Crossover between integer and fractional vortex lattices in coherently coupled two-component Bose-Einstein condensates

We study effects of the internal coherent (Rabi) coupling in vortex lattices in two-component BECs under rotation. We find how the vortex lattices without the Rabi coupling known before are connected to the Abrikosov lattice of integer vortices with increasing the Rabi coupling. We find that 1) for small Rabi couplings, fractional vortices in triangular or square lattice for small or large inter-component coupling constitute hexamers or tetramers, namely multi-dimer bound states made of six or four vortices, respectively, 2) these bound states are broken into a set of dimers at intermediate Rabi couplings, and 3) vortices change their partners in various ways depending on the inter-component coupling to organize themselves for constituting the Abrikosov lattice of integer vortices at strong Rabi couplings.