Nonequilibrium depinning transition of ac driven vortices with random pinning

Abstract

We study the critical behavior associated with transient dynamics of vortices near the depinning transition by an ac drive. From ${I}_{{\rm{ac}},{\rm{e}}}$ – ${V}_{{\rm{ac}},{\rm{e}}}$ curves at different frequencies f , where ${I}_{{\rm{ac}},{\rm{e}}}$ and ${V}_{{\rm{ac}},{\rm{e}}}$ are the effective current and voltage of sinusoidal waveform, respectively, we clearly identify the characteristic voltage separating linear and nonlinear regimes, from which we can estimate the mean diameter of the pinning potential. We also measure the time evolution of the voltage V _ac ( t ) for a disordered initial vortex configuration in response to the ac drive I _ac ( t ) of square waveform, and find a gradual increase in the amplitude $| {V}_{{\rm{ac}}}(t)| $ towards the steady-state voltage, indicative of dynamic ordering. The relaxation time $\tau (f)$ against $| {I}_{{\rm{ac}}}(f)| $ shows a power-law divergence at the f -dependent depinning threshold ${I}_{{\rm{ac}},{\rm{d}}}(f)$ . When plotted against $| {I}_{{\rm{ac}}}(f)| -{I}_{{\rm{ac}},{\rm{d}}}(f)$ , all $\tau (f)$ 's collapse on a single line with a critical exponent of 1.4, which almost coincides with the value for the dc depinning transition. These results indicate that the critical behavior of the depinning transition is observed not only for the dc drive but also for the ac drive, further demonstrating the universality of the nonequilibrium depinning transition.

Keywords

• vortex pinning
• deformation and plasticity
• dynamic critical phenomena
• 74.25.Wx
• 62.20.F-
• 64.60.Ht