Topological superfluids under engineered nanofluidic confinement: new order parameters and exotic excitations

在绝对零度附近的液氦实验对凝聚态物理中许多核心概念的发展起到了关键作用。超流的发现给了我们第一个p波超流体，一个非传统超导电性的模型，其中的配对破坏了母正常金属的对称性。从那时起，国际材料发现计划推出了许多新的非常规超导体。从形式上讲，超流3He的相可以被视为量子真空，与粒子物理学和宇宙学有相似之处(参见《氦液滴中的宇宙》，G.E.Volovik)。最近，凝聚态系统的拓扑学(在动量空间中)与对称性破缺的概念一起，作为一种强大的分类方案被广泛应用。简单的拓扑学真理(例如，球面不能连续变形为环面)在应用于复杂的相互作用的量子系统时具有强大的影响，因为它指向了必须存在的现象，而与微观细节无关；强大的保护是由拓扑不可侵犯的约束赋予的。这可能会导致电子设备的运行依赖于量子力学定律的方式不受环境干扰，这一愿景得到了微软的Q站和相关程序的支持。在这个程序中，我们将研究限制在规则纳米格子几何中的氦-3的拓扑超流，作为一个模型系统来加深我们对拓扑量子物质的理解。我们的实验将利用我们在量子纳米流体方面取得的最新技术突破，以及基于超导量子干涉器件(SQUID)对进动磁场信号的检测而开发的灵敏的核磁共振技术。超流3He在厚度为库珀对直径数量级的板状空腔中的限制，对超流有序有深远的影响，并有望稳定新的超流物质状态。3He的可压缩性允许对直径进行压力调节，从而改变有效的限制条件。规则的几何形状可以用特征良好的表面来制造，这些表面可以通过镀氦-4薄膜进行原位调谐。这种精细的几何控制和可调性，再加上理想的超流3He材料质量，以及高度发达的微观模型，提供了严格的理论-实验界面。具有不同拓扑的相在不同的条件下是稳定的，我们将映射我们的新控制参数限制对这些相的影响。我们将量化表面粗糙引起的无序的作用，以及量子尺寸效应的重要性。这些拓扑超流体在腔体的表面或边缘、磁区壁上和涡旋上支持新的激发。这些激发的精确性质取决于超流基态是保持还是破坏了时间反转对称性。在B相表面，他们正在传播Majorana费米子，我们将寻找这些费米子作为项目的一部分。该项目具有强大的国际合作维度，无论是实验还是理论，与美国的康奈尔大学和西北大学，以及德国的PTB(柏林)密切合作，并利用我们在欧洲Microkelvin合作的成员资格。我们将通过哈伯德理论联盟的访客计划和工作坊，与英国和国际上关于拓扑量子物质的其他计划建立联系。该计划预计将导致对拓扑量子物质，特别是拓扑超流/超导的基本见解。它将推动超低温和纳米科学相结合的新仪器的创新，以及具有广泛适用性的新的SQUID核磁共振技术。

Experiments on liquid helium-three near the absolute zero of temperature have played a key role in the development of many central concepts in condensed matter physics. The discovery of superfluid 3He gave us the first p-wave superfluid, a model for unconventional superconductivity, in which the pairing breaks the symmetry of the parent normal metal. Since that time the international programme of materials discovery has thrown up many new unconventional superconductors. And formally the phases of superfluid 3He can be regarded as a quantum vacua, with parallels in particle physics and cosmology [see "The Universe in a Helium Droplet", G.E.Volovik].Recently the topology (in momentum space) of condensed matter systems has been widely applied as a powerful scheme for their classification, alongside the concept of broken symmetry. The simple truths of topology (eg a sphere's surface cannot be continuously deformed into that of a torus) have a powerful impact when applied to complex interacting quantum systems, by pointing to phenomena that must be there, independent of microscopic details; robust protection is conferred by the inviolable constraints of topology. This may lead to electronic devices whose operation relies on the laws of quantum mechanics in a way immune to environmental disturbance, a vision that is supported by Microsoft's Station Q and associated programmes.In this programme we will study the topological superfluidity of helium-three confined in regular nanofabricated geometries, as a model system to further our understanding of topological quantum matter. Our experiments will exploit the recent technical breakthroughs we have made in quantum nanofluidics, and the development of sensitive NMR techniques based on the detection of the precessing magnetic signal by SQUIDs (Superconducting Quantum Interference Devices).Confinement of superfluid 3He in a slab-like cavity of thickness of order the diameter of the Cooper pairs, has a profound effect on the superfluid order and is expected to stabilize new superfluid states of matter. The compressibility of 3He allows the pair diameter to be pressure-tuned, varying the effective confinement. Regular geometries can be fabricated with well-characterized surfaces, which can be tuned in situ by plating with a helium-4 film. This exquisite geometrical control and tuneability, coupled to the ideal material qualities of superfluid 3He, and highly developed microscopic models provide a rigorous theory-experiment interface. Phases with different topologies are expected to be stable under different conditions, and we will map the effect of our new control parameter, confinement, on these phases. We will quantify the role of disorder, arising from surface roughness, and the importance of quantum size effects. These topological superfluids support novel excitations at the faces or edges of the cavity, at domain walls and vortices. The precise character of these excitations depends on whether the superfluid ground state preserves or breaks time reversal symmetry. At the surface of the B-phase they are propagating Majorana fermions, and we will search for these as part of the project.This project has a strong international collaborative dimension, both experimental and theoretical, closely partnering with Cornell and Northwestern in the USA, and PTB (Berlin) in Germany, and exploiting our membership of the European Microkelvin Collaboration. We will connect with other programmes on topological quantum matter in the UK and internationally, enhanced by the Hubbard Theory Consortium, through its visitors programmes and workshops.The project is expected to lead to fundamental insights into topological quantum matter and topological superfluidity/superconductivity in particular. It will drive the innovation of new instrumentation at the new frontier combining ultra-low temperatures and nanoscience, and new SQUID NMR techniques of broad applicability.