2,847 research outputs found
Correlated hopping of bosonic atoms induced by optical lattices
In this work we analyze a particular setup with ultracold atoms trapped in
state-dependent lattices. We show that any asymmetry in the contact interaction
translates into one of two classes of correlated hopping. After deriving the
effective lattice Hamiltonian for the atoms, we obtain analytically and
numerically the different phases and quantum phase transitions. We find for
weak correlated hopping both Mott insulators and charge density waves, while
for stronger correlated hopping the system transitions into a pair superfluid.
We demonstrate that this phase exists for a wide range of interaction
asymmetries and has interesting correlation properties that differentiate it
from an ordinary atomic Bose-Einstein condensate.Comment: 24 pages with 9 figures, to appear in New Journal of Physic
Dynamic regimes of fluids simulated by multiparticle-collision dynamics
We investigate the hydrodynamic properties of a fluid simulated with a
mesoscopic solvent model. Two distinct regimes are identified, the `particle
regime' in which the dynamics is gas-like, and the `collective regime' where
the dynamics is fluid-like. This behavior can be characterized by the Schmidt
number, which measures the ratio between viscous and diffusive transport.
Analytical expressions for the tracer diffusion coefficient, which have been
derived on the basis of a molecular-chaos assumption, are found to describe the
simulation data very well in the particle regime, but important deviations are
found in the collective regime. These deviations are due to hydrodynamic
correlations. The model is then extended in order to investigate self-diffusion
in colloidal dispersions. We study first the transport properties of heavy
point-like particles in the mesoscopic solvent, as a function of their mass and
number density. Second, we introduce excluded-volume interactions among the
colloidal particles and determine the dependence of the diffusion coefficient
on the colloidal volume fraction for different solvent mean-free paths. In the
collective regime, the results are found to be in good agreement with previous
theoretical predictions based on Stokes hydrodynamics and the Smoluchowski
equation.Comment: 15 pages, 15 figure
Quantum computation with unknown parameters
We show how it is possible to realize quantum computations on a system in
which most of the parameters are practically unknown. We illustrate our results
with a novel implementation of a quantum computer by means of bosonic atoms in
an optical lattice. In particular we show how a universal set of gates can be
carried out even if the number of atoms per site is uncertain.Comment: 3 figure
Power law tails of time correlations in a mesoscopic fluid model
In a quenched mesoscopic fluid, modelling transport processes at high
densities, we perform computer simulations of the single particle energy
autocorrelation function C_e(t), which is essentially a return probability.
This is done to test the predictions for power law tails, obtained from mode
coupling theory. We study both off and on-lattice systems in one- and
two-dimensions. The predicted long time tail ~ t^{-d/2} is in excellent
agreement with the results of computer simulations. We also account for finite
size effects, such that smaller systems are fully covered by the present theory
as well.Comment: 11 pages, 12 figure
Trains, tails and loops of partially adsorbed semi-flexible filaments
Polymer adsorption is a fundamental problem in statistical mechanics that has
direct relevance to diverse disciplines ranging from biological lubrication to
stability of colloidal suspensions. We combine experiments with computer
simulations to investigate depletion induced adsorption of semi-flexible
polymers onto a hard-wall. Three dimensional filament configurations of
partially adsorbed F-actin polymers are visualized with total internal
reflection fluorescence microscopy. This information is used to determine the
location of the adsorption/desorption transition and extract the statistics of
trains, tails and loops of partially adsorbed filament configurations. In
contrast to long flexible filaments which primarily desorb by the formation of
loops, the desorption of stiff, finite-sized filaments is largely driven by
fluctuating filament tails. Simulations quantitatively reproduce our
experimental data and allow us to extract universal laws that explain scaling
of the adsorption-desorption transition with relevant microscopic parameters.
Our results demonstrate how the adhesion strength, filament stiffness, length,
as well as the configurational space accessible to the desorbed filament can be
used to design the characteristics of filament adsorption and thus engineer
properties of composite biopolymeric materials
Simulations of thermophoretic nanoswimmers
We consider a nanodimer in solution with asymmetric thermal properties that shows self-propelled motion. One monomer of the nanodimer can be heated to a fixed temperature producing a radially symmetric temperature gradient. The thermophoretic properties of the second monomer produce then a propulsion against or toward the heated particle, such that the nanodimer becomes a puller or pusher nanoswimmer. We combine our simulation measurements with a theoretical analysis that satisfactorily characterizes the self-propelled velocity with the temperature gradient, and the thermophoretic properties of the bead
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