  We consider the two-species Vicsek model (TSVM) consisting of two kinds of
self-propelled particles, A and B, that tend to align with particles from the
same species and to antialign with the other. The model shows a flocking
transition that is reminiscent of the original Vicsek model: it has a
liquid-gas phase transition and displays micro-phase-separation in the
coexistence region where multiple dense liquid bands propagate in a gaseous
background. The interesting features of the TSVM are the existence of two kinds
of bands, one composed of mainly A particles and one mainly of B particles, the
appearance of two dynamical states in the coexistence region: the PF (parallel
flocking) state in which all bands of the two species propagate in the same
direction, and the APF (antiparallel flocking) state in which the bands of
species A and species B move in opposite directions. When PF and APF states
exist in the low-density part of the coexistence region they perform stochastic
transitions from one to the other. The system size dependence of the transition
frequency and dwell times show a pronounced crossover that is determined by the
ratio of the band width and the longitudinal system size. Our work paves the
way for studying multispecies flocking models with heterogeneous alignment
interactions.
