Quench dynamics and Townes soliton formation in two-dimensional Bose gases

Predicting the evolution of many-body systems under attractive interactions is a challenging task, owing to the instability to collapse. Bright solitons are remarkable stationary states, established when the self-focusing effect responsible for collapse is exactly compensated by the wave dispersion. In two-dimensional (2D) Bose gases, however, such intricate balance cannot be fulfilled except at a critical norm known as the Townes threshold – only at which matter-wave bright solitons can form. By quenching the atomic interaction from repulsive to attractive via a Feshbach resonance \footnote{Cheng-An Chen and Chen-Lung Hung, \textbf{arXiv}:1907.12550 (2019).}, we report the observation of Townes solitons formation through the manifestation of modulational instability that results in the amplification of density wave disturbances and fragmentation of a 2D sample. Our high-resolution density measurements in space and time domain reveal detailed information about the formation process, and demonstrate multiple universal behaviors in association with the formation of a stationary state in an attractive many-body system.