多层超导体的理论场极限
Theoretical Field Limits for Multi-Layer Superconductors
摘要：
SIS结构--在这项工作中，我们表明，事实上$B_{c1}$ = 0的SIS结构。我们计算的过热场$B_{sh}$，我们表明，它可以稍微提高使用SIS结构，但只有在一个小范围的膜厚度，只有当膜和散装是不同的材料。我们还表明，使用多层而不是一个单一的厚层是有害的，因为这减少了$B_{sh}$的膜。我们计算了由于涡旋渗透以上的$B_{sh}$的薄膜的耗散，并发现它是无法管理的SRF应用程序。然而，我们发现，如果在序参量的相位中引入梯度，SIS结构可能能够屏蔽大的DC和低频场。我们认为，SIS结构是没有好处的SRF腔，但由于最近的实验表明，低表面电阻性能以上$B_{c1}$的超导体与小的相干长度制成的腔，我们认为，增强$B_{c1}$是没有必要的，和替代材料的体膜显示出很大的希望。
英文摘要：
The SIS structure---a thin superconducting film on a bulk superconductor separated by a thin insulating film---was propsed as a method to protect alternative SRF materials from flux penetration by enhancing the first critical field $B_{c1}$. In this work, we show that in fact $B_{c1}$ = 0 for a SIS structure. We calculate the superheating field $B_{sh}$, and we show that it can be enhanced slightly using the SIS structure, but only for a small range of film thicknesses and only if the film and the bulk are different materials. We also show that using a multilayer instead of a single thick layer is detrimental, as this decreases $B_{sh}$ of the film. We calculate the dissipation due to vortex penetration above the $B_{sh}$ of the film, and find that it is unmanageable for SRF applications. However, we find that if a gradient in the phase of the order parameter is introduced, SIS structures may be able to shield large DC and low frequency fields. We argue that the SIS structure is not beneficial for SRF cavities, but due to recent experiments showing low-surface-resistance performance above $B_{c1}$ in cavities made of superconductors with small coherence lengths, we argue that enhancement of $B_{c1}$ is not necessary, and that bulk films of alternative materials show great promise.
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