Polaron formation by electron polarization in two-dimensional transition metal halides

An electron in solid can be dressed by the lattice distortions of surroundings, forming a localized composite quasiparticle called small polaron, whose formation has been customarily attributed to the electron-phonon couplings that the ion polarization traps the excess electron. Here we present a theory of electron-polarization induced small polaron, in which the carrier localization happens spontaneously and drives subsequent ion relaxation. This mechanism of polaron formation is qualitatively different than the Mott-Stoneham picture in that there is no need to overcome a kinetic barrier for the carrier to self-trap to form a polaron.Through a combination of first-principles theory and model Hamiltonian, we show that this is the mechanism for polaron formation in the monolayer two-dimensional transition metal halides, CrI₂, CoCl₂and CoBr₂. These findings may explain the exceptional stability and manipulability of polarons in this class of materials by scanning tunneling microscopy.