Rotational states of an asymmetric vortex pair with mass imbalance in binary condensates

Abstract

We consider massive vortices in binary condensates, where the immiscibility condition entails the trapping of the minority component in the vortex cores of the majority component. We study such vortices by means of a 2D point-like model, and show how the relevant dynamical equations exhibit vortex-pair solutions characterized by different vortex masses and circular orbits of different radii $a$ and $b$. These solutions are validated by the simulations of the Gross-Pitaevskii equations for binary condensates. After examining the properties of vortex-pair rotational frequency $Ω$ as a function of the vortex masses for a given pair geometry, we define the rotational-state diagram $\cal D$, describing all the possible vortex-pair solutions in terms of the orbit radii at given $Ω$. This includes solutions with equal-mass pairs but $a \ne b$ or with one of the two masses (or both) equal to zero. Also, we analytically find the minimum value of $Ω$ for the existence of such solutions, and obtain numerically the critical frequency $Ω_c$ below which $\cal D$ changes its structure and the transition to an unstable vortex-pair regime takes place. Our work highlights an indirect measurement scheme to infer the vortex masses from orbits radii $a$ and $b$, and a link between the vortex masses and the vortex-pair small-oscillation properties.