Stable regions and singular trajectories in chaotic soft wall billiards
  We present numerical and experimental results for the development of islands of stability in atom-optics billiards with soft walls. As the walls are soften, stable regions appear near singular periodic trajectories in converging (focusing) and dispersing billiards, and are surrounded by areas of "stickiness" in phase-space. The size of these islands depends on the softness of the potential in a very sensitive way. 
Ariel Kaplan, Nir Friedman, Mikkel Andersen, and Nir Davidson