479 research outputs found
Dynamics of coreless vortices and rotation-induced dissipation peak in superfluid films on rotating porous substrates
We analyze dynamics of 3D coreless vortices in superfluid films covering
porous substrates. The 3D vortex dynamics is derived from the 2D dynamics of
the film. The motion of a 3D vortex is a sequence of jumps between neighboring
substrate cells, which can be described, nevertheless, in terms of
quasi-continuous motion with average vortex velocity. The vortex velocity is
derived from the dissociation rate of vortex-antivortex pairs in a 2D film,
which was developed in the past on the basis of the Kosterlitz-Thouless theory.
The theory explains the rotation-induced dissipation peak in torsion-oscillator
experiments on He films on rotating porous substrates and can be used in
the analysis of other phenomena related to vortex motion in films on porous
substrates.Comment: 8 pages, 3 figures submitted to Phys. Rev.
Osmotic compression of droplets of hard rods: A computer simulation study
By means of computer simulations we study how droplets of hard, rod-like
particles optimize their shape and internal structure under the influence of
the osmotic compression caused by the presence of spherical particles that act
as depletion agents. At sufficiently high osmotic pressures the rods that make
up the drops spontaneously align to turn them into uniaxial nematic liquid
crystalline droplets. The nematic droplets or "tactoids" that are formed this
way are not spherical but elongated, resulting from the competition between the
anisotropic surface tension and the elastic deformation of the director field.
In agreement with recent theoretical predictions we find that sufficiently
small tactoids have a uniform director field, whilst large ones are
characterized by a bipolar director field. From the shape and director-field
transformation of the droplets we are able to estimate the surface anchoring
strength and an average of the elastic constants of the hard-rod nematic
Magnetic Force Exerted by the Aharonov-Bohm Line
The problem of the scattering of a charge by the Aharonov-Bohm (AB) flux line
is reconsidered in terms of finite width beams. It is shown that despite the
left-right symmetry in the AB scattering cross-section, the charge is scattered
asymmetrically. The asymmetry (i.e. magnetic force) originates from almost
forward scattering within the angular size of the incident wave. In the
paraxial approximation, the real space solution to the scattering problem of a
beam is found as well as the scattering S-matrix. The Boltzmann kinetics and
the Landau quantization in a random AB array are considered.Comment: 5 pages, RevTeX. Discussions of paraxial approximation to the
Aharonov-Bohm solution (Cornu spiral) and S-matrix, are extended. References
are adde
The vortex depinning transition in untwinned YBaCuO using complex impedance measurements
We present surface impedance measurement of the vortex linear response in a
large untwinned YBCO crystal. The depinning spectra obtained over a broad
frequency range (100 Hz- 30 MHz) are those of a surface pinned vortex lattice
with a free flux flow resistivity (two modes response). The critical current in
the "Campbell" like regime and the flux flow resistivity in the dissipative
regime are extracted. Those two parameters are affected by the first order
transition, showing that this transition may be related to the electronic state
of vortices.Comment: to be published in the proceedings of M2S RI
Domain Walls in a Tetragonal Chiral p-Wave Superconductor
Domain walls in a tetragonal chiral p-wave superconductors with broken time
reversal symmetry are analyzed in the framework of the Ginsburg-Landau theory.
The energy and the jump of the magnetic induction on the wall were determined
for different types of walls as functions of the parameters of the
Ginzburg-Landau theory and orientation of the domain wall with respect to the
crystallographic axes. We discuss implications of the analysis for
, where no stray magnetic fields from domain walls were detected
experimentally.Comment: 8 pages, 2 figure
Novel A-B type oscillations in a 2-D electron gas in inhomogenous magnetic fields
We present results from a quantum and semiclassical theoretical study of the
and resistivities of a high mobility 2-D electron gas
in the presence of a dilute random distribution of tubes with magnetic flux
and radius , for arbitrary values of and . We
report on novel Aharonov-Bohm type oscillations in and ,
related to degenerate quantum flux tube resonances, that satisfy the selection
rule , with an integer. We discuss possible
experimental conditions where these oscillations may be observed.Comment: 11 pages REVTE
Alignment transition in a nematic liquid crystal due to field-induced breaking of anchoring
We report on the alignment transition of a nematic liquid crystal from
initially homeotropic to quasi-planar due to field-induced anchoring breaking.
The initial homeotropic alignment is achieved by Langmuir-Blodgett monolayers.
In this geometry the anchoring strength can be evaluated by the Frederiks
transition technique. Applying an electric field above a certain threshold
provokes turbulent states denoted DSM1 and DSM2. While DSM1 does not affect the
anchoring, DSM2 breaks the coupling between the surface and the liquid crystal:
switching off the field from a DSM2 state does not immediately restore the
homeotropic alignment. Instead, we obtain a quasi-planar metastable alignment.
The cell thickness dependence for the transition is related to theComment: 7 pages, LaTeX2e article, 4 figures, 7 EPS files, added references,
accepted for publication in Europhysics Letter
Computer simulations of nematic drops: Coupling between drop shape and nematic order
We perform Monte Carlo computer simulations of nematic drops in equilibrium with their vapor
using a Gay-Berne interaction between the rod-like molecules. To generate the drops, we initially
perform NPT simulations close to the nematic-vapor coexistence region, allow the system to equilibrate
and subsequently induce a sudden volume expansion, followed with NVT simulations. The
resultant drops coexist with their vapor and are generally not spherical but elongated, have the rodlike
particles tangentially aligned at the surface and an overall nematic orientation along the main
axis of the drop. We find that the drop eccentricity increases with increasing molecular elongation,
κ. For small κ the nematic texture in the drop is bipolar with two surface defects, or boojums, maximizing
their distance along this same axis. For sufficiently high κ, the shape of the drop becomes
singular in the vicinity of the defects, and there is a crossover to an almost homogeneous texture; this
reflects a transition from a spheroidal to a spindle-like dro
On Multistep Bose-Einstein Condensation in Anisotropic Traps
Multistep Bose-Einstein condensation of an ideal Bose gas in anisotropic
harmonic atom traps is studied. In the presence of strong anisotropy realized
by the different trap frequency in each direction, finite size effect dictates
a series of dimensional crossovers into lower-dimensional excitations. Two-step
condensation and the dynamical reduction of the effective dimension can appear
in three separate steps. When the multistep behavior occurs, the occupation
number of atoms excited in each dimension is shown to behave similarly as a
function of the temperature.Comment: 26 pages, 7 figures, revised version, to appear in Jour. Phys.
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