114 research outputs found
Flexoelectricity and pattern formation in nematic liquid crystals
We present in this paper a detailed analysis of the flexoelectric instability
of a planar nematic layer in the presence of an alternating electric field
(frequency ), which leads to stripe patterns (flexodomains) in the
plane of the layer. This equilibrium transition is governed by the free energy
of the nematic which describes the elasticity with respects to the
orientational degrees of freedom supplemented by an electric part. Surprisingly
the limit is highly singular. In distinct contrast to the
dc-case, where the patterns are stationary and time-independent, they appear at
finite, small periodically in time as sudden bursts. Flexodomains are
in competition with the intensively studied electro-hydrodynamic instability in
nematics, which presents a non-equilibrium dissipative transition. It will be
demonstrated that is a very convenient control parameter to tune
between flexodomains and convection patterns, which are clearly distinguished
by the orientation of their stripes
Galactic Rotation Parameters from Data on Open Star Clusters
Currently available data on the field of velocities Vr, Vl, Vb for open star
clusters are used to perform a kinematic analysis of various samples that
differ by heliocentric distance, age, and membership in individual structures
(the Orion, Carina--Sagittarius, and Perseus arms). Based on 375 clusters
located within 5 kpc of the Sun with ages up to 1 Gyr, we have determined the
Galactic rotation parameters
Wo =-26.0+-0.3 km/s/kpc,
W'o = 4.18+-0.17 km/s/kpc^2,
W''o=-0.45+-0.06 km/s/kpc^3, the system contraction parameter K = -2.4+-0.1
km/s/kpc, and the parameters of the kinematic center Ro =7.4+-0.3 kpc and lo =
0+-1 degrees. The Galactocentric distance Ro in the model used has been found
to depend significantly on the sample age. Thus, for example, it is 9.5+-0.7
kpc and 5.6+-0.3 kpc for the samples of young (50 Myr)
clusters, respectively. Our study of the kinematics of young open star clusters
in various spiral arms has shown that the kinematic parameters are similar to
the parameters obtained from the entire sample for the Carina-Sagittarius and
Perseus arms and differ significantly from them for the Orion arm. The
contraction effect is shown to be typical of star clusters with various ages.
It is most pronounced for clusters with a mean age of 100 Myr, with the
contraction velocity being Kr = -4.3+-1.0 km/s.Comment: 14 pages, 4 figures, 2 table
Anomalous Negative Magnetoresistance Caused by Non-Markovian Effects
A theory of recently discovered anomalous low-field magnetoresistance is
developed for the system of two-dimensional electrons scattered by hard disks
of radius randomly distributed with concentration For small magnetic
fields the magentoresistance is found to be parabolic and inversely
proportional to the gas parameter, With increasing field the magnetoresistance becomes linear
in a good agreement with the
experiment and numerical simulations.Comment: 4 pages RevTeX, 5 figure
Evolution of the Velocity Ellipsoids in the Thin Disk of the Galaxy and the Radial Migration of Stars
Data from the revised Geneva--Copenhagen catalog are used to study the
influence of radial migration of stars on the age dependences of parameters of
the velocity ellipsoids for nearby stars in the thin disk of the Galaxy,
assuming that the mean radii of the stellar orbits remain constant. It is
demonstrated that precisely the radial migration of stars, together with the
negative metallicity gradient in the thin disk,are responsible for the observed
negative correlation between the metallicities and angular momenta of nearby
stars, while the angular momenta of stars that were born at the same
Galactocentric distances do not depend on either age or metallicity. (abridged)Comment: Astronomy Reports, Vol. 86 No. 9, P.1117-1126 (2009
Flexoelectricity in an oxadiazole bent-core nematic liquid crystal
