Ryan Doran

Vortices in one- and two-component superfluid systems

Superfluids, such as those formed by ultra-cold atomic Bose-Einstein Condensates (BECs), have incredible properties, such as the ability to flow without viscous effects, and the fact that vorticity is quantized.

Although the problem of superfluid flow past a potential barrier is a well-studied problem in BECs, fewer studies have considered the case of superfluid flow through a disordered potential. We consider the case of a superfluid in a channel with multiple point-like barriers, randomly placed to form a disordered potential. We identify the relationship between the relative position of two point-like barriers, and the critical velocity for vortex nucleation of this arrangement, before considering a system with many obstacles. We then study how the flow of a superfluid in a point-like disordered potential is arrested through the nucleation of vortices and the breakdown of superfluidity. We then consider the vortex decay rate as the width of the barriers and show that vortex pinning becomes an important effect.

We then turn our attention to a two-component BEC in the immiscible limit. In such a system, if vortices are formed in a "majority" component, atoms in the "minority" component will fill the vortex cores, modifying the vortex profile. We show that a variational approach can be employed to approximate the vortex profile for a range of atom numbers in the in-filling component, and that these solutions are stable to small perturbations. We then consider the dynamics of these in-filled vortices.