Electronic Polaron in Strongly-correlated Transition Metal Halides

Formation of polarons has been customarily attributed to electron-phonon coupling in solids. Herein, we present a theory of electronic polarons, in which spontaneous carrier localization occurs solely because of electron-electron interaction. Through a combination of first-principles calculation and scanning tunneling spectroscopy we demonstrate the existence of electronic polarons in strongly correlated two-dimensional transition metal halides, such a CrI2. While lattice relaxation occurs at the polaron, we stress that this is a secondary effect in response to spontaneous carrier localization. This alters our fundamental understanding of small polarons and indicates that electron-phonon coupling may not be a prerequisite for their formation.