1,395 research outputs found
The Surface Electroclinic Effect near the First Order Smectic-A*--Smectic-C* transition
We analyze the surface electroclinic effect (SECE) in a material that
exhibits a first order bulk smectic- (Sm-) -- smectic-
(Sm-) transition. The effect of a continuously varying degree of
enantiomeric excess on the SECE is also investigated. We show that due to the
first order nature of the bulk Sm- -- Sm- transition, the SECE can be
unusually strong and that as enantiomeric excess is varied, a jump in surface
induced tilt is expected. A theoretical state map, in enantiomeric excess -
temperature space, features a critical point which terminates a line of first
order discontinuities in the surface induced tilt. This critical point is
analogous to that found for the phase diagram (in electric field - temperature
space) for the bulk electroclinic effect. Analysis of the decay of the surface
induced tilt, as one moves from surface into bulk shows that for sufficiently
high surface tilt the decay will exhibit a well defined spatial kink within
which it becomes especially rapid. We also propose that the SECE is
additionally enhanced by the de Vries nature (i.e. small layer shrinkage at the
bulk Sm-A* -- Sm-C* transition) of the material. As such the SECE provides a
new means to characterize the de Vries nature of a material. We discuss the
implications for using these materials in device applications and propose ways
to investigate the predicted features experimentally
De Vries Behavior in Smectics near a Biaxiality Induced Smectic A - Smectic C Tricritical Point
We show that a generalized Landau theory for the smectic A and C phases
exhibits a biaxiality induced AC tricritical point. Proximity to this
tricritical point depends on the degree of orientational order in the system;
for sufficiently large orientational order the AC transition is 3D XY-like,
while for sufficiently small orientational order, it is either tricritical or
1st order. We investigate each of the three types of AC transitions near
tricriticality and show that for each type of transition, small orientational
order implies de Vries behavior in the layer spacing, an unusually small layer
contraction. This result is consistent with, and can be understood in terms of,
the "diffuse cone" model of de Vries. Additionally, we show that birefringence
grows upon entry to the C phase. For a continuous transition, this growth is
more rapid the closer the transition is to tricriticality. Our model also
predicts the possibility of a nonmontonic temperature dependence of
birefringence
Elasticity, fluctuations and vortex pinning in ferromagnetic superconductors: A "columnar elastic glass"
We study the elasticity, fluctuations and pinning of a putative spontaneous
vortex solid in ferromagnetic superconductors. Using a rigorous thermodynamic
argument, we show that in the idealized case of vanishing crystalline pinning
anisotropy the long-wavelength tilt modulus of such a vortex solid vanishes
identically, as guaranteed by the underlying rotational invariance. The
vanishing of the tilt modulus means that, to lowest order, the associated
tension elasticity is replaced by the softer, curvature elasticity. The effect
of this is to make the spontaneous vortex solid qualitatively more susceptible
to the disordering effects of thermal fluctuations and random pinning. We study
these effects, taking into account the nonlinear elasticity, that, in three
dimensions, is important at sufficiently long length scales, and showing that a
``columnar elastic glass'' phase of vortices results. This phase is controlled
by a previously unstudied zero-temperature fixed point and it is characterized
by elastic moduli that have universal strong wave-vector dependence out to
arbitrarily long length scales, leading to non-Hookean elasticity. We argue
that, although translationally disordered for weak disorder, the columnar
elastic glass is stable against the proliferation of dislocations and is
therefore a topologically ordered {\em elastic} glass. As a result, the
phenomenology of the spontaneous vortex state of isotropic magnetic
superconductors differs qualitatively from a conventional,
external-field-induced mixed state. For example, for weak external fields ,
the magnetic induction scales {\em universally} like , with .Comment: Minor editorial changes, version to be published in PRB, 39 pages, 7
figure
"Soft" Anharmonic Vortex Glass in Ferromagnetic Superconductors
Ferromagnetic order in superconductors can induce a {\em spontaneous} vortex
(SV) state. For external field , rotational symmetry guarantees a
vanishing tilt modulus of the SV solid, leading to drastically different
behavior than that of a conventional, external-field-induced vortex solid. We
show that quenched disorder and anharmonic effects lead to elastic moduli that
are wavevector-dependent out to arbitrarily long length scales, and non-Hookean
elasticity. The latter implies that for weak external fields , the magnetic
induction scales {\em universally} like , with
. For weak disorder, we predict the SV solid is a
topologically ordered vortex glass, in the ``columnar elastic glass''
universality class.Comment: minor corrections; version published in PR
Pattern stabilization through parameter alternation in a nonlinear optical system
We report the first experimental realization of pattern formation in a
spatially extended nonlinear system when the system is alternated between two
states, neither of which exhibits patterning. Dynamical equations modeling the
system are used for both numerical simulations and a weakly nonlinear analysis
of the patterned states. The simulations show excellent agreement with the
experiment. The nonlinear analysis provides an explanation of the patterning
under alternation and accurately predicts both the observed dependence of the
patterning on the frequency of alternation, and the measured spatial
frequencies of the patterns.Comment: 12 pages, 5 figures. To appear in PR
Hydrodynamics of Polar Liquid Crystals
Starting from a microscopic definition of an alignment vector proportional to the polarization, we discuss the hydrodynamics of polar liquid crystals with local
C1v-symmetry. The free energy for polar liquid crystals differs from that of nematic liquid crystals (D1h) in that it contains terms violating the n −n symmetry. First we show that these Z2-odd terms induce a general splay instability of a uniform polarized state in a range of parameters. Next we use the general Poissonbracket formalism to derive the hydrodynamic equations of the system in the polarized state. The structure of the linear hydrodynamic modes confirms the existence of the splay instability
A Discotic Disguised as a Smectic: A Hybrid Columnar Bragg Glass
We show that discotics, lying deep in the columnar phase, can exhibit an
x-ray scattering pattern which mimics that of a somewhat unusual smectic liquid
crystal. This exotic, new glassy phase of columnar liquid crystals, which we
call a ``hybrid columnar Bragg glass'', can be achieved by confining a columnar
liquid crystal in an anisotropic random environment of e.g., strained aerogel.
Long-ranged orientational order in this phase makes {\em single domain} x-ray
scattering possible, from which a wealth of information could be extracted. We
give detailed quantitative predictions for the scattering pattern in addition
to exponents characterizing anomalous elasticity of the system.Comment: 4 RevTeX pgs, 2 eps figures. To appear in PR
Viscoelasticity from a Microscopic Model of Dislocation Dynamics
It is shown that the dynamics of a two-dimensional crystal with a finite concentration of dislocations, as well as vacancy and interstitial defects, is governed by the hydrodynamic equations of a viscoelastic medium. At the longest length scales the viscoelasticity is described by the simplest Maxwell model, whose shear and compressional relaxation times are obtained in terms of microscopic quantities, including the density of free dislocations. At short length scales, bond orientational order effects become important and lead to wavevector dependent corrections to the relaxation times
- …