Papers by Alejandro Kolton
Physical review letters, Jan 11, 2014
Magnetic-field-driven domain wall motion in an ultrathin Pt/Co(0.45 nm)/Pt ferromagnetic film wi... more Magnetic-field-driven domain wall motion in an ultrathin Pt/Co(0.45 nm)/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, ψ=0.15, and the Larkin length crossover exponent, ϕ=0.24, fit well with the numerical predictions.
Physical Review B, 2011
The motion of a domain wall in a two-dimensional medium is studied by taking into account the int... more The motion of a domain wall in a two-dimensional medium is studied by taking into account the internal elastic degrees of freedom of the wall and geometrical pinning produced by both 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. Exact calculation as a function of the geometry of the sample 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.
Physical Review Letters, Oct 1, 2009
We study the nonstationary dynamics of an elastic interface in a disordered medium at the depinni... more We study the nonstationary dynamics of an elastic interface in a disordered medium at the depinning transition. We compute the two-time response and correlation functions, found to be universal and characterized by two independent critical exponents. We find a good agreement between two-loop functional renormalization group calculations and molecular dynamics simulations for the scaling forms, and for the response aging exponent thetaR. We also describe a dynamical dimensional crossover, observed at long times in the relaxation of a finite system. Our results are relevant for the nonsteady driven dynamics of domain walls in ferromagnetic films and contact lines in wetting.
Pinning of vortex lines by array of nanoparticles embedded inside superconductors became the most... more Pinning of vortex lines by array of nanoparticles embedded inside superconductors became the most efficient practical way to achieve high critical currents. In this situation pinning occurs via trapping of the vortex-line segments and the critical current is determined by the typical length of trapped segment. To verify analytical estimates and develop a quantitative description of strong pinning, we use large-scale numerical simulations. We study the dependence of the critical force on the density of pins in the regime of independently pinned lines, statistical properties of trapped lines, and suppression of the apparent critical force by thermal fluctuations. This work is supported by the Center for Emergent Superconductivity funded by the U.S. DOE, Office of Science, under Award No. DE-AC0298CH1088.
We study the slow dynamics of an elastic string in a two dimensional pinning landscape by means o... more We study the slow dynamics of an elastic string in a two dimensional pinning landscape by means of Langevin dynamics simulations. We find that the Velocity-Force characteristics are well described by the creep formula predicted from phenomenological scaling arguments. However, at strong disorder, the creep exponent μ and the roughness ζ of the string display a clear deviation from the values μ 1/4 and ζ 2/3 expected assuming a quasi-equilibrium-nucleation picture of the creep motion. We also analyzed the non-stationary relaxation of the string towards the steady state. We identify a slowly growing length L(T,F,t) separating equilibrated and non-equilibrated length scales during the relaxation. For equilibrated lengths, l < L, we find a roughness ζ 2/3 at F=0 while for small F > 0 an ``excess'' of roughness ζ> 2/3 is always observed.
We find mode-locking steps in simulated current-voltage characteristics of ac-driven vortex latti... more We find mode-locking steps in simulated current-voltage characteristics of ac-driven vortex lattices with {\it random} pinning. For low frequencies there is mode-locking above a finite ac force amplitude, while for large frequencies there is mode-locking for any small ac force. This is correlated with the nature of temporal order in the different regimes in the absence of ac drive. The mode-locked state is a frozen solid pinned in the moving reference of frame, and the depinning from the step shows plastic flow and hysteresis.
Physical Review B, 2009
We study the thermally assisted relaxation of a directed elastic line in a two dimensional quench... more We study the thermally assisted relaxation of a directed elastic line in a two dimensional quenched random potential by solving numerically the Edwards-Wilkinson equation and the Monte Carlo dynamics of a solid-on-solid lattice model. We show that the aging dynamics is governed by a growing correlation length displaying two regimes: an initial thermally dominated power-law growth which crosses over, at a static temperature-dependent correlation length LT ∼ T 3 , to a logarithmic growth consistent with an algebraic growth of barriers. We present a scaling arguments to deal with the crossover-induced geometrical and dynamical effects. This analysis allows to explain why the results of most numerical studies so far have been described with effective power-laws and also permits to determine the observed anomalous temperature-dependence of the characteristic growth exponents. We argue that a similar mechanism should be at work in other disordered systems. We generalize the Family-Vicsek stationary scaling law to describe the roughness by incorporating the waiting-time dependence or age of the initial configuration. The analysis of the two-time linear response and correlation functions shows that a well-defined effective temperature exists in the power-law regime. Finally, we discuss the relevance of our results for the slow dynamics of vortex glasses in High-Tc superconductors.
