4,391 research outputs found
Nematic - Isotropic Transition in Porous Media - a Monte Carlo Study
We propose a lattice model to simulate the influence of porous medium on the
Nematic - Isotropic transition of liquid crystal confined to the pores. The
effects of pore size and pore connectivity are modelled through a disorder
parameter. Monte Carlo calculations based on the model leads to results that
compare well with experiments.Comment: 11 pages; 4 figure
Re-equilibration after quenches in athermal martensites:Conversion-delays for vapour to liquid domain-wall phases
Entropy barriers and ageing states appear in martensitic
structural-transition models, slowly re-equilibrating after temperature
quenches, under Monte Carlo dynamics. Concepts from protein folding and ageing
harmonic oscillators turn out to be useful in understanding these
nonequilibrium evolutions. We show how the athermal, non-activated delay time
for seeded parent-phase austenite to convert to product-phase martensite,
arises from an identified entropy barrier in Fourier space. In an ageing state
of low Monte Carlo acceptances, the strain structure factor makes
constant-energy searches for rare pathways, to enter a Brillouin zone `golf
hole' enclosing negative energy states, and to suddenly release entropically
trapped stresses. In this context, a stress-dependent effective temperature can
be defined, that re-equilibrates to the quenched bath temperature.Comment: 11 pages, 12 figures. Under process with Phys. Rev. B (2015
Complex free energy landscapes in biaxial nematics and role of repulsive interactions : A Wang - Landau study
General quadratic Hamiltonian models, describing interaction between crystal
molecules (typically with symmetry) take into account couplings
between their uniaxial and biaxial tensors. While the attractive contributions
arising from interactions between similar tensors of the participating
molecules provide for eventual condensation of the respective orders at
suitably low temperatures, the role of cross-coupling between unlike tensors is
not fully appreciated. Our recent study with an advanced Monte Carlo technique
(entropic sampling) showed clearly the increasing relevance of this cross term
in determining the phase diagram, contravening in some regions of model
parameter space, the predictions of mean field theory and standard Monte Carlo
simulation results. In this context, we investigated the phase diagrams and the
nature of the phases therein, on two trajectories in the parameter space: one
is a line in the interior region of biaxial stability believed to be
representative of the real systems, and the second is the extensively
investigated parabolic path resulting from the London dispersion approximation.
In both the cases, we find the destabilizing effect of increased cross-coupling
interactions, which invariably result in the formation of local biaxial
organizations inhomogeneously distributed. This manifests as a small, but
unmistakable, contribution of biaxial order in the uniaxial phase.The free
energy profiles computed in the present study as a function of the two dominant
order parameters indicate complex landscapes, reflecting the difficulties in
the ready realization of the biaxial phase in the laboratory.Comment: 23 pages, 12 figure
Haldane Exclusion Statistics and the Boltzmann Equation
We generalize the collision term in the one-dimensional Boltzmann-Nordheim
transport equation for quasiparticles that obey the Haldane exclusion
statistics. For the equilibrium situation, this leads to the ``golden rule''
factor for quantum transitions. As an application of this, we calculate the
density response function of a one-dimensional electron gas in a periodic
potential, assuming that the particle-hole excitations are quasiparticles
obeying the new statistics. We also calculate the relaxation time of a nuclear
spin in a metal using the modified golden rule.Comment: version accepted for publication in J. of Stat. Phy
End wall flows in rotors and stators of a single stage compressor
A computer code for solving the parabolized Navier-Stokes equations for internal flows was developed. Oscillations that develop in the calculation procedure are discussed. The measurements made in the hub and annulus wall boundary layers are summarized. The flow in the hub wall boundary layer, starting ahead of the inlet guide vanes to the inlet of the rotor is traced
- …