The cross-over from Townes solitons to droplets in a 2D Bose mixture

TL;DR

This paper investigates the transition from Townes solitons to quantum droplets in a two-dimensional Bose mixture, revealing conditions for localized states and their dynamics in the non-miscible regime.

Contribution

It introduces a theoretical framework describing the crossover from Townes solitons to droplets in 2D Bose mixtures, identifying thresholds and effective descriptions for localized states.

Findings

Localized wave packets exist above a critical atom number.

Effective single-field equations describe the system up to the droplet regime.

Specific parameter ranges support localized excitation modes.

Abstract

When two Bose-Einstein condensates -- labelled 1 and 2 -- overlap spatially, the equilibrium state of the system depends on the miscibility criterion for the two fluids. Here, we theoretically focus on the non-miscible regime in two spatial dimensions and explore the properties of the localized wave packet formed by the minority component 2 when immersed in an infinite bath formed by component 1. We address the zero-temperature regime and describe the two-fluid system by coupled classical field equations. We show that such a wave packet exists only for an atom number $N_2$ above a threshold value corresponding to the Townes soliton state. We identify the regimes where this localized state can be described by an effective single-field equation up to the droplet case, where component 2 behaves like an incompressible fluid. We study the near-equilibrium dynamics of the coupled fluids, which reveals specific parameter ranges for the existence of localized excitation modes.