Extreme Spontaneous Deformations of Active Crystals

TL;DR

This paper reveals that active two-dimensional crystals can undergo extremely large spontaneous deformations while maintaining order, challenging equilibrium theories and highlighting the role of particle axis persistence.

Contribution

It demonstrates that active crystals can sustain large deformations with long-range order, exceeding equilibrium bounds, due to persistent particle axes, supported by elastic theory and effective temperatures.

Findings

Active crystals exhibit deformations beyond the 1/3 exponent limit.

Long-range bond order persists despite large deformations.

Effective temperatures quantify deformation and bond-order fluctuations.

Abstract

We demonstrate that two-dimensional crystals made of active particles can experience extremely large spontaneous deformations without melting. Using particles mostly interacting via pairwise repulsive forces, we show that such active crystals maintain long-range bond order and algebraically-decaying positional order, but with an exponent $\eta$ not limited by the $\tfrac{1}{3}$ bound given by the (equilibrium) KTHNY theory. We rationalize our findings using linear elastic theory and show the existence of two well-defined effective temperatures quantifying respectively large-scale deformations and bond-order fluctuations. The root of these phenomena lies in the sole time-persistence of the intrinsic axes of particles, and they should thus be observed in many different situations.