arXiv subjects

A. B. Kolton

Publications and source records attributed to A. B. Kolton.

33 records · Page 2Linked to original sources

arXiv2012

Thermal rounding of the depinning transition in ultrathin Pt/Co/Pt films

We perform a scaling analysis of the mean velocity of extended magnetic domain walls driven in ultrathin Pt/Co/Pt ferromagnetic films with perpendicular anisotropy, as a function of the applied external field for different film-thicknesses. We find that the scaling of the experimental data around the thermally rounded depinning transition is consistent with the universal depinning exponents theoretically expected for elastic interfaces described by the one-dimensional quenched Edwards-Wilkinson equation. In particular, values for the depinning exponent $β$ and thermal rounding exponent $ψ$ are tested and the present analysis of the experimental data is compatible with $β=0.33$ and $ψ=0.2$, in agreement with numerical simulations.

cond-mat.dis-nn↗

arXiv2012

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 $V \sim T^ψ$, with $ψ$ the thermal rounding exponent. We show that the computed value of the thermal rounding exponent, $ψ= 0.15$, 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.

cond-mat.dis-nn↗

arXiv2012

Joint probability distributions and fluctuation theorems

We derive various exact results for Markovian systems that spontaneously relax to a non-equilibrium steady-state by using joint probability distributions symmetries of different entropy production decompositions. The analytical approach is applied to diverse problems such as the description of the fluctuations induced by experimental errors, for unveiling symmetries of correlation functions appearing in fluctuation-dissipation relations recently generalised to non-equilibrium steady-states, and also for mapping averages between different trajectory-based dynamical ensembles. Many known fluctuation theorems arise as special instances of our approach, for particular two-fold decompositions of the total entropy production. As a complement, we also briefly review and synthesise the variety of fluctuation theorems applying to stochastic dynamics of both, continuous systems described by a Langevin dynamics and discrete systems obeying a Markov dynamics, emphasising how these results emerge from distinct symmetries of the dynamical entropy of the trajectory followed by the system For Langevin dynamics, we embed the "dual dynamics" with a physical meaning, and for Markov systems we show how the fluctuation theorems translate into symmetries of modified evolution operators.

cond-mat.stat-mech↗

arXiv2011

Levy ratchets with dichotomic random flashing

Additive symmetric Lévy noise can induce directed transport of overdamped particles in a static asymmetric potential. We study, numerically and analytically, the effect of an additional dichotomous random flashing in such Lévy ratchet system. For this purpose we analyze and solve the corresponding fractional Fokker-Planck equations and we check the results with Langevin simulations. We study the behavior of the current as function of the stability index of the Lévy noise, the noise intensity and the flashing parameters. We find that flashing allows both to enhance and diminish in a broad range the static Lévy ratchet current, depending on the frequencies and asymmetry of the multiplicative dichotomous noise, and on the additive Lévy noise parameters. Our results thus extend those for dichotomous flashing ratchets with Gaussian noise to the case of broadly distributed noises.

cond-mat.stat-mech↗

arXiv2010

Crossed-ratchet effects and domain wall geometrical pinning

The motion of a domain wall in a two dimensional medium is studied taking into account the internal elastic degrees of freedom of the wall and geometrical pinning produced both by holes and sample boundaries. This study is used to analyze the geometrical conditions needed for optimizing crossed ratchet effects in periodic rectangular arrays of asymmetric holes, recently observed experimentally in patterned ferromagnetic films. Geometrical calculations and numerical simulations have been used to obtain the anisotropic critical fields for depinning flat and kinked walls in rectangular arrays of triangles. The aim is to show with a generic elastic model for interfaces how to build a rectifier able to display crossed ratchet effects or effective potential landscapes for controlling the motion of interfaces or invasion fronts.

cond-mat.mtrl-sci↗

arXiv2010

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.

cond-mat.dis-nn↗

arXiv2010

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.

cond-mat.stat-mech↗

arXiv2009

The mechanisms of spatial and temporal earthquake clustering

The number of earthquakes as a function of magnitude decays as a power law. This trend is usually justified using spring-block models, where slips with the appropriate global statistics have been numerically observed. However, prominent spatial and temporal clustering features of earthquakes are not reproduced by this kind of modeling. We show that when a spring-block model is complemented with a mechanism allowing for structural relaxation, realistic earthquake patterns are obtained. The proposed model does not need to include a phenomenological velocity weakening friction law, as traditional spring-block models do, since this behavior is effectively induced by the relaxational mechanism as well. In this way, the model provides also a simple microscopic basis for the widely used phenomenological rate-and-state equations of rock friction.