Physica C: Superconductivity, 2000
We present molecular dynamics simulations of driven vortices in layered superconductors in the pr... more We present molecular dynamics simulations of driven vortices in layered superconductors in the presence of an external homogeneous force and point disorder. We use a model introduced by J.R..Clem for describing 3D vortex lines as stacks of 2D pancake vortices where only magnetic interactions are considered and the Josephson interlayer coupling is neglected. We numerically evaluate the long-range magnetic interaction between pancake vortices exactly. We analyze the vortex correlation along the field direction on (c-axis). We find that above the critical current, in the "plastic flow" regime, pancakes are completely uncorrelated in the c-direction. When increasing the current, there is an onset of correlation along the c-axis at the transition from plastic flow to a moving smectic phase. This transition coincides with the peak in the differential resistance.
Papers in Physics, 2010
We numerically study the geometry of a driven elastic string at its sample-dependent depinning th... more We numerically study the geometry of a driven elastic string at its sample-dependent depinning threshold in random-periodic media. We find that the anisotropic finite-size scaling of the average square width w 2 and of its associated probability distribution are both controlled by the ratio k = M/L ζ dep , where ζ dep is the random-manifold depinning roughness exponent, L is the longitudinal size of the string and M the transverse periodicity of the random medium. The rescaled average square width w 2 /L 2ζ dep displays a non-trivial single minimum for a finite value of k. We show that the initial decrease for small k reflects the crossover at k ∼ 1 from the random-periodic to the random-manifold roughness. The increase for very large k implies that the increasingly rare critical configurations, accompanying the crossover to Gumbel critical-force statistics, display anomalous roughness properties: a transverse-periodicity scaling in spite that w 2 M , and subleading corrections to the standard random-manifold longitudinal-size scaling. Our results are relevant to understanding the dimensional crossover from interface to particle depinning. *
Lecture Notes in Physics, 2006
Physical Review B, 2000
... Alejandro B. Kolton,1 Daniel Dominguez,1 Cynthia J. Olson,2 and Niels Gr?h-Jensen3,4 ? Centro... more ... Alejandro B. Kolton,1 Daniel Dominguez,1 Cynthia J. Olson,2 and Niels Gr?h-Jensen3,4 ? Centro Atomico Bariloche, 8400 San Carlos de Bariloche, Rio Negro, Argentina 2 Department of Physics, University of California, Davis, California 95616 3 Department of Applied ...
Physical review letters, Jan 25, 2002
We study numerically correlation and response functions in nonequilibrium driven vortex lattices ... more We study numerically correlation and response functions in nonequilibrium driven vortex lattices with random pinning. From a generalized fluctuation-dissipation relation, we calculate an effective transverse temperature in the fluid moving phase. We find that the effective temperature decreases with increasing driving force and becomes equal to the equilibrium melting temperature when the dynamic transverse freezing occurs. We also discuss how the effective temperature can be measured experimentally from a generalized Kubo formula.
Physical Review B, 2003
We study, analytically and numerically, phase locking of driven vortex lattices in fully-frustrat... more We study, analytically and numerically, phase locking of driven vortex lattices in fully-frustrated Josephson junction arrays at zero temperature. We consider the case when an ac current is applied perpendicular to a dc current. We observe phase locking, steps in the current-voltage characteristics, with a dependence on external ac-drive amplitude and frequency qualitatively different from the Shapiro steps, observed when the ac and dc currents are applied in parallel. Further, the critical current increases with increasing transverse ac-drive amplitude, while it decreases for longitudinal ac-drive. The critical current and the phase-locked current step width, increase quadratically with (small) amplitudes of the ac-drive. For larger amplitudes of the transverse ac-signal, we find windows where the critical current is hysteretic, and windows where phase locking is suppressed due to dynamical instabilities. We characterize the dynamical states around the phase-locking interference condition in the IV curve with voltage noise, Lyapunov exponents and Poincaré sections. We find that zero temperature phase-locking behavior in large fully frustrated arrays is well described by an effective four plaquette model.
