Kibble-Zurek mechanism in polariton graphene.

We study the formation of topological defects (quantum vortices) in a 2D polariton condensate at the $\Gamma$ point of a honeycomb lattice via the Kibble-Zurek mechanism. We demonstrate that the vortices can be pinned to the lattice, which prevents their recombination and allows their observation and counting in cw experiments. A scaling exponent of $0.95\pm0.05$ is observed, in agreement with the mean-field predictions for a 2D system with lattice-induced linear dispersion.