Domain walls and bubble droplets in immiscible binary Bose gases

The existence and stability of domain walls (DWs) and bubble-droplet (BD) states in binary mixtures ofquasi-one-dimensional ultracold Bose gases with inter- and intraspecies repulsive interactions is considered.Previously, DWs were studied by means of coupled systems of Gross-Pitaevskii equations (GPEs) with cubicterms, which model immiscible binary Bose-Einstein condensates (BECs). We address immiscible BECs withtwo- and three-body repulsive interactions, as well as binary Tonks–Girardeau (TG) gases, using systems ofGPEs with cubic and quintic nonlinearities for the binary BEC, and coupled nonlinear Schr¨odinger equationswith quintic terms for the TG gases. Exact DW solutions are found for the symmetric BEC mixture, with equalintraspecies scattering lengths. Stable asymmetric DWs in the BEC mixtures with dissimilar interactions in thetwo components, as well as of symmetric and asymmetric DWs in the binary TG gas, are found by means ofnumerical and approximate analytical methods. In the BEC system, DWs can be easily put in motion by phaseimprinting. Combining a DW and anti-DW on a ring, we construct BD states for both the BEC and TG models.These consist of a dark soliton in one component (the “bubble”), and a bright soliton (the “droplet”) in the other.In the BEC system, these composite states are mobile, too.