Superfluidity in Helium Nanodroplets

氦是一种在许多方面都很特殊的元素。与其他元素不同，氦可以在绝对零度（0 K，-273 °C，-460 °F）下保持液态。在这个范围内（大约2K及以下），氦展示了一些只能用量子物理学描述的独特性质。自然界中存在两种不同质量的氦原子，即所谓的同位素：He-3和He-4。虽然从外表上看，它们看起来很相似，但He-3和He-4在低温区的行为非常不同。低于2K时，He-4失去了所有的粘性，可以在液体内部自发地产生小的旋转龙卷风，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在低温区保持正常流体状态。本计画利用线性粒子加速器以x光拍摄液态氦奈米大小液滴的快照，以探讨其超流体性质。图像的超流He-4液滴含有量子涡旋进行了比较，那些正常的流体He-3液滴。此外，获得了含有两种类型氦的混合物的液滴的图像，以了解同位素如何相互作用。该项目为本科生和研究生提供了参与尖端科学的机会，并获得了加强美国STEM劳动力的技术技能。技术摘要氦滴长期以来一直被认为是探索独立、孤立超流体中涡度的理想系统。该小组最近已经证明了这种研究的可行性，使用单个亚微米大小的氦滴，通过使用自由电子激光的X射线散射进行询问。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量子涡旋和形状变形的阵列。该项目将量子液滴的x射线研究扩展到新的方向，例如胶囊状液滴的旋转，这涉及量子涡旋和毛细管波的协同作用。该项目还探测了由玻色子He-4和费米子He-3的混合物制成的旋转纳米液滴中的量子相分离。此外，液滴被用作实验容器来解决所谓的汤普森问题，找到一个球体上N个单位电荷的最小能量配置，这个问题在实验上仍然没有解决。最后，得到了旋转超流He-4和非超流He-3液滴的形状，并进行了比较。拟议的实验将提供基准信息的结构，稳定性和旋转动力学的自由旋转的超流体和非superfluid droplets.This奖项反映了NSF的法定使命，并已被认为是值得的支持，通过评估使用基金会的智力价值和更广泛的影响审查标准。

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.