Shell-shaped BEC with mixtures*

Currently there is a huge interest in the properties of hollow Bose-Einstein condensates (BECs) [Phys. Rev. A 98, 013609 (2018)], which are commonly realized with radio-frequency (rf) dressing potentials. As an alternative method we propose to use a dual-species mixture for the preparation of such hollow BECs. By tuning the interspecies interaction strength and the number of atoms we can realize a ground state where one atomic species is centered in the middle, while the other species forms a shell around it. In order to obtain the width and the form of the shell as well as the spectrum of the collective excitations, from the filled to a hollow BEC, we solve the Gross-Pitaevskii equation both numerically and within the Thomas-Fermi approach. For the hollow case the inter-species boundary leads to a change to the collective excitation spectrum. This new method could be more robust for the creation of bubble BECs than the method of rf dressing potentials.