准粒子的暂时冷却和超临界电流突然接通后超导桥电压响应的延迟
Temporary cooling of quasiparticles and delay in voltage response of superconducting bridges after abrupt switching on the supercritical current
摘要：
我们重新讨论了超导桥突然接通超临界电流$I>I_c$后的动态响应问题。与以前的理论工作相比，我们考虑到空间梯度，并使用局部温度的方法和准粒子的分布函数的动力学方程。在这两种模型中，电压响应中的有限延迟时间$t_d$与由于电流抑制超导序参量而引起的准粒子的暂时冷却有关。我们发现，$t_d$有不同的值和不同的温度依赖性所考虑的模型。事实证明，即使是很小的不均匀性的存在下，在桥或散装引线/接触的{ \it同质}桥的两端有利于超导序参量$|Δ|$在动态响应期间的局部抑制。与空间均匀模型相比，由于非平衡准粒子从局部抑制$|Δ|$的区域扩散，它导致延迟时间的减少。在电流分布在桥上空间不均匀的情况下，延迟时间主要与在电流密度最大的位置（在$I\sim I_c$和对于不太宽的膜）处第一涡旋成核所需的时间有关。我们还发现，一个短的交流电流脉冲（正弦波）与零时间平均可能会导致非零的时间平均电压响应，其符号取决于交流电流的相位。
英文摘要：
We revisit the problem of the dynamic response of a superconducting bridge after abruptly switching on the supercritical current $I>I_c$. In contrast to previous theoretical works we take into account spatial gradients and use both the local temperature approach and the kinetic equation for the distribution function of quasiparticles. In both models the finite delay time $t_d$ in the voltage response is connected with temporary cooling of quasiparticles due to the suppression of the superconducitng order parameter by current. We find that $t_d$ has different values and different temperature dependencies in the considered models. In turns out that the presence of even small inhomogeneities in the bridge or of bulk leads/contacts at the ends of the {\it homogenous} bridge favors a local suppression of the superconducting order parameter $|Δ|$ during the dynamic response. It results in a decrease of the delay time, in comparison with the spatially uniform model, due to the diffusion of nonequilibrium quasiparticles from the region with locally suppressed $|Δ|$. In case the current distribution is spatially nonuniform across the bridge the delay time is mainly connected with the time needed for the nucleation of the first vortex at the position where the current density is maximal (at $I\sim I_c$ and for not very wide films). We also find that a short alternating current pulse (sinusoid like) with zero time-average may result in a nonzero time-averaged voltage response where its sign depends on the phase of the ac current.
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