Superfluidity in Helium Nanodroplets

University Of Southern California, Los Angeles CA

Investigators

Abstract

Non-Technical Abstract Helium 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 Abstract Helium 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.