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

Abstract

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. Planned Impact: The premise of our International Network is to radically advance the experimental and theoretical science of unconventional superconductivity at oxide interfaces. By establishing the core teams in Cambridge and Kyoto Universities as research flagships and hubs in the area of unconventional superconductivity and oxide interfaces, we will use our Network to inspire a global research effort in this field with the broad aim of underpinning the application of new superconducting/quantum phenomena. By exploring new methods to control the pairing state of the unconventional superconductors we hope to greatly enhance the understanding of novel mechanisms of superconductivity. The aims and objectives set out in detail the expected outcomes of this research. Although in the short-term these advances will mainly impact on the academic community, there are identified long-term application areas for this research. For example, the European roadmap on superconductive electronics highlights the importance of unconventional devices for controlling the phase shift in digital circuits and requirement for much lower power computing for data centres. In the latter case, superconducting spintronics could help to minimise significantly this power consumption problem by offering energy efficient spintronic circuits in which complex circuit operation will be possible with minimal ohmic heating. For integrated circuits, particularly those operating within large-scale installations such as data centres, these power gains would greatly exceed the additional cooling overhead required to reach the superconducting state. In the USA, IARPA has recently launched a cryogenic computing initiative targeting exactly this issue. From a basis anchored in fundamental theory and basic experiments, the principles, methodology and programmatic output of the Network can therefore realistically inform and direct the technical development of a new generation of superconducting and quantum technologies. Aspects of this research also have the potential to impact on the distant prospect of quantum computing via devices based on Majorana fermions. Strategic roadsmapping and horizon scanning will extend well beyond the duration of this grant. Emerging and maturing superconducting and quantum technologies will in the longer term affect many industries, including electronics, communications, sensing and security. Through the Cambridge and Kyoto hubs, our capability and expertise will be continuously promoted including to companies identified as longer-term beneficiaries. Spin-out formation and licensing of IP for commercial uptake of our findings will lead to direct economic impact in the UK and Japan, as the academic research base strength will attract industrial R&D presence around it. Early-career researcher training through the Network will become a source of highly skilled workforce for companies developing superconducting and quantum technologies en-route to market, filling the rising need for interdisciplinary and specialised graduates and ensuring first-hand transfer of knowledge and know-how. Through public outreach our cutting-edge research and capability will enthuse future generations of scientists and technologists and feed into positive perception of quantum technologies (hence the National QT Programme) in the general public.