764 research outputs found
Smectic Liquid Crystals: Materials with One-Dimensional, Periodic Order
Smectic liquid crystals are materials formed by stacking deformable, fluid
layers. Though smectics prefer to have flat, uniformly-spaced layers, boundary
conditions can impose curvature on the layers. Since the layer spacing and
curvature are intertwined, the problem of finding minimal configurations for
the layers becomes highly nontrivial. We discuss various topological and
geometrical aspects of these materials and present recent progress on finding
some exact layer configurations. We also exhibit connections to the study of
certain embedded minimal surfaces and briefly summarize some important open
problems.Comment: 16 page
Roughening Transition in a Moving Contact Line
The dynamics of the deformations of a moving contact line on a disordered
substrate is formulated, taking into account both local and hydrodynamic
dissipation mechanisms. It is shown that both the coating transition in contact
lines receding at relatively high velocities, and the pinning transition for
slowly moving contact lines, can be understood in a unified framework as
roughening transitions in the contact line. We propose a phase diagram for the
system in which the phase boundaries corresponding to the coating transition
and the pinning transition meet at a junction point, and suggest that for
sufficiently strong disorder a receding contact line will leave a
Landau--Levich film immediately after depinning. This effect may be relevant to
a recent experimental observation in a liquid Helium contact line on a Cesium
substrate [C. Guthmann, R. Gombrowicz, V. Repain, and E. Rolley, Phys. Rev.
Lett. {\bf 80}, 2865 (1998)].Comment: 16 pages, 6 encapsulated figure
Dissipation in Dynamics of a Moving Contact Line
The dynamics of the deformations of a moving contact line is studied assuming
two different dissipation mechanisms. It is shown that the characteristic
relaxation time for a deformation of wavelength of a contact line
moving with velocity is given as . The velocity
dependence of is shown to drastically depend on the dissipation
mechanism: we find for the case when the dynamics is governed
by microscopic jumps of single molecules at the tip (Blake mechanism), and
when viscous hydrodynamic losses inside the moving
liquid wedge dominate (de Gennes mechanism). We thus suggest that the debated
dominant dissipation mechanism can be experimentally determined using
relaxation measurements similar to the Ondarcuhu-Veyssie experiment [T.
Ondarcuhu and M. Veyssie, Nature {\bf 352}, 418 (1991)].Comment: REVTEX 8 pages, 9 PS figure
Number of distinct sites visited by N random walkers on a Euclidean lattice
The evaluation of the average number S_N(t) of distinct sites visited up to
time t by N independent random walkers all starting from the same origin on an
Euclidean lattice is addressed. We find that, for the nontrivial time regime
and for large N, S_N(t) \approx \hat S_N(t) (1-\Delta), where \hat S_N(t) is
the volume of a hypersphere of radius (4Dt \ln N)^{1/2},
\Delta={1/2}\sum_{n=1}^\infty \ln^{-n} N \sum_{m=0}^n s_m^{(n)} \ln^{m} \ln N,
d is the dimension of the lattice, and the coefficients s_m^{(n)} depend on the
dimension and time. The first three terms of these series are calculated
explicitly and the resulting expressions are compared with other approximations
and with simulation results for dimensions 1, 2, and 3. Some implications of
these results on the geometry of the set of visited sites are discussed.Comment: 15 pages (RevTex), 4 figures (eps); to appear in Phys. Rev.
Supercurrent in a mesoscopic proximity wire
Recent experiments on the proximity induced supercurrent in mesoscopic normal
wires revealed a surprising temperature dependence. They suggest clean-limit
behavior although the wires are strongly disordered. We demonstrate that this
unexpected scaling is actually contained in the conventional description of
diffusive superconductors and find excellent agreement with the experimental
results. In addition we propose a SQUID-like proximity structure for further
experimental investigations of the effects in question.Comment: 6 pages LaTeX, 4 postscript figures; to appear in J. Low Temp. Phys.
(Proceedings of WSS '96
A Scenario to the Anomalous Hall Effect in the Mixed State of Superconductors
We argue that the motion of vacancies in a pinned vortex lattice may dominate
the contribution to the Hall effect in an appropriate parameter regime for a
superconductor. Based on this consideration a model is constructed to explain
the anomalous Hall effect without any modification of the basic vortex dynamic
equation. Quantitative predictions are obtained. Present model can be directly
tested by an observation of the vacancy motion.Comment: latex, 6 pages (Presented at the Miami High Tc Conf., Jan 5-11, 1995.
To appear at J. Supercond.
Spin diffusion of the t-J model
The spin-diffusion constant of the 2D model is calculated for the first
time using an analytical approach at high temperatures and a recently-developed
numerical method based on the Lanczos technique combined with random sampling
in the intermediate temperature regime. A simple relation, ,
between spin conductivity and spin diffusion is established and used to
calculate the latter. In the high-temperature and low-doping limit the
calculated diffusion constant agrees with known results for the Heisenberg
model. At small hole doping, increases approximately linearly with
doping, which leads us to an important conclusion that hopping processes
enhance spin diffusion at high temperatures. At modest hole doping, , diffusion exhibits a nonmonotonic temperature dependence, which
indicates anomalous spin dynamics at small frequencies.Comment: 12 pages with figure
Freezing of Spinodal Decompostion by Irreversible Chemical Growth Reaction
We present a description of the freezing of spinodal decomposition in
systems, which contain simultaneous irreversible chemical reactions, in the
hydrodynamic limit approximation. From own results we conclude, that the
chemical reaction leads to an onset of spinodal decomposition also in the case
of an initial system which is completely miscible and can lead to an extreme
retardation of the dynamics of the spinodal decomposition, with the probability
of a general freezing of this process, which can be experimetally observed in
simultaneous IPN formation.Comment: 10 page
Études de magnétisme réalisées avec des neutrons
This paper describes two series of experiments on neutron scattering by magnetic media. Series A gives the antiferromagnetic structure of FeCl 2, and the effect of a magnetic field on the spin orientations as measured with a crystal spectrometer. In series B, the critical magnetic scattering of iron near the Curie Point has been investigated with a time of flight spectrometer. As a result one gets both the range of the spin correlations and the scattering constant which is related to their decay in time.On décrit deux expériences de diffusion des neutrons par des substances magnétiques. Dans la première, on étudie au moyen d'un spectromètre à cristal la structure antiferromagnétique de FeCl2 et l'action d'un champ magnétique extérieur sur la disposition des moments magnétiques. La seconde, effectuée avec un spectromètre à temps de vol, a pour objet la diffusion critique au voisinage du point de Curie du fer. Elle permet de déterminer les deux paramètres caractérisant les fluctuations d'aimantation, la partie des corrélations et leur constante de diffusion
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