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arXiv 1107.5599cond-mat.supr-con
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圆形棘齿通道中旋涡运动的修正

Rectification of Vortex Motion in a Circular Ratchet Channel

N. S. Lin, T. W. Heitmann, K. Yu, B. L. T. Plourde, V. R. Misko

摘要:

我们研究了在Corbino装置中由外部电流I驱动的非对称环形通道中的涡旋动力学。非对称势流可以在无外力驱动的情况下对涡的运动进行整流,形成净流,即棘轮效应。在外加交流电流的情况下,电势可以矫正通道中的旋涡运动,并诱导直流净流。我们发现,涡的净流量强烈地依赖于涡密度和驱动电流的频率。根据密度,我们区分由通道的每个单元内的势能景观确定的“单涡旋”整流区(低密度)(即,“硬”和“易”运动方向)和“多涡”或“集体”整流(高密度)。驱动交流电流的频率决定了涡流在一个周期内可能移动的距离。对于高频电流,涡旋仅在三角形单元中振荡。对于低频率,涡角速度$ω$几乎线性地增加,直到驱动力在硬方向上达到最大摩擦力。此外,涡旋数和细胞数之间的可互换性导致阶梯式$ω-I$曲线。除了整数步之外,即,除了在单涡情况下发现的大台阶外,我们还发现了对应于涡和三角形单元数之间分数比的分数台阶。当涡的净流量达到最大值时,不同的电流的主频率和分数频率被发现,当涡的密度较低时,与频率成正比。我们已经进行了初步的测量装置上包含一个单一的弱钉扎圆形棘轮通道的Corbino几何形状,并观察到大量的非对称涡流响应。

英文摘要:

We study the dynamics of vortices in an asymmetric ring channel driven by an external current I in a Corbino setup. The asymmetric potential can rectify the motion of vortices and cause a net flow without any unbiased external drive, which is called ratchet effect. With an applied ac current, the potential can rectify the motion of vortices in the channel and induce a dc net flow. We show that the net flow of vortices strongly depends on vortex density and frequency of the driving current. Depending on the density, we distinguish a "single-vortex" rectification regime (low density) determined by the potential-energy landscape inside each cell of the channel (i.e., "hard" and "easy" directions of motion) and "multi-vortex", or "collective", rectification (high density) when the interaction between vortices becomes important. The frequency of the driving ac current determines a possible distance that a vortex could move during one period. For high frequency current, vortices only oscillate in the triangular cell. For low frequency, the vortex angular velocity $ω$ increases nearly linearly until the driving force reaches the maximum friction force in the hard direction. Furthermore, the commensurability between the number of vortices and the number of cells results in a stepwise $ω-I$ curve. Besides the integer steps, i.e., the large steps found in the single vortex case, we also found fractional steps corresponding to fractional ratio between the numbers of vortices and triangular cells. The principal and fractional frequencies for different currents are found, when the net flow of vortices reaches the maximum that is proportional to the frequency when the density of vortices is low. We have performed preliminary measurements on a device containing a single weak-pinning circular ratchet channel in a Corbino geometry and observed a substantial asymmetric vortex response.

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