Collective excitation of a shell-shaped Bose-Einstein condensate

We investigate the hollowing transition of a shell-shaped Bose-Einstein condensate (BEC) with collective excitation. The shell is created using a double species BEC in the immiscible regime, with the hollowness of the shell BEC controlled by tuning the repulsive interspecies interaction by a Feshbach resonance. Our study reveals two distinct monopole modes in which the two condensates oscillate in-phase and out-of-phase relative to each other, respectively. While the frequency of the in-phase mode remains largely constant, the frequency of the out-of-phase mode changes significantly, providing a clear signature of the topology change from a filled to a hollow condensate. Furthermore, we observe a strong dependence of the critical point of the hollowing transition on the number ratio of the two species. Our findings offer a comprehensive understanding of the topology change in this curved quantum gas system and pave the way for future research into quantum many-body phenomena in curved spaces.