Alejandro B. Kolton

HALL NOISE AND TRANSVERSE FREEZING IN DRIVEN VORTEX LATTICES

We study driven vortex lattices in superconducting thin films. Above the critical force F{sub c} we find two dynamical phase transitions at F{sub p} and F{sub t} , which could be observed in simultaneous noise measurements of the longitudinal and Hall voltage. At F{sub p} 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 F{sub t} there is a sharp transition to a frozen transverse solid, where the Hall noise falls abruptly and vortex motion is localized in the transverse direction. {copyright} {ital 1999} {ital The American Physical Society}

Mode locking in ac-driven vortex lattices with random pinning.

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.

Dynamics below the Depinning Threshold in Disordered Elastic Systems

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 1. 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.

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 3D-2D transition from vortex lines to decoupled pancakes as a function of relative interlayer coupling strength, with an accompanying large increase in the critical current reminiscent 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.

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(0.45  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.

Theory and simulations on strong pinning of vortex lines by nanoparticles

The pinning of vortex lines by an array of nanoparticles embedded inside superconductors has become the most efficient practical way to achieve high critical currents. In this scenario, pinning occurs via trapping of the vortex-line segments, and the critical current is determined by the typical length of the trapped segments. To verify analytical estimates and develop a quantitative description of strong pinning, we numerically simulated isolated vortex lines driven through an array of nanoparticles. We found that the critical force grows roughly as the square root of the pin density and that it is strongly suppressed by thermal noise. The configurations of pinned lines are strongly anisotropic; displacements in the drive directions are much larger than those in the transverse direction. Moreover, we found that the roughening index for the longitudinal displacements exceeds 1. This indicates that the local stresses in the critical region increase with the total line length and that the elastic description breaks down in the thermodynamic limit. Thermal noise reduces the anisotropy of displacements in the critical regions and straightens the lines.

Creep dynamics of elastic manifolds via exact transition pathways

We study the steady state of driven elastic strings in disordered media below the depinning threshold. In the low-temperature limit, for a fixed sample, the steady state is dominated by a single configuration, which we determine exactly from the transition pathways between metastable states. We obtain the dynamical phase diagram in this limit. At variance with a thermodynamic phase transition, the depinning transition is not associated with a divergent length scale of the steady state below threshold, but only of the transient dynamics. We discuss the distribution of barrier heights, and check the validity of the dynamic phase diagram at small but finite temperatures using Langevin simulations. The phase diagram continues to hold for broken statistical tilt symmetry. We point out the relevance of our results for experiments of creep motion in elastic interfaces.

Creep Motion of an Elastic String in a Random Potential

We study the creep motion 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. We analyze the creep exponent mu and the roughness exponent zeta. Two regimes are identified: when the temperature is larger than the strength of the disorder, we find mu approximately 1/4 and zeta approximately 2/3, in agreement with the quasi-equilibrium-nucleation picture of creep motion; on the contrary, when lowering the temperature enough, the values of mu and zeta increase, showing a strong violation of the latter picture.

Effective temperature in driven vortex lattices with random pinning.

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.

Dynamics of Disordered Elastic Systems

In these notes we present a brief review of the dynamical properties of interfaces in a disordered environment. We focus in particular on the response of such systems to a very small external force, and the corresponding very slow motion it entails, so called creep. We discuss various general theoretical aspects of this problem and consider in detail the case of a one dimensional interface (domain wall).