Negative Poisson ratio in a two-dimensional ‘‘isotropic’’ solid

A two-dimensional system of hard cyclic hexamers is shown to display an anomalous (negative) Poisson ratio in a high-density crystalline phase. The anomaly can be reproduced qualitatively by a simple free-volume (FV) approximation. The FV approximation and a lattice model indicate the crucial role of broken mirror symmetry for this phenomenon.