# The role of topological defects in the two-stage melting and elastic behavior of active Brownian particles

## TL;DR

This study investigates how topological defects influence the melting process and elastic properties of active Brownian particles, revealing deviations from classical theories and showing that activity shifts the solid regime without altering elastic constants.

## Contribution

It demonstrates that active particle melting involves defect dynamics different from KTHNY theory and shows elastic constants are unaffected by activity, only shifting the solid phase.

## Key findings

- Crystalline states melt into hexatic states without dislocation unbinding.
- Elastic constants of active solids match those of passive solids.
- Activity shifts the stable solid regime to higher densities.

## Abstract

We find that crystalline states of repulsive active Brownian particles at high activity melt into a hexatic state but this transition is not driven by an unbinding of bound dislocation pairs as suggested by the Kosterlitz-Thouless-Halperin-Nelson-Young (KTHNY) theory. Upon reducing the density, the crystalline state melts into a high-density hexatic state devoid of any defects. Decreasing the density further, the dislocations proliferate and introduce plasticity in the system, nevertheless maintaining the hexatic state, but eventually melting into a fluid state. Remarkably, the elastic constants of active solids are equal to those of their passive counterparts, as the swim contribution to the stress tensor is negligible in the solid state. The sole effect of activity is that the stable solid regime shifts to higher densities. Furthermore, discontinuities in the elastic constants as a function of density correspond to changes in the defect concentrations rather than to the solid-hexatic transition.