Entropy production of active Brownian particles across the liquid, hexatic and solid phases

Due to its inherent intertwinement with irreversibility, entropy production is a prime observable to monitor in systems of active particles. With this premise, we numerically study entropy production across the liquid, hexatic and solid phases of a two-dimensional system of active Brownian particles at both average and fluctuation level. We find that the trend of the average entropy production as a function of density is not affected by any discontinuity, but rather its derivative marks the hexatic-solid transition density with a pronounced change. Concerning fluctuations, we instead find that distributions show peculiar tail structures, which we physically interpreted by looking at microscopic configurations. Qualitative and quantitative analysis revealed that particles in regions of low local order generate tail values according to different dynamical mechanisms: they move towards empty regions or bounce back and forth into close neighbours. Finally, we propose a simple single-particle model able to reproduce these peculiar structures by incorporating key dynamical aspects of the original one through an intermittent harmonic potential.