Quantum vortices in helium nanodroplets

Quantum vortices have been observed experimentally in various fields of physics, including superconductors, Bose-Einstein condensates, modern cosmology, superfluid helium in the bulk as well as in droplets. Quantum vortices are among the most dramatic features of superfluidity. The objective of the present proposal is the experimental investigation of quantized vortices in helium nanodroplets. Whereas in theoretical studies, quantum vortices have been predicted for droplets that contain as few as 300 He atoms, in experiments million times larger droplets are required to exhibit this macroscopic signature of superfluidity. However, not all droplets larger than these critical sizes exhibit signatures of vortices. The current understanding of the formation of quantized vortices is a stochastic process that is highly irreproducible and thus, every droplet may have a different size, angular momentum and subsequently different vorticity. Preparatory studies performed in a collaborating laboratory in Leicester indicate that collisions of helium droplets with surfaces at grazing incidence form quantum vortices in helium droplets. Such collisions introduce angular momentum into helium droplets in a well-defined way, and this strategy will be employed in the present project to explore quantum vortices in both neutral and charged droplets. Experiments are proposed to explore the angular momentum pickup upon surface impact of helium droplets under different surface conditions (material, impact angle and temperature) as well as the size of droplets. Utilizing an optical microscope, the angular distortion of rotating micrometer sized helium droplets will be investigated. Furthermore, it is planned to determine the location of charge centers in multiply charged helium droplets with and without quantized vortices. And finally we will explore the potential of rotating helium droplets to bread superconducting nanowires. Three PhD students will perform the proposed experiments adapting existing and constructing new setups in the initial phase of this project. The PhD students will be supported and guided by the applicant and experienced postdoctoral researchers working in the group in Innsbruck. Furthermore, international collaborations with research groups in the UK, France and Italy as well as a collaboration with the Department of Physical Chemistry of the University of Innsbruck will contribute to the success of this project.