The extension of the concept of topological insulators to systems, where new mechanisms may provide topological protection is very important due to a number of potential practical application of such materials. Among such systems are polaritonic systems that are especially attractive because they allow to study nonlinear topological effects.

In our Letter we show that linear and nonlinear topological edge states can be observed in an important new setting: Lieb lattices of microcavity pillars. Spin-orbit coupling of cavity photons and Zeeman splitting in the magnetic fields renders such polariton states unidirectional. We found that nonlinear edge states in Lieb insulators are exceptionally robust that allowed us to obtain topological dark edge solitons travelling along the surface of the lattice over long time intervals almost without radiation.

We believe that our results will be interesting for broad audience of Physical Review Letters, since results obtained in the Letter are relevant for a number of other areas of physics beyond polaritons.