Topological interface physics in spinor Bose-Einstein condensates

We present an experimentally viable scheme whereby the physics of coherent interfaces between topologically distinct regions can be studied in an atomic quantum gas~[1]. The interface engineering is achieved using the internal spin structures of atoms together with local control over interaction strengths. We consider a coherent interface between polar and ferromagnetic regions of a spin-1 Bose-Einstein condensate and show that defects representing different topologies can connect continuously across the boundary~[1,2]. We show that energy minimization leads to nontrivial interface-crossing defect structures, demonstrating how the method can be used to study stability properties of field-theoretical solitons. We demonstrate, e.g., the formation of a half-quantum vortex arch, an \emph{Alice arch,} on the interface, exhibiting the topological charge of a point defect. We also demonstrate an energetically stable connection of a coreless vortex to two half-quantum vortices. Our method can be extended to study interface physics in spin-2 and spin-3 BECs with richer phenomenology, or in strongly correlated optical-lattice systems.\\[4pt] [1] M.~O.~Borgh and J.~Ruostekoski, Phys.\ Rev.\ Lett.\ \textbf{109,} 015302 (2012)\\[0pt] [2] M.~O.~Borgh and J.~Ruostekoski, arXiv:1212.2147 (2012)