Superfluidity in Helium Nanodroplets

该项目重点研究由氦制成的纳米液滴的性质，氦在温度降至绝对零度（-460°F）时仍保持液态。在足够低的温度下，液滴没有任何粘度，在这一点上它们被称为超流体。超流体是一种量子现象，自20世纪初被发现以来，它一直是一个非常富有成果的研究领域。有关超流体的理论预测，旋转的超流体液滴包含一组漩涡，但这些漩涡在很长一段时间内没有被观测到。最近，首席研究员和他的学生开发了一种新的技术，通过对液滴进行x射线辐射来成像液滴中的涡流，并证明了液滴可以在密集阵列中维持许多涡流。该技术的进一步发展及其在不同种类液滴上的应用将为研究超流体液滴的性质提供新的信息，这对有限量子系统的物理学具有重要的基础意义。此外，对高密度涡旋的研究将有助于更深入地理解普通流体的物理特性和极端湍流下涡旋的影响，这适用于飞机和船舶的高速运动。此外，该项目还为本科生和研究生提供了参与尖端科学和后续出版物的机会，从而加强了美国的STEM劳动力。技术摘要本项目主要研究氦纳米液滴中观尺度的量子流体力学。液滴是由加压氦在真空中在t10 K下膨胀产生的，并通过直线相干光源自由电子激光器（XFEL）的x射线相干衍射成像进行了研究。本工作揭示了不同形状和角动量的4He液滴中旋涡的三维排列以及嵌入质量对旋涡运动学的影响。此外，本工作正在将XFEL实验扩展到3He和混合3He- 4he液滴，目的是研究旋转混合液滴中的相分离，包括可能观察到非平衡超流体相，以及整洁的非超流体3He液滴中的旋转。所提出的实验将为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提供基准信息。与这些实验一起开发的液滴相干衍射成像算法将能够提供从衍射模式实时图像重建的第一个实例。

Nontechnical AbstractThis project focuses on the properties of nanoscopic droplets made from helium, which remains liquid at temperatures down to absolute zero (-460 °F). At low enough temperatures, the droplets lack any viscosity, at which point they are said to be superfluid. Superfluidity is a quantum phenomenon, and it has been a very fruitful field of study since its discovery in the early 1900's. Theories pertaining to superfluids predicted that a rotating superfluid droplet contains a collection of vortices, but such vortices evaded observation for a long time. Recently, the principal investigator and his students have developed a novel technique for imaging vortices in the droplets, by subjecting the droplets to x-ray radiation, and proved that droplets can sustain many vortices in dense arrays. Further development of this technique and its applications to different kinds of droplets will provide new information on the properties of superfluid droplets, which are of fundamental importance for the physics of finite quantum systems. Moreover, studying vortices at high density will contribute to a deeper understanding of the physics of ordinary fluids and the effect of vortices at extreme turbulence, which is applicable to the motion of planes and ships at high velocity. Additionally, this project provides opportunities for undergraduate and graduate students to take part in cutting-edge science and ensuing publications, which strengthens the US STEM workforce.Technical AbstractThis project focuses on the study of quantum hydrodynamics at the meso-scale in helium nanodroplets. The droplets are produced upon expansion of pressurized helium in vacuum at T 10 K and are interrogated via x-ray coherent diffraction imaging using the Linac Coherent Light Source free electron laser (XFEL). This work uncovers the three-dimensional arrangement of vortices in the 4He droplets of different shape and angular momentum and the effect of the embedded mass on vortex kinematics. In addition, the work is expanding the XFEL experiments to 3He and mixed 3He-4He droplets with the objective of studying phase separation in rotating mixed droplets, including possible observation of the non-equilibrium superfluid phase, and rotation in the neat non-superfluid 3He droplet. The proposed experiments will provide benchmark information on the structure, stability and rotational dynamics of free rotating super- and non-superfluid droplets. The droplet coherent diffraction imaging algorithm developed alongside these experiments will be able to provide the first instance of real-time image reconstructions from diffraction patterns.