823 research outputs found
Miniature grinder for solid specimens
Machine grinds fines to appropriate micron sizes with the least biological trauma and greatest degree of reproducibility. Device controls destruction of material so that recovery of microorganisms is as great as possible and protects operation and grinding products from exogenous contamination
Numerical study of domain coarsening in anisotropic stripe patterns
We study the coarsening of two-dimensional smectic polycrystals characterized
by grains of oblique stripes with only two possible orientations. For this
purpose, an anisotropic Swift-Hohenberg equation is solved. For quenches close
enough to the onset of stripe formation, the average domain size increases with
time as . Further from onset, anisotropic pinning forces similar to
Peierls stresses in solid crystals slow down defects, and growth becomes
anisotropic. In a wide range of quench depths, dislocation arrays remain mobile
and dislocation density roughly decays as , while chevron boundaries
are totally pinned. We discuss some agreements and disagreements found with
recent experimental results on the coarsening of anisotropic electroconvection
patterns.Comment: 8 pages, 11 figures. Phys. Rev E, to appea
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
Spatio-temporal patterns in inclined layer convection
This paper reports on a theoretical analysis of the rich variety of spatio-temporal patterns observed recently in inclined layer convection at medium Prandtl number when varying the inclination angle γ and the Rayleigh number R. The present numerical investigation of the inclined layer convection system is based on the standard Oberbeck–Boussinesq equations. The patterns are shown to originate from a complicated competition of buoyancy driven and shear-flow driven pattern forming
mechanisms. The former are expressed as longitudinal convection rolls with their axes oriented parallel to the incline, the latter as perpendicular transverse rolls. Along with conventional methods to study roll patterns and their stability, we employ direct numerical simulations in large spatial domains, comparable with the experimental ones. As a result, we determine the phase diagram of the characteristic complex 3-D convection patterns above onset of convection in the γ–R plane, and find
that it compares very well with the experiments. In particular we demonstrate that interactions of specific Fourier modes, characterized by a resonant interaction of their wavevectors in the layer plane, are key to understanding the pattern morphologies
Specific heat and thermal conductivity in the vortex state of the two-gap superconductor MgB_2
The specific heat coefficient gamma_s(H) and the electronic thermal
conductivity kappa_{es}(H) are calculated for Abrikosov's vortex lattice by
taking into account the effects of supercurrent flow and Andreev scattering.
First we solve the gap equation for the entire range of magnetic fields. We
take into account vertex corrections due to impurity scattering calculated in
the Born approximation. The function gamma_s(H)/gamma_n increases from zero and
becomes approximately linear above H/H_{c2} \sim 0.1. The dependence on
impurity scattering is substantially reduced by the vertex corrections. The
upward curvature of kappa_{es}(H)/kappa_{en}, which is caused by decreasing
Andreev scattering for increasing field, is reduced for increasing impurity
scattering. We also calculate the temperature dependence of the scattering
rates 1/tau_{ps}(H) of a phonon and 1/tau_{es}(H) of a quasiparticle due to
quasiparticle and phonon scattering, respectively. At low temperatures the
ratio tau_{pn}/tau_{ps}(H) increases rapidly to one as H tends to H_{c2} which
yields a rapid drop in the phononic thermal conductivity kappa_{ph}. Our
results are in qualitative agreement with the experiments on the two-gap
superconductor MgB_2.Comment: 12 pages, 5 figures, additions to figures 1, 2, and 3. Accepted by
Phys. Rev.
Hall Effect in the mixed state of moderately clean superconductors
The Hall conductivity in the mixed state of a clean () type-II
s-wave superconductor is determined from a microscopic calculation within a
quasiclassical approximation. We find that below the superconducting transition
the contribution to the transverse conductivity due to dynamical fluctuations
of the order parameter is compensated by the modification of the quasiparticle
contribution. In this regime the nonlinear behaviour of the Hall angle is
governed by the change in the effective quasiparticle scattering rate due to
the reduction in the density of states at the Fermi level. The connection with
experimental results is discussed
On retracts, absolute retracts, and folds in cographs
Let G and H be two cographs. We show that the problem to determine whether H
is a retract of G is NP-complete. We show that this problem is fixed-parameter
tractable when parameterized by the size of H. When restricted to the class of
threshold graphs or to the class of trivially perfect graphs, the problem
becomes tractable in polynomial time. The problem is also soluble when one
cograph is given as an induced subgraph of the other. We characterize absolute
retracts of cographs.Comment: 15 page
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