In phase transition dynamics involving symmetry breaking, topological defects
can be spontaneously created but it is suppressed in a spatially inhomogeneous
system due to the spreading of the ordered phase information. We demonstrate
the defect suppression effect in a trapped atomic Bose gas which is quenched
into a superfluid phase. The spatial distribution of created defects is
measured for various quench times and it is shown that for slower quenches, the
spontaneous defect production is relatively more suppressed in the sample's
outer region with higher atomic density gradient. The power-law scaling of the
local defect density with the quench time is enhanced in the outer region,
which is consistent with the Kibble-Zurek mechanism including the causality
effect due to the spatial inhomogeneity of the system. This work opens an
avenue in the study of nonequilibrium phase transition dynamics using the
defect position information.Comment: 6 pages, 4 figure
