In multiband superconductors, each superconducting condensate supportsvortices with fractional quantum flux. In the ground state, vortices indifferent bands are spatially bounded together to form a composite vortex,carrying one quantum flux \Phi_0. Here we predict dissociation of the compositevortices lattice in the flux flow state due to the disparity of the vortexviscosity and flux of the vortex in different bands. For a small drivingcurrent, composite vortices start to deform, but the constituting vortices indifferent bands move with the same velocity. For a large current, compositevortices dissociate and vortices in different bands move with differentvelocities. The dissociation transition shows up as an increase of flux flowresistivity. In the dissociated phase, Shapiro steps are developed when an accurrent is superimposed with a dc current.
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