Nature https://doi.org/c9gv (2019)

Polarons are quasiparticles resulting from the coupling between a single impurity, usually an electronic charge carrier, and a surrounding bath of particles. The impurity repelling or attracting nearby particles modifies the background potential, which, in turn, affects the physical properties of the impurity. Although the presence of polarons has been inferred from macroscopic transport and spectroscopic measurements of various materials, their microscopic details, such as the internal structure, have never been confirmed experimentally.

This goal has now been achieved by Joannis Koepsell and co-workers who have reported a direct observation of magnetic polarons in a doped Fermi–Hubbard system realized by an ultracold-atom quantum simulator. The full single-site spin and density resolution on the lattice allowed the tracking of a local distortion of the magnetic correlations upon impurity doping, yielding a kind of real-space image of the polaron. The authors were able to derive the size of the polaron based on the range within which the impurity retains its impact on the environment.