We have determined experimentally the magnitude of the difference in the splay and bend flexoelectric coefficients, |e1 - e3|, of an oxadiazole bent-core liquid crystal by measuring the critical voltage for the formation of flexodomains together with their wave number. The coefficient |e1 - e3| is found to be a factor of 2-3 times higher than in most conventional calamitic nematic liquid crystals, varying from 8 pCm-1 to 20 pCm-1 across the ∼60 K - wide nematic regime. We have also calculated the individual flexoelectric coefficients e1 and e3, with the dipolar and quadrupolar contributions of the bent-core liquid crystal by combining density functional theory calculations with a molecular field approach and atomistic modelling. Interestingly, the magnitude of the bend flexoelectric coefficient is found to be rather small, in contrast to common expectations for bent-core molecules. The calculations are in excellent agreement with the experimental values, offering an insight into how molecular parameters contribute to the flexoelectric coefficients and illustrating a huge potential for the prediction of flexoelectric behaviour in bent-core liquid crystals
Electron-electron interaction at decreasing
The contribution of the electron-electron interaction to conductivity is
analyzed step by step in gated GaAs/InGaAs/GaAs heterostructures with different
starting disorder. We demonstrate that the diffusion theory works down to , where is the Fermi quasimomentum, is the mean free
paths. It is shown that the e-e interaction gives smaller contribution to the
conductivity than the interference independent of the starting disorder and its
role rapidly decreases with decrease.Comment: 5 pages, 6 figure
Galactic Kinematics from OB3 Stars with Distances determined from Interstellar Ca II Lines
Based on data for 102 OB3 stars with known proper motions and radial
velocities, we have tested the distances derived by Megier et al. from
interstellar Ca II spectral lines. The internal reconciliation of the distance
scales using the first derivative of the angular velocity of Galactic rotation
{\Omega}'0 and the external reconciliation with Humphreys's distance scale for
OB associations refined by Mel'nik and Dambis show that the initial distances
should be reduced by \approx 20%. Given this correction, the heliocentric
distances of these stars lie within the range 0.6-2.6 kpc. A kinematic analysis
of these stars at a fixed Galactocentric distance of the Sun, R0=8 kpc, has
allowed the following parameters to be determined:(1) the solar peculiar
velocity components (U_o,V_o,W_o)=(8.9,10.3,6.8)\pm(0.6,1.0,0.4) km/s;(2) the
Galactic rotation parameters {\Omega}_o=-31.5\pm0.9 km/s/kpc,
{\Omega}'_o=+4.49\pm0.12 km/s/kpc^2, {\Omega}"_o=-1.05\pm0.38 km/s/kpc^3, (the
corresponding Oort constants are A=17.9\pm0.5 km/s/kpc, B=-13.6\pm1.0 km/s/kpc
and the circular rotation velocity of the solar neighborhood is |V_o|=252\pm14
km/s); (3) the spiral density wave parameters, namely: the perturbation
amplitudes for the radial and azimuthal velocity components, respectively, f_R
= -12.5\pm1.1 km/s and f_{\theta}=2.0\pm1.6 km/s; the pitch angle for the
two-armed spiral pattern i=-5.3\pm0.3 degrees, with the wavelength of the
spiral density wave at the solar distance being {\lambda}=2.3\pm0.2 kpc; the
Sun's phase in the spiral wave {\chi}_o=-91\pm4 degrees.Comment: 14 pages, 4 figures, 3 table
Global existence and full regularity of the Boltzmann equation without angular cutoff
We prove the global existence and uniqueness of classical solutions around an
equilibrium to the Boltzmann equation without angular cutoff in some Sobolev
spaces. In addition, the solutions thus obtained are shown to be non-negative
and in all variables for any positive time. In this paper, we study
the Maxwellian molecule type collision operator with mild singularity. One of
the key observations is the introduction of a new important norm related to the
singular behavior of the cross section in the collision operator. This norm
captures the essential properties of the singularity and yields precisely the
dissipation of the linearized collision operator through the celebrated
H-theorem
Emission of partial dislocations from triple junctions of grain boundaries in nanocrystalline materials
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