Physical Review B, 2001
For a vortex lattice moving in a periodic array we show analytically and numerically that a new t... more For a vortex lattice moving in a periodic array we show analytically and numerically that a new type of phase locking occurs in the presence of a longitudinal dc driving force and a transverse ac driving force. This phase locking is distinct from the Shapiro step phase locking found with longitudinal ac drives. We show that an increase in critical current and a fundamental phase locked step width scale with the square of the driving ac amplitude. Our results should carry over to other systems such as vortex motion in Josephson-junction arrays.
Physical Review B, 2002
We find mode-locking steps in simulated current-voltage characteristics of driven vortex lattices... more We find mode-locking steps in simulated current-voltage characteristics of driven vortex lattices with random pinning when an applied ac-current is perpendicular to the dc-current. For low frequencies there is mode-locking only above a non-zero threshold ac force amplitude, while for large frequencies there is mode-locking for any small ac force. This is consistent with the nature of transverse temporal order in the different regimes in the absence of an applied ac-drive. For large frequencies the magnitude of the fundamental mode-locked step depends linearly with the ac force amplitude.
Any decomposition of the total trajectory entropy production for markovian systems have a joint p... more Any decomposition of the total trajectory entropy production for markovian systems have a joint probability distribution satisfying a generalized detailed fluctuation theorem, without relying in dual probability distributions, when all the contributing terms are odd with respect to time reversal. We show that several fluctuation theorems for perturbed non-equilibrium steady states are unified and arise as simple particular cases of
Physical Review B, 2006
We study numerically the relaxation of a driven elastic string in a two dimensional pinning lands... more We study numerically the relaxation of a driven elastic string in a two dimensional pinning landscape. The relaxation of the string, initially flat, is governed by a growing length L(t) separating the short steady-state equilibrated lengthscales, from the large lengthscales that keep memory of the initial condition. We find a macroscopic short time regime where relaxation is universal, both above and below the depinning threshold, different from the one expected for standard critical phenomena. Below the threshold, the zero temperature relaxation towards the first pinned configuration provides a novel, experimentally convenient way to access all the critical exponents of the depinning transition independently.
Physical Review Letters, 2001
We find mode-locking steps in simulated current-voltage characteristics of ac-driven vortex latti... more We find mode-locking steps in simulated current-voltage characteristics of ac-driven vortex lattices with random pinning. For low frequencies there is mode-locking above a finite ac force amplitude, while for large frequencies there is mode-locking for any small ac force. This is correlated with the nature of temporal order in the different regimes in the absence of ac drive. The mode-locked state is a frozen solid pinned in the moving reference of frame, and the depinning from the step shows plastic flow and hysteresis.
Physical Review Letters, 2006
We study the steady-state low-temperature dynamics of an elastic line in a disordered medium belo... more We study the steady-state low-temperature dynamics of an elastic line in a disordered medium below the depinning threshold. Analogously to the equilibrium dynamics, in the limit T → 0, the steady state is dominated by a single configuration which is occupied with probability one. We develop an exact algorithm to target this dominant configuration and to analyze its geometrical properties as a function of the driving force. The roughness exponent of the line at large scales is identical to the one at depinning. No length scale diverges in the steady state regime as the depinning threshold is approached from below. We do find, a divergent length, but it is associated only with the transient relaxation between metastable states.
Physical Review Letters, 1999
We study driven vortices lattices in superconducting thin films. Above the critical force Fc we f... more We study driven vortices lattices in superconducting thin films. Above the critical force Fc we find two dynamical phase transitions at Fp and Ft, which could be observed in simultaneous noise measurements of the longitudinal and the Hall voltage. At Fp there is a transition from plastic flow to smectic flow where the voltage noise is isotropic (Hall noise = longitudinal noise) and there is a peak in the differential resistance. At Ft there is a sharp transition to a frozen transverse solid where the Hall noise falls down abruptly and vortex motion is localized in the transverse direction.
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Papers by Alejandro Kolton