Dynamics of shock waves in a superfluid unitary Fermi gas
We study the formation and dynamics of shock waves initiated by a repulsive potential in a superfluid unitary Fermi gas by using the order-parameter equation. In the theoretical framework, the regularization process of shock waves mediated by the quantum pressure term is purely dispersive. Our results show good agreement with the experiment of Joseph {\it et al}. [Phys. Rev. Lett. {\bf 106}, 150401 (2011)]. We reveal that the boxlike-shaped density peak observed in the experiment consists of many vortex rings due to the transverse instability of the dispersive shock wave. In addition, we study the transition from a sound wave to subsonic shock waves by increasing the strength of the repulsive potential and show a strong qualitative change in the propagation speed of the wavefronts. In the relatively small strength regime, the speed decreases below the sound speed with increasing the strength as a scaling behavior, while in the large regime the speed remains almost unchanged, which is found to be the same expansion speed of the proliferation of the vortex rings.