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

接近绝对零度的液态氦-3实验在凝聚态物理学许多核心概念的发展中发挥了关键作用。超流体3 He的发现给了我们第一个p波超流体，一个非常规超导的模型，其中配对打破了母体正常金属的对称性。从那时起，国际材料发现计划已经推出了许多新的非传统超导体。在形式上，超流3 He的相可以被看作是量子真空，在粒子物理学和宇宙学中也有类似之处[参见“氦滴中的宇宙”，G. E. Volovik]。最近，凝聚态系统的拓扑（在动量空间中）已经被广泛应用，作为它们分类的有力方案，以及对称性破缺的概念。拓扑学的简单真理（例如球体的表面不能连续变形为环面）在应用于复杂的相互作用量子系统时具有强大的影响，通过指向必须存在的现象，独立于微观细节;拓扑学的不可侵犯的约束赋予了强大的保护。这可能会导致电子设备的操作依赖于量子力学定律的方式免疫环境干扰，这是由微软的Station Q和相关programmes.In这个程序的支持的愿景，我们将研究的拓扑超流性的氦-3限制在规则的纳米制造的几何形状，作为一个模型系统，以进一步我们的理解拓扑量子物质。我们的实验将利用我们最近在量子纳米流体方面取得的技术突破，以及基于SQUID检测旋进磁信号的灵敏NMR技术的发展（超导量子干涉器件）。超流3 He在厚度为库珀对直径量级的板状腔中的限制，对超流秩序有着深远的影响，并有望稳定物质的新超流状态。3 He的可压缩性允许对直径进行压力调谐，从而改变有效约束。规则的几何形状可以用良好表征的表面来制造，其可以通过用氦-4膜电镀来原位调整。这种精致的几何控制和调谐，再加上超流3 He的理想材料质量，以及高度发达的微观模型提供了严格的理论-实验接口。具有不同拓扑结构的相在不同条件下是稳定的，我们将映射我们的新控制参数约束对这些相的影响。我们将量化无序的作用，所产生的表面粗糙度，和量子尺寸效应的重要性。这些拓扑超流体支持在腔的表面或边缘，在域壁和涡旋的新颖的激发。这些激发的精确特征取决于超流基态是否保持或破坏时间反演对称性。在B相的表面，他们正在传播马约拉纳费米子，我们将寻找这些作为项目的一部分。这个项目有一个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，并利用我们的欧洲Microkelvin合作组织的成员资格。我们将与英国和国际上其他拓扑量子物质项目建立联系，并通过哈伯德理论联盟的访客项目和研讨会加强联系。该项目预计将导致对拓扑量子物质和拓扑超流性/超导性的基本见解。它将在结合超低温和纳米科学的新前沿推动新仪器的创新，以及具有广泛适用性的新SQUID NMR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.