Visualization study of vortex-line dynamics in a magnetically levitated helium-4 superfluid drop

【摘要】量子涡旋就像受量子力学定律支配的小龙卷风，对于理解超流氦、超导体、液晶、中子星甚至早期宇宙中的宇宙弦等系统的行为都很重要。本项目探索超流液氦小液滴（~1cm）中量子漩涡的行为。磁悬浮技术与高速列车和电梯正在探索的技术相同，可以在所有3个维度上移动，它被用来漂浮液滴，消除壁面的影响，漩涡的运动是用激光照射时发出荧光的示踪分子成像的。这些研究将帮助我们更好地理解这种奇怪形式的量子物质的行为，并将有利于超导体和液晶等先进材料的研究。该项目还将在低温物理、流体动力学和先进激光技术等技术重要领域培养新一代科学家。研究小组计划在国家高磁场实验室的各种教育和推广项目中进行涉及磁悬浮和超流氦的演示，向公众介绍这些科学概念。技术摘要：本项目解决了与量子化涡旋线动力学相关的有趣的基本问题。目标包括深入研究涡旋的出现如何影响旋转超流体液滴的稳定性，以及深入研究无壁环境中涡旋缠结的演变。在超流体氦-4中，涡线可以通过捕获在涡线上的示踪粒子成像直接可视化。然而，在低温下在氦中生产示踪剂并对捕获的示踪剂进行成像仍然具有挑战性，并且容器壁通常会影响涡线运动。本项目采用悬浮氦-4液滴作为工作系统，通过快速蒸发冷却和可控液滴旋转产生涡流。这些漩涡可以用亚稳态He2分子示踪剂修饰，可以使用激光诱导荧光技术成像。对旋转氦-4液滴的涡旋结构的研究有助于解释所观察到的液滴形态，并允许推导旋转超流体液滴的稳定性图。由于悬浮氦气液滴可以作为其他超流体液滴系统（如中子星）的模型，因此本研究获得的结果可能会引起广泛的兴趣。可视化氦滴中涡旋缠结的演变有助于研究量子湍流研究中的核心问题，如极化涡旋束的存在和涡旋上开尔文波的产生；这些研究可以直接促进我们对量子湍流特性的认识。

Nontechnical AbstractQuantum vortices, which are like little tornados governed by the laws of quantum mechanics, are important in understanding the behavior of systems such as superfluid helium, superconductors, liquid crystals, neutrons stars and perhaps even cosmic strings in the early universe. This project explores the behavior of quantum vortices in small (~1cm) droplets of superfluid liquid helium. Magnetic levitation, which is the same technique being explored for high speed trains and elevators that can move in all 3 dimensions, is used to float the droplet and eliminate the influence of the walls and the motion of the vortices is imaged using tracer molecules which fluoresce when illuminated with laser light. These studies will help us better understand the behavior of this strange form of quantum matter and will benefit studies of advanced materials such as superconductors and liquid crystals. This project will also train a new generation of scientists in technologically important fields such as low-temperature physics, fluid dynamics and advanced laser technologies. The research team plans to conduct demonstrations involving magnetic levitation and superfluid helium in various educational and outreach programs at the National High Magnetic Field Laboratory to introduce these scientific concepts to the general public.Technical AbstractThis project addresses intriguing fundamental problems relevant to the dynamics of quantized vortex lines. The objectives include a thorough investigation on how the appearance of vortices can affect the stability of a rotating superfluid drop and an in-depth study of the evolution of a vortex tangle in a wall-free environment. In superfluid helium-4, vortex lines can be directly visualized by imaging tracer particles trapped on the lines. However, producing tracers in helium at low temperatures and imaging the trapped tracers remains challenging, and the container walls can often affect the vortex-line motion. This project employs a levitated helium-4 drop as the working system, in which the vortices can be produced via fast evaporative cooling and controllable drop rotation. These vortices can be decorated with metastable He2 molecular tracers which can be imaged using a laser-induced fluorescence technique. The study of the vortex configuration in a rotating helium-4 drop helps explain the observed drop morphology and allows the derivation of the stability diagram of rotating superfluid drops. Since a levitated helium drop may serve as a model for other superfluid drop systems, such as neutron stars, the results obtained in this study may generate broad interest. Visualizing the evolution of a vortex tangle in a helium drop can facilitate the study of central questions in quantum turbulence research, such as the existence of polarized vortex bundles and the generation of Kelvin waves on vortices; these studies can directly advance our knowledge about the characteristics of quantum turbulence.