Figure 2

Disorder - order transition. (a) Scalar order parameter versus packing density plot for 400 × 400 system size at βε = 2.0. The equilibrium system (E, solid line) shows continuous isotropic to nematic state transition with increasing density. The active system goes from the isotropic (I) state to the locally ordered inhomogeneous mixed (IM, ×) state. In the high density regime, the system shows bistability between the IM state and the homogeneous globally ordered (HO, •) state. (b) The I to IM transition at low temperature occurs with a jump in S where the particles form bands (BS). Distribution of the scalar order parameter near the I - BS transition at (βε, C) = (2.0, 0.37) shows two peaks. (c) Finite size scaling of S for both the HO and the IM state at (βε, C) = (2.0, 0.76). (d) Order parameter time series show that the active system flips in between the HO and the IM state in the bistable regime. Two time series are shown for two different parameter values in the high density regime.