Vortex Lattice in Rotating Prolate ⁴He Droplets

So far, lattices of quantum vortices have been observed in axially symmetric, rotating superfluid ⁴He or rotating Bose-Einstein condensates (BECs). Vortex configurations in rotating superfluids lacking axial symmetry have been discussed theoretically, but experimental observation of such systems proves to be challenging. Here, we present a study of vortices in microscopic free, prolate superfluid ⁴He droplets rotating around their short axis. The vortices were doped with Xe atoms and studied via coherent x-ray scattering at the LCLS free-electron laser. It was found that the vortices form a distorted lattice within the droplet. We compare the shapes of classical droplets executing rigid body rotation to rotating prolate superfluid droplets, the angular momentum of which has contributions from quantum vortices and potential flow.