  The self-trapping of exciton-polariton condensates is demonstrated and
explained by the formation of a new polaron-like state. Above the polariton
lasing threshold, local variation of the lattice temperature provides the
mechanism for an attractive interaction between polaritons. Due to this
attraction, the condensate collapses into a small bright spot. Its position and
momentum variances approach the Heisenberg quantum limit. The self-trapping
does not require either a resonant driving force or a presence of defects. The
trapped state is stabilized by the phonon-assisted stimulated scattering of
excitons into the polariton condensate. While the formation mechanism of the
observed self-trapped state is similar to the Landau-Pekar polaron model, this
state is populated by several thousands of quasiparticles, in a strike contrast
to the conventional single-particle polaron state.
