Spontaneous defect formation and causality effect in inhomogeneous Bose gases

Topological defects may be spontaneously created during a phase transition involving symmetry breaking. In a system with density inhomogeneity, the critical point is reached locally then the front of the transition spreads through the system with a finite velocity. The formation of defects can be suppressed if the spread of the transition front is slower than the spread of ordered phase information. We experimentally investigate this suppression of defect formation in an inhomogeneous, trapped Bose gas that is rapidly cooled into a superfluid phase. The results indicate that spontaneous defect production is more suppressed in the outer region of the sample where the atomic density gradient is higher. Furthermore, the power-law scaling of the local defect density with respect to the cooling time is enhanced in the outer region.