66 research outputs found
Two harmonically coupled Brownian particles in random media
We study the behaviour of two Brownian particles coupled by an elastic
harmonic force in a quenched disordered medium. We found that to first order in
disorder strength, the relative motion weakens (with respect to the reference
state of a Brownian particle with the double mass) the effect of the quenched
forces on the centre of mass motion of the Brownian particles, so that the
motion will become less subdiffusive (superdiffusive) for potential
(solenoidal) disorder. The mean-square relative distance between the particles
behaves in a different way depending of whether the particles are free to move
or one particle is anchored in the space. While the effect of nonpotential
disorder consists in increasing the mean-square distance in both cases, the
potential disorder decreases the mean-square distance, when the particles are
free to move, and increases it when one particle is anchored in the space.Comment: 8 pages, 3 figure
Size and doping effects on the coercive field of ferroelectric nanoparticles
A microscopic model for describing ferroelectric nanoparticles is proposed
which allows us to calculate the polarization as a function of an external
electric field, the temperature, the defect concentration and the particle
size. The interaction of the constituents of the material, arranged in layers,
depends on both the coupling strength at the surface and that of defect shells
in addition to the bulk values. The analysis is based on an Ising model in a
transverse field, modified in such a manner to study the influence of size and
doping effects on the hysteresis loop of the nanoparticles. Using a Green
function technique in real space we find the coercive field, the remanent
polarization and the critical temperature which differ significantly from the
bulk behavior. Depending on the varying coupling strength due to the kind of
doping ions and the surface configuration, the coercive field and the remanent
polarization can either increase or decrease in comparison to the bulk
behavior. The theoretical results are compared with a variety of different
experimental data.Comment: 16 pages, 7 figure
Competing Glauber and Kawasaki Dynamics
Using a quantum formulation of the master equation we study a kinetic Ising
model with competing stochastic processes: the Glauber dynamics with
probability and the Kawasaki dynamics with probability . Introducing
explicitely the coupling to a heat bath and the mutual static interaction of
the spins the model can be traced back exactly to a Ginzburg Landau functional
when the interaction is of long range order. The dependence of the correlation
length on the temperature and on the probability is calculated. In case
that the spins are subject to flip processes the correlation length disappears
for each finite temperature. In the exchange dominated case the system is
strongly correlated for each temperature.Comment: 9 pages, Revte
Fluctuation effects in the theory of microphase separation of diblock copolymers in the presence of an electric field
We generalize the Fredrickson-Helfand theory of the microphase separation in
symmetric diblock copolymer melts by taking into account the influence of a
time-independent homogeneous electric field on the composition fluctuations
within the self-consistent Hartree approximation. We predict that electric
fields suppress composition fluctuations, and consequently weaken the
first-order transition. In the presence of an electric field the critical
temperature of the order-disorder transition is shifted towards its mean-field
value. The collective structure factor in the disordered phase becomes
anisotropic in the presence of the electric field. Fluctuational modulations of
the order parameter along the field direction are strongest suppressed. The
latter is in accordance with the parallel orientation of the lamellae in the
ordered state.Comment: 16 page
Computer simulations of two-dimensional melting with dipole-dipole interactions
We perform molecular dynamics and Monte Carlo simulations of two-dimensional
melting with dipole-dipole interactions. Both static and dynamic behaviors are
examined. In the isotropic liquid phase, the bond orientational correlation
length 6 and susceptibility 6 are measured, and the data are fitted to the
theoretical ansatz. An algebraic decay is detected for both spatial and
temporal bond orientational correlation functions in an intermediate
temperature regime, and it provides an explicit evidence for the existence of
the hexatic phase. From the finite-size scaling analysis of the global bond
orientational order parameter, the disclination unbinding temperature Ti is
estimated. In addition, from dynamic Monte Carlo simulations of the positional
order parameter, we extract the critical exponents at the dislocation unbinding
temperature Tm. All the results are in agreement with those from experiments
and support the Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) theory.Comment: 23 pages, 12figure
Phase Ordering Dynamics of Theory with Hamiltonian Equations of Motion
Phase ordering dynamics of the (2+1)- and (3+1)-dimensional theory
with Hamiltonian equations of motion is investigated numerically. Dynamic
scaling is confirmed. The dynamic exponent is different from that of the
Ising model with dynamics of model A, while the exponent is the same.Comment: to appear in Int. J. Mod. Phys.
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