cond-mat.dis-nn↗

arXiv2008

Thermal Effects in the dynamics of disordered elastic systems

Many seemingly different macroscopic systems (magnets, ferroelectrics, CDW, vortices,..) can be described as generic disordered elastic systems. Understanding their static and dynamics thus poses challenging problems both from the point of view of fundamental physics and of practical applications. Despite important progress many questions remain open. In particular the temperature has drastic effects on the way these systems respond to an external force. We address here the important question of the thermal effect close to depinning, and whether these effects can be understood in the analogy with standard critical phenomena, analogy so useful to understand the zero temperature case. We show that close to the depinning force temperature leads to a rounding of the depinning transition and compute the corresponding exponent. In addition, using a novel algorithm it is possible to study precisely the behavior close to depinning, and to show that the commonly accepted analogy of the depinning with a critical phenomenon does not fully hold, since no divergent lengthscale exists in the steady state properties of the line below the depinning threshold.

cond-mat.dis-nn↗

arXiv2007

Voltage rectification effects in mesoscopic superconducting triangles: experiment and modelling

The interaction of externally applied currents with persistent currents induced by magnetic field in a mesoscopic triangle is investigated. As a consequence of the superposition of these currents, clear voltage rectification effects are observed. We demonstrate that the amplitude of the rectified signal strongly depends on the configurations of the current leads with the lowest signal obtained when the contacts are aligned along a median of the triangle. When the contacts are aligned off-centered compared to the geometrical center, the voltage response shows oscillations as a function of the applied field, whose sign can be controlled by shifting the contacts. These results are in full agreement with theoretical predictions for an analogous system consisting of a closed loop with a finite number of identical Josephson junctions.

cond-mat.supr-con↗

arXiv2007

Crossed ratchet effects for magnetic domain wall motion

We study both experimentally and theoretically the driven motion of domain walls in extended amorphous magnetic films patterned with a periodic array of asymmetric holes. We find two crossed ratchet effects of opposite sign that change the preferred sense for domain wall propagation, depending on whether a flat or a kinked wall is moving. By solving numerically a simple $ϕ^4$-model we show that the essential physical ingredients for this effect are quite generic and could be realized in other experimental systems involving elastic interfaces moving in multidimensional ratchet potentials.

cond-mat.mtrl-sci↗

arXiv2007

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 $v\sim T^ψ$, where $ψ$ is the thermal exponent. Using the sample-dependent value of the critical force, we precisely evaluate the value of $ψ$ directly from the temperature dependence of the velocity, obtaining the value $ψ= 0.15 \pm 0.01$. 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 $ξ\propto v^{-ν/β}$, where $ν$ and $β$ 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.

cond-mat.stat-mech↗

arXiv2005

Dynamics of disordered elastic systems

We review in these notes some dynamical properties of interfaces in random media submitted to an external force. We focuss in particular to the response to a very small force (so called creep motion) and discuss various theoretical aspects of this problem. We consider in details in particular the case of a one dimensional interface (domain wall).

cond-mat.dis-nn↗

arXiv2000

Driven vortices in 3D layered superconductors: Dynamical ordering along the c-axis

We study a 3D model of driven vortices in weakly coupled layered superconductors with strong pinning. Above the critical force $F_c$, we find a plastic flow regime in which pancakes in different layers are uncoupled, corresponding to a pancake gas. At a higher $F$, there is an ``smectic flow'' regime with short-range interlayer order, corresponding to an entangled line liquid. Later, the transverse displacements freeze and vortices become correlated along the c-axis, resulting in a transverse solid. Finally, at a force $F_s$ the longitudinal displacements freeze and we find a coherent solid of rigid lines.

cond-mat.supr-con↗

arXiv2000

Static and dynamic coupling transitions of vortex lattices in disordered anisotropic superconductors

We use three-dimensional molecular dynamics simulations of magnetically interacting pancake vortices to study vortex matter in disordered, highly anisotropic materials such as BSCCO. We observe a sharp 2D-3D transition from vortex lines to decoupled pancakes as a function of relative interlayer coupling strength, with an accompanying large increase in the critical current remniscent of a second peak effect. We find that decoupled pancakes, when driven, simultaneously recouple and order into a crystalline-like state at high drives. We construct a dynamic phase diagram and show that the dynamic recoupling transition is associated with a double peak in dV/dI.

cond-mat.supr-con↗