Superfluidity in Helium Nanodroplets

非技术摘要氦是一种在许多方面都很特殊的元素。与任何其他元素不同，氦可以保持液态，直到绝对零度(0K，-273°C，-460°F)。在这个区域(大约2K及以下)，氦表现出一些只能用量子物理描述的独特性质。自然界中存在两种氦原子，即不同质量的氦-3和氦-4。虽然表面上看起来很相似，但He-3和He-4在低温条件下的表现截然不同。在2K以下，He-4失去了所有的粘度，可以自发地在液体中产生微小的旋转龙卷风，称为量子涡旋。在这种状态下，氦-4被称为超流体。然而，由于He-3和He-4之间的量子力学差异，He-3在低温区域保持正常的流体状态。该项目通过使用线性粒子加速器用X射线拍摄纳米液氦液滴的快照，来研究它们的物理超流体性质。将含有量子涡旋的超流体He-4液滴的图像与正常流体He-3液滴的图像进行了比较。此外，还获得了含有两种氦混合物的液滴图像，以了解同位素是如何相互作用的。该项目为本科生和研究生提供了参与尖端科学的机会，并获得了增强美国STEM工作能力的技术技能。技术摘要长期以来，氦液滴一直被认为是探索独立、孤立超流体中涡度的理想系统。该小组最近证明了使用单个亚微米大小的氦液滴进行此类研究的可行性，这些液滴是通过使用自由电子激光进行X射线散射而获得的。液滴在离开喷嘴后经历旋转，根据液滴的角动量的大小表现出一系列量子涡旋和形状变形。该项目将量子液滴的X射线研究扩展到新的方向，如胶囊状液滴的旋转，这涉及到量子涡旋和毛细管波的协同作用。该项目还探索了由玻色子He-4和费米子He-3的混合物组成的旋转纳米液滴中的量子相分离。此外，这些液滴被用作实验容器来解决所谓的汤普森问题，找到一个球上N个单位电荷的最小能量配置，这在实验上仍然没有解决。最后，得到了旋转超流体He-4液滴和非超流体He-3液滴的形状，并进行了比较。拟议的实验将提供关于自由旋转的超流体和非超流体液滴的结构、稳定性和旋转动力学的基准信息。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Non-Technical AbstractHelium is an element that is exceptional in many respects. Unlike any other elements, helium can remain liquid down to absolute zero temperature (0 K, -273 °C, -460 °F). In this regime (roughly 2 K and below), helium demonstrates some unique properties that can only be described by quantum physics. Two kinds of helium atoms exist in nature, so called isotopes with different mass: He-3 and He-4. While on the outside they seem similar, He-3 and He-4 behave very differently in the low temperature regime. Below 2K, He-4 loses all viscosity and can spontaneously create small, rotating tornados inside the liquid called quantum vortices. In this state, helium-4 is known as a superfluid. He-3, however, remains in the normal fluid state in the low temperature regime due to a quantum mechanical difference between He-3 and He-4. This project investigates physical superfluid properties in nano-sized droplets of liquid helium by taking snapshots of them with x-rays using a linear particle accelerator. Images of superfluid He-4 droplets containing quantum vortices are compared to those of normal fluid He-3 droplets. Additionally, images of droplets containing a mixture of both types of helium are obtained to see how the isotopes interact. This project provides opportunities for undergraduate and graduate students to take part in cutting-edge science and acquire technical skills which strengthen the US STEM workforce.Technical AbstractHelium droplets have long been considered ideal systems to explore vorticity in self-contained, isolated superfluids. This group has recently shown the feasibility of such studies using single, sub-micrometer sized helium droplets interrogated via x-ray scattering using a free electron laser. Droplets undergo rotation after exiting the nozzle, manifesting an array of quantum vortices and shape deformations depending on the magnitude of the droplet’s angular momentum. This project expands the x-ray study of quantum droplets to new directions such as the rotation of capsule-shaped droplets, which involves a concerted action of quantum vortices and capillary waves. The project also probes quantum phase separation in rotating nanodroplets made of mixtures of the bosonic He-4 and the fermionic He-3. Additionally, the droplets are used as an experimental vessel to tackle the so-called Thompson problem, finding the minimum energy configuration of N unit charges on a sphere, which remains unsolved experimentally. Finally, the shapes of rotating superfluid He-4 and non-superfluid He-3 droplets are obtained and compared. The proposed experiments will provide benchmark information on the structure, stability, and rotational dynamics of free rotating super- and non-superfluid droplets.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.