Phase-dependent kink collisions and dual critical-velocity branches in the complex sine-Gordon model
The complex sine-Gordon (CSG) model contains an internal phase degree of freedom that strongly modifies the dynamics of its solitary-wave solutions. We present a numerical study of complex kink--kink collisions and determine how the final state depends jointly on the initial velocity and relative phase. In contrast with the elastic collisions of the real sine-Gordon model, the CSG system exhibits scattering, capture, long-lived bion formation, breather-like states, and emission of radiative profiles. The simulations reveal two distinct phase-dependent branches of critical velocity. In one branch, increasing the initial velocity promotes capture, whereas in the other it restores scattering. This dual structure highlights the rich velocity--phase dependence of the collision dynamics. We also compute the energy carried by radiative profiles and examine extreme values of the energy density, kinetic and gradient contributions, and field modulus at the collision center. These quantities show sharp transitions at critical points and provide sensitive diagnostics of phase-controlled dynamics. These results suggest that the relative phase behaves as an effective internal degree of freedom that plays an important role in the collision dynamics of complex solitons.