimization process.Comment: 30 page
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