Vortex Plastic Motion in Twinned Superconductors

Abstract

We present simulations, without electrodynamical assumptions, of $B(x,y,H(t)), M(H(t))$, and $J_c(H(t))$, in hard superconductors, for a variety of twin-boundary pinning potential parameters, and for a range of values of the density and strength of the pinning sites. We numerically solve the overdamped equations of motion of up to 10^4 flux-gradient-driven vortices which can be temporarily trapped at $\sim 10^6$ pinning centers. These simulations relate macroscopic measurements (e.g., M(H), ``flame'' shaped $B(x,y)$ profiles) with the underlying microscopic pinning landscape and the plastic dynamics of individual vortices

Dynamic vortex phases and pinning in superconductors with twin boundaries

“…Our results in this work complement our previous simulational work on twin-boundaries [17], where we considered only the case of very slow driving that occurs as a magnetic field is increased. In Ref.…”

“…The ratio f T B p /f p is expected to vary as a function of temperature. In the case predicted for low T [1] where f T B p /f p < 1, the twin boundary acts as an easy flow channel for certain angles [17]. On the other hand, at higher T , f T B p /f p > 1, and the twin acts as a barrier to flux flow.…”

“…Twin boundaries are a very common defect found in Y Ba 2 Cu 3 O 7−x (YBCO) and their pinning properties have been extensively studied using Bitter decoration [2], torque magnetometry [3], magnetization [4][5][6][7] transport [8], magneto-optical imaging [9][10][11][12][13][14], and theoretical studies [15][16][17]. Many of the earlier experiments on twinned YBCO samples found conflicting evidence for the role of twin boundaries in vortex pinning.…”

“…In particular, the magneto-optical measurements by Duran et al [9] had shown that twin boundaries act as areas of reduced pinning that allow easy flux penetration, whereas studies by Vlasko-Vlasov et al [10] found the twin boundaries to be barriers to flux motion. Further magneto-optical studies [11][12][13][14], systematic computer simulations [17], and transport measurements [8] have shown that these conflicting results can be resolved when the direction of the Lorentz force with respect to the twin boundary is considered. The twin boundary (TB) acts as an easyflow channel when the Lorentz force is parallel to the twin, but acts as a strong barrier for forces perpendicular to the TB.…”

“…A very systematic simulational study, using samples with of the order of a million pinning sites, by Groth et al [17] of the angular dependence of the Lorentz force with respect to the twin boundary showed that, when the angle between the Lorentz force and the twin is large, a portion of the vortices get trapped inside the twin. This produces a pile-up effect leading to a higher density of vortices on one side of the twin in agreement with observations by several groups including, for example, Vlasko-Vlasov et al [10], Welp et al [12], and Wijngaarden et al [14].…”