4,963 research outputs found
Phase transitions of nematic rubbers
Single crystal nematic elastomers undergo a transition from a strongly
ordered phase N to an "isotropic" phase I. We show that: (a) samples produced
under tension by the Finkelmann procedure are intrinsically anisotropic and
should show a small (temperature dependent) birefringence in the high
temperature I phase. (b) for the I->Ntransition via cooling there is a spinodal
limit but for the N->I transition via heating there is no soft mode at the
standard spinodal temperature. (c) the N->I transition is reminiscent of a
martensitic transformation: nucleation of the I phase should occur in the form
of platelets, making a well defined angle with the director.Comment: 7 pages, 3 figures (To appear in Europhys. Lett.
Pacman percolation: a model for enzyme gel degradation
We study a model for the gel degradation by an enzyme, where the gel is
schematized as a cubic lattice, and the enzyme as a random walker, that cuts
the bonds over which it passes. The model undergoes a (reverse) percolation
transition, which for low density of enzymes falls in a universality class
different from random percolation. In particular we have measured a gel
fraction critical exponent beta=1.0+-0.1, in excellent agreement with
experiments made on the real system.Comment: 4 pages, 7 eps figure
Pseudo-Casimir force in confined nematic polymers
We investigate the pseudo-Casimir force in a slab of material composed of
nematically ordered long polymers. We write the total mesoscopic energy
together with the constraint connecting the local density and director
fluctuations and evaluate the corresponding fluctuation free energy by standard
methods. It leads to a pseudo-Casimir force of a different type than in the
case of standard, short molecule nematic. We investigate its separation
dependence and its magnitude and explicitly derive the relevant limiting cases.Comment: 7 pages, 2 figure
Theory of point contact spectroscopy in electron-doped cuprates
In the hole-doped -wave cuprate superconductor, due to the
midgap surface state (MSS), a zero bias conductance peak (ZBCP) is widely
observed in [110] interface point contact spectroscopy (PCS). However, ZBCP of
this geometry is rarely observed in the electron-doped cuprates, even though
their pairing symmetry is still likely the -wave. We argue
that this is due to the coexistence of antiferromagnetic (AF) and the
superconducting (SC) orders. Generalizing the Blonder-Tinkham-Klapwijk (BTK)
formula to include an AF coupling, it is shown explicitly that the MSS is
destroyed by the AF order. The calculated PCS is in good agreement with the
experiments.Comment: 5 pages, 2 figures. Replaced with published versio
Devil's Staircase and Disordering Transitions in Sliding Vortices and Wigner Crystals on Random Substrates with Transverse Driving
Using numerical simulations we show that, in the presence of random quenched
disorder, sliding superconducting vortices and Wigner crystals pass through a
variety of dynamical phases when an additional transverse driving force is
applied. If the disorder is weak, the driven particles form a moving lattice
and the transverse response shows a devil's staircase structure as the net
driving force vector locks with the symmetry directions of the moving lattice,
in agreement with the predictions of Le Doussal and Giamarchi. For strong
disorder, and particularly for smoothly varying potential landscapes, the
transverse response consists of a sequence of disordering transitions with an
intervening formation of stable channel structures.Comment: 7 pages, 6 postscript figures. Version to appear in Phys. Rev.
Chirality transfer and stereo-selectivity of imprinted cholesteric networks
Imprinting of cholesteric textures in a polymer network is a method of
preserving a macroscopically chiral phase in a system with no molecular
chirality. By modifying the elastics properties of the network, the resulting
stored helical twist can be manipulated within a wide range since the
imprinting efficiency depends on the balance between the elastics constants and
twisting power at network formation. One spectacular property of phase
chirality imprinting is the created ability of the network to adsorb
preferentially one stereo-component from a racemic mixture. In this paper we
explore this property of chirality transfer from a macroscopic to the molecular
scale. In particular, we focus on the competition between the phase chirality
and the local nematic order. We demonstrate that it is possible to control the
subsequent release of chiral solvent component from the imprinting network and
the reversibility of the stereo-selective swelling by racemic solvents
Hamiltonian dynamics of homopolymer chain models
The Hamiltonian dynamics of chains of nonlinearly coupled particles is
numerically investigated in two and three dimensions. Simple, off-lattice
homopolymer models are used to represent the interparticle potentials. Time
averages of observables numerically computed along dynamical trajectories are
found to reproduce results given by the statistical mechanics of homopolymer
models. The dynamical treatment, however, indicates a nontrivial transition
between regimes of slow and fast phase space mixing. Such a transition is
inaccessible to a statistical mechanical treatment and reflects a bimodality in
the relaxation of time averages to corresponding ensemble averages. It is also
found that a change in the energy dependence of the largest Lyapunov exponent
indicates the theta-transition between filamentary and globular polymer
configurations, clearly detecting the transition even for a finite number of
particles.Comment: 11 pages, 8 figures, accepted for publication in Physical Review
Stereo-selective swelling of imprinted cholesteric networks
Molecular chirality, and the chiral symmetry breaking of resulting
macroscopic phases, can be topologically imprinted and manipulated by
crosslinking and swelling of polymer networks. We present a new experimental
approach to stereo-specific separation of chiral isomers by using a cholesteric
elastomer in which a helical director distribution has been topological
imprinted by crosslinking. This makes the material unusual in that is has a
strong phase chirality, but no molecular chirality at all; we study the nature
and parameters controlling the twist-untwist transition. Adding a racemic
mixture to the imprinted network results in selective swelling by only the
component of ``correct'' handedness. We investigate the capacity of demixing in
a racemic environment, which depends on network parameters and the underlying
nematic order
Does changing the pulling direction give better insight into biomolecules?
Single molecule manipulation techniques reveal that the mechanical resistance
of a protein depends on the direction of the applied force. Using a lattice
model of polymers, we show that changing the pulling direction leads to
different phase diagrams. The simple model proposed here indicates that in one
case the system undergoes a transition akin to the unzipping of a
sheet, while in the other case the transition is of a shearing (slippage)
nature. Our results are qualitatively similar to experimental results. This
demonstrates the importance of varying the pulling direction since this may
yield enhanced insights into the molecular interactions responsible for the
stability of biomolecules.Comment: RevTeX v4, 10 pages with 6 eps figure
Semi-soft Nematic Elastomers and Nematics in Crossed Electric and Magnetic Fields
Nematic elastomers with a locked-in anisotropy direction exhibit semi-soft
elastic response characterized by a plateau in the stress-strain curve in which
stress does not change with strain. We calculate the global phase diagram for a
minimal model, which is equivalent to one describing a nematic in crossed
electric and magnetic fields, and show that semi-soft behavior is associated
with a broken symmetry biaxial phase and that it persists well into the
supercritical regime. We also consider generalizations beyond the minimal model
and find similar results.Comment: 4 pages, 3 figure
- …