International network to explore novel superconductivity at advanced oxide superconductor/magnet interfaces and in nanodevices

这个由日本、韩国、意大利和英国的世界领先的实验和理论小组组成的国际网络将领导一个计划，在氧化物超导体与磁性材料和纳米设备的界面上探索新的超导电性。通过更好地了解材料性能和工艺，我们的愿景是实现对氧化物界面超导对称性的完全控制，并将剑桥和京都建立为探索先进氧化物界面和非传统超导科学的全球枢纽。在过去的十年里，对传统(S波)超导体和铁磁体之间界面上的非常规超导电性的理解有了快速的发展[自然物理11,307(2015)]。一个亮点是英国申请者对三重态邻近效应的实验演示[Science 329，59(2010)]，这需要通过自旋混合界面将自旋-单态库珀对转变为自旋-三重态对。另一个例子是在纳米线/超导器件中可能产生电子-复合粒子-反粒子。这样的Majorana费米子也有望处于自旋三重态超导体的半量子磁通状态，日本核心成员(Maeno)使用超导微环获得了这一点的第一个证据[科学311,186(2011)]。与此同时，对超流氦和Sr2RuO4(SRO)等化合物中非传统超导电性的理解也取得了显著进展[例如Science 331,186-188(2011)和Science 344,283(2014)]。SRO中的三重态(p波)是偶频的，在概念上不同于S波超导体中铁磁体诱导的奇频三重态配对。然而，有理论预测，表面可以诱导出不同的配对态，这增加了通过SRO和传统超导体之间的界面工程邻近效应耦合不同超导态的可能性。该网络的主要目的之一是以SRO和铁磁/超导结构为模型系统，研究不同超导对称性之间的耦合。理论上预测，SrO和铁磁/超导体杂化材料的表面可以支持诱导的奇频三重态，因此应该存在与传统超导体的邻近效应，这将产生或增强SRO晶体或薄膜的超导电性。实现这一点将有助于详细研究SRO中的电子配对态和不同超导序参数的混合，这是以前在单晶样品中无法实现的。要在氧化物界面连接这些新的超导态，进一步的材料开发是关键。全球对非常规超导电性的兴趣和最近的高影响力实现可能会带来变革性的科学，同时提供低温计算和加密的新范式。为了领导这项研究，剑桥和京都的中心将汇集不同的专业，包括超导、氧化物薄膜和晶体生长、材料表征(XMCD、低能Muon光谱、泵浦-探测太赫兹光谱、角度分辨光电发射、电子显微镜)、纳米制造和理论。该网络的成员将与博士生、PDRA和调查人员密切合作，在成员小组之间进行例行研究访问。同样重要的是，该网络将通过组织会议和学生研讨会以及研究访问，积极与更广泛的科学界接触，总体目标是引发一项长期的全球努力，将基础科学推向应用。

This International Network of world-leading experimental and theoretical groups in Japan, South Korea, Italy and the UK will lead a programme to explore novel superconductivity at oxide superconductor interfaces with magnetic materials and in nanodevices. Through a better understanding of materials properties and processing, our vision is to realise full control over superconducting symmetry at oxide interfaces and to setup Cambridge and Kyoto as a global hubs to explore the science of advanced oxide interfaces and unconventional superconductivity. The past decade has seen rapid developments in the understanding of unconventional superconductivity at the interface between conventional (s-wave) superconductors and ferromagnets [Nature Physics 11, 307 (2015)]. A highlight was the experimental demonstration of a triplet proximity effect by the UK applicants [Science 329, 59 (2010)], which required the transformation of spin-singlet Cooper pairs by a spin-mixing interface into spin-triplet pairs. Another example is a possible creation of electron-composite particle-antiparticles in nanowire/superconductor devices. Such Majorana Fermions are also expected in a half-quantum flux state of a spin-triplet superconductor, and the Japanese core member (Maeno) obtained the first evidence for this using superconducting micro-rings [Science 311, 186 (2011)]. In parallel, the understanding of unconventional superconductivity in superfluid helium and in compounds such as Sr2RuO4 (SRO) [e.g. Science 331, 186-188 (2011) and Science 344, 283 (2014)] has also seen dramatic advances. The triplet (p-wave) state in SRO is even-frequency and is conceptually different to the odd-frequency triplet pairing induced in ferromagnets on s-wave superconductors. However, there are theoretical predictions that alternative pairing states can be induced at surfaces, which raise the prospect of coupling different superconducting states via interface-engineered proximity effects between SRO and conventional superconductors. One of the key aims of the Network is to investigate the coupling of different superconducting symmetries taking SRO and ferromagnet/superconductor structures as a model system. There are theoretical predictions that the surface of SRO and ferromagnet/superconductor hybrids can support an induced odd-frequency triplet-state and so there should be a proximity effect from the conventional superconductor which would create or enhance the superconductivity in a SRO crystal or thin-film. Achieving this will enable detailed studies of the electron pairing state in SRO and the mixing of different superconducting order parameters, which have not previously been possible with single crystal samples.To bridge these novel superconducting states at oxide interfaces, further materials developments are critical. The global interest in unconventional superconductivity and recent high-impact realisations could lead to transformative science and simultaneously offer new paradigms of cryogenic computing and encryption. To lead this research, the Cambridge and Kyoto hubs will bring together different specialities including superconductivity, thin-film and crystal growth of oxides, materials characterization (XMCD, low energy muon spectroscopy, pump-probe terahertz spectroscopy, angle-resolved photoemission, electron microscopy), nanofabrication and theory. The members of the Network will work closely together with PhD students, PDRAs and investigators undertaking routine research visits between the member groups. As importantly the Network will actively engage with the wider scientific community through the organisation of conferences and student workshops, and research visits with the overarching aim of triggering a long-term global effort to lift basic science to application.