Shell-shaped Bose-Einstein condensates realized with dual-species mixtures

Abstract

Confining Bose-Einstein condensates (BECs) in shell-shaped trapping potentials enables the generation of hollow quasi-2D topologies with superfluid properties. Motivated by recent microgravity experiments, these shell-shaped BECs are nowadays actively studied both theoretically and experimentally, with radio-frequency (rf) dressing being the main trapping mechanism under study. Here we present an alternative approach that utilizes the repulsive interaction in a dual-species mixture to achieve shell-shaped BECs, which could be realized in the future BECCAL mission. In contrast to the rf case, which relies on a dynamical transition from a filled to a hollow condensate, the mixture approach is based on realizing the shell structure as the ground state of the system, where one species is located at the center of the trap surrounded by the other and kept in place by the repulsive inter-species interaction. We compare both approaches by analyzing the initial states, the free expansion dynamics, and the collective excitation spectrum with analytical and numerical methods. In all three categories the mixture performs similar to the rf approach. Moreover, the interaction-driven expansion of the mixture allows to increase the size of the shell during time-of-flight without distorting its shape and therefore magnifying the dynamics within the shell; a mechanism not realizable in the rf case. We conclude by performing a feasibility analysis for both approaches that takes residual gravitational effects and possible trap asymmetries into account, which currently are the main obstacles to experimentally realize shell-shaped BECs.