Townes soliton and beyond: Non-miscible Bose mixtures in 2D

In these lecture notes, we discuss the physics of a two-dimensional binary mixture of Bose gases at zero temperature, close to the point where the two fluids tend to demix. We are interested in the case where one of the two fluids (the bath) fills the whole space, while the other one (the minority component) contains a finite number of atoms. We discuss under which condition the minority component can form a stable, localized wave packet, which we relate to the celebrated "Townes soliton". We discuss the formation of this soliton and the transition towards a droplet regime that occurs when the number of atoms in the minority component is increased. Our investigation is based on a macroscopic approach based on coupled Gross-Pitaevskii equations, and it is complemented by a microscopic analysis in terms of bath-mediated interactions between the particles of the minority component.
Comment: 24 pages, 8 figures. Proceedings of the International School of Physics "Enrico Fermi", Course 211 - Quantum Mixtures with Ultra-cold Atoms, July 2022, directors: Rudolf Grimm, Massimo Inguscio, Sandro Stringari