Excited states of two-dimensional solitons supported by spin-orbit coupling and field-induced dipole-dipole repulsion

Abstract

It was recently found that excited states of semivortex and mixed-mode solitons are unstable in spin-orbit-coupled Bose-Einstein condensates (BECs) with contact interactions. We demonstrate a possibility to stabilize such excited states in a setting based on repulsive dipole-dipole interactions induced by a polarizing field, oriented perpendicular to the plane in which the dipolar BEC is trapped. The strength of the field is assumed to grow in the radial direction $\ensuremath{\sim}{r}^{4}$. Excited states of semivortex solitons have vorticities $S$ and $S+1$ in their two components, each being an eigenstate of the angular momentum. They are fully stable up to $S=5$. The excited state of mixed-mode solitons feature interweaving necklace structures with opposite fractional values of the angular momentum in the two components. They are stable if they are built of dominant angular harmonics $\ifmmode\pm\else\textpm\fi{}S$, with $S\ensuremath{\le}4$. The characteristics and stability of these two types of previously unknown higher-order solitons are systematically analyzed. Their characteristic size is $\ensuremath{\sim}10\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{m}$, with the number of atoms $\ensuremath{\lesssim}{10}^{5}$.