122 research outputs found

### Dynamics stabilization and transport coherency in a rocking ratchet for cold atoms

Cold atoms in optical lattices have emerged as an ideal system to investigate
the ratchet effect, as demonstrated by several recent experiments. In this work
we analyze theoretically two aspects of ac driven transport in cold atoms
ratchets. We first address the issue of whether, and to which extent, an ac
driven ratchet for cold atoms can operate as a motor. We thus study
theoretically a dissipative motor for cold atoms, as obtained by adding a load
to a 1D non-adiabatically driven rocking ratchet. We demonstrate that a current
can be generated also in the presence of a load, e.g. the ratchet device can
operate as a motor. Correspondingly, we determine the stall force for the
motor, which characterizes the range of loads over which the device can operate
as a motor, and the differential mobility, which characterizes the response to
a change in the magnitude of the load. Second, we compare our results for the
transport in an ac driven ratchet device with the transport in a dc driven
system. We observe a peculiar phenomenon: the bi-harmonic ac force stabilizes
the dynamics, allowing the generation of uniform directed motion over a range
of momentum much larger than what is possible with a dc bias. We explain such a
stabilization of the dynamics by observing that a non-adiabatic ac drive
broadens the effective cooling momentum range, and forces the atom trajectories
to cover such a region. Thus the system can dissipate energy and maintain a
steady-state energy balance. Our results show that in the case of a
finite-range velocity-dependent friction, a ratchet device may offer the
possibility of controlling the particle motion over a broader range of momentum
with respect to a purely biased system, although this is at the cost of a
reduced coherency

### Transverse rectification of disorder-induced fluctuations in a driven system

We study numerically the overdamped motion of particles driven in a two
dimensional ratchet potential. In the proposed design, of the so-called
geometrical-ratchet type, the mean velocity of a single particle in response to
a constant force has a transverse component that can be induced by the presence
of thermal or other unbiased fluctuations. We find that additional quenched
disorder can strongly enhance the transverse drift at low temperatures, in
spite of reducing the transverse mobility. We show that, under general
conditions, the rectified transverse velocity of a driven particle fluid is
equivalent to the response of a one dimensional flashing ratchet working at a
drive-dependent effective temperature, defined through generalized Einstein
relations.Comment: 4.5 pages, 3 fig

### Non-equilibrium relaxation of an elastic string in random media

We study the relaxation of an elastic string in a two dimensional pinning
landscape using Langevin dynamics simulations. The relaxation of a line,
initially flat, is characterized by a growing length, $L(t)$, separating the
equilibrated short length scales from the flat long distance geometry that keep
memory of the initial condition. We find that, in the long time limit, $L(t)$
has a non--algebraic growth, consistent with thermally activated jumps over
barriers with power law scaling, $U(L) \sim L^\theta$.Comment: 2 pages, 1 figure, Proceedings of ECRYS-2005 International Workshop
on Electronic Crysta

### Uniqueness of the thermodynamic limit for driven disordered elastic interfaces

We study the finite size fluctuations at the depinning transition for a
one-dimensional elastic interface of size $L$ displacing in a disordered medium
of transverse size $M=k L^\zeta$ with periodic boundary conditions, where
$\zeta$ is the depinning roughness exponent and $k$ is a finite aspect ratio
parameter. We focus on the crossover from the infinitely narrow ($k\to 0$) to
the infinitely wide ($k\to \infty$) medium. We find that at the thermodynamic
limit both the value of the critical force and the precise behavior of the
velocity-force characteristics are {\it unique} and $k$-independent. We also
show that the finite size fluctuations of the critical force (bias and
variance) as well as the global width of the interface cross over from a
power-law to a logarithm as a function of $k$. Our results are relevant for
understanding anisotropic size-effects in force-driven and velocity-driven
interfaces.Comment: 10 pages, 12 figure

### Random-Manifold to Random-Periodic Depinning of an Elastic Interface

We study numerically the depinning transition of driven elastic interfaces in
a random-periodic medium with localized periodic-correlation peaks in the
direction of motion. The analysis of the moving interface geometry reveals the
existence of several characteristic lengths separating different length-scale
regimes of roughness. We determine the scaling behavior of these lengths as a
function of the velocity, temperature, driving force, and transverse
periodicity. A dynamical roughness diagram is thus obtained which contains, at
small length scales, the critical and fast-flow regimes typical of the
random-manifold (or domain wall) depinning, and at large length-scales, the
critical and fast-flow regimes typical of the random-periodic (or
charge-density wave) depinning. From the study of the equilibrium geometry we
are also able to infer the roughness diagram in the creep regime, extending the
depinning roughness diagram below threshold. Our results are relevant for
understanding the geometry at depinning of arrays of elastically coupled thin
manifolds in a disordered medium such as driven particle chains or vortex-line
planar arrays. They also allow to properly control the effect of transverse
periodic boundary conditions in large-scale simulations of driven disordered
interfaces.Comment: 19 pages, 10 figure

### Thermal rounding exponent of the depinning transition of an elastic string in a random medium

We study numerically thermal effects at the depinning transition of an
elastic string driven in a two-dimensional uncorrelated disorder potential. The
velocity of the string exactly at the sample critical force is shown to behave
as $V \sim T^\psi$, with $\psi$ the thermal rounding exponent. We show that the
computed value of the thermal rounding exponent, $\psi = 0.15$, is robust and
accounts for the different scaling properties of several observables both in
the steady-state and in the transient relaxation to the steady-state. In
particular, we show the compatibility of the thermal rounding exponent with the
scaling properties of the steady-state structure factor, the universal
short-time dynamics of the transient velocity at the sample critical force, and
the velocity scaling function describing the joint dependence of the
steady-state velocity on the external drive and temperature

### Thermal rounding of the depinning transition

We study thermal effects at the depinning transition by numerical simulations
of driven one-dimensional elastic interfaces in a disordered medium. We find
that the velocity of the interface, evaluated at the critical depinning force,
can be correctly described with the power law $v\sim T^\psi$, where $\psi$ is
the thermal exponent. Using the sample-dependent value of the critical force,
we precisely evaluate the value of $\psi$ directly from the temperature
dependence of the velocity, obtaining the value $\psi = 0.15 \pm 0.01$. By
measuring the structure factor of the interface we show that both the
thermally-rounded and the T=0 depinning, display the same large-scale geometry,
described by an identical divergence of a characteristic length with the
velocity $\xi \propto v^{-\nu/\beta}$, where $\nu$ and $\beta$ are respectively
the T=0 correlation and depinning exponents. We discuss the comparison of our
results with previous estimates of the thermal exponent and the direct
consequences for recent experiments on magnetic domain wall motion in
ferromagnetic thin films.Comment: 6 pages, 3 figure

### Pinning dependent field driven domain wall dynamics and thermal scaling in an ultrathin Pt/Co/Pt magnetic film

Magnetic field-driven domain wall motion in an ultrathin Pt/Co(0.45nm)/Pt
ferromagnetic film with perpendicular anisotropy is studied over a wide
temperature range. Three different pinning dependent dynamical regimes are
clearly identified: the creep, the thermally assisted flux flow and the
depinning, as well as their corresponding crossovers. The wall elastic energy
and microscopic parameters characterizing the pinning are determined. Both the
extracted thermal rounding exponent at the depinning transition, $\psi=$0.15,
and the Larkin length crossover exponent, $\phi=$0.24, fit well with the
numerical predictions.Comment: 5 pages, 4 figure

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