Superconducting spin currents

在几乎所有的超导体中，携带电荷的电子对都处于所谓的“单线态”，在这种状态下，两个电子的量子自旋是反平行的。无论材料是高温氧化物超导体（在这种情况下配对对称性被描述为d波）还是具有s波配对的传统金属超导体，这都是正确的。只有少数已知的化合物表现出所谓的p波超导性，其中电子自旋在一对中是平行的，因此处于“三重态”。在过去的五年中，越来越多的证据表明，单线态超导体和铁磁体之间的接近耦合有时会在铁磁体内产生三重态对——证据是，超电流可以通过铁磁材料，其长度尺度太大，单线态对无法生存。今年早些时候，我们与另外两个国际团队一起，在展示如何以可控的方式创造这种三重态方面取得了突破。总之，这些结果为三重态超导的一个丰富的新领域开辟了道路，在这个领域中，超电流携带自旋的潜在能力可以与标准自旋电子学（“自旋电子学”）相结合。在这个项目中，剑桥的实验团队将与布里斯托尔的凝聚态理论小组合作，巩固他们在这一领域的领先地位，并探索如何通过设备内的磁性元件控制三重态电流，从而可以直接测量自旋超电流。除了展示超导自旋电子器件外，该项目还旨在通过开发预测具有良好p波耦合但本身不是超导体的材料来研究创造人工p波超导体的潜力。这个项目的结果将不可避免地激发更广泛的科学界对非常规超导和自旋电子学的兴趣。我们相信，通过建立产生和控制自旋超电流的基本过程，这项工作将为利用这一新现象的重要新研究方向铺平道路。

In almost all superconductors the pairs of electrons which carry the charge are in the so-called "singlet" state in which the quantum spin of the two electrons is anti-parallel. This is true whether the materials are high temperature oxide superconductors in which case the pairing symmetry is described as d-wave, or conventional metallic superconductors which have s-wave pairing. There are only a few known compounds which show so-called p-wave superconductivity in which the electron spins within a pair are parallel and hence in a "triplet" state.During the past five years there has been increasing evidence that proximity coupling between singlet superconductors and ferromagnets can sometimes generate triplet pairs within the ferromagnet - the evidence being that supercurrents can be passed through ferromagnetic materials over length scales which are simply too large for singlet pairs to survive. Earlier this year, in parallel with two other international groups, we made a breakthrough in demonstrating how this triplet state can be created in a controlled way. Together, the results have opened the way for a rich new field of triplet superconductivity in which the potential ability of a supercurrent to carry spin can be allied with standard spin electronics ("spintronics").In this project the experimental team at Cambridge will develop an existing collaboration with the Condensed Matter Theory Group at Bristol to cement their lead in this field and to explore how triplet currents can be controlled by magnetic elements within a device so that the spin supercurrent can be directly measured. As well as demonstrating superconducting spintronic devices, this project also aims to investigate the potential of creating artificial p-wave superconductors by exploiting materials which are predicted to have a favourable p-wave coupling but which are not themselves superconductors. The results from this programme will inevitably stimulate the broader scientific community interested in unconventional superconductivity and spintronics. We believe that, by establishing the fundamental processes at work in generating spin supercurrents and controlling them, this work will pave the way for important new research directions exploiting this novel phenomenon.