  We perform Brownian dynamics simulations in two dimensions to study the
collective behavior of circle swimmers, which are driven by both, an
(effective) translational and rotational self-propulsion, and interact via
steric repulsion. We find that active rotation generally opposes
motility-induced clustering and phase separation, as demonstrated by a
narrowing of the coexistence region upon increase of the propulsion angular
velocity. Moreover, although the particles are intrinsically assigned to rotate
counterclockwise, a novel state of clockwise vortices emerges at an optimal
value of the effective propulsion torque. We propose a simple gear-like model
to capture the underlying mechanism of the clockwise vortices.
