Spin and charge dynamics of an interacting gas of small (bi)polarons across the mobility edge

We performed electron spin resonance (ESR) and optical studies on a model system that is used to investigate the discontinuous metal-insulator transition in a deformable lattice. The ESR signal below the mobility edge has no correlation to the conductivity and arises purely from a minority of magnetic polarons. Above the percolation threshold of the system and below the mobility edge, the intensity of the ESR signal shows a decrease suggesting that small polarons form paired states, i.e., small bipolarons, with increasing interactions. The conducting transition is marked by the appearance of an asymmetric ESR line shape simultaneously with a decrease in the small bipolaron optical absorption band. The delocalized states result from a partial dissociation of the small bipolarons. Immediately above the mobility edge, competing phases of delocalized (extended) states, small polarons, and small bipolarons coexist. Based on the experimental results, we propose a phase diagram that adds to the theoretical studies and highlight that the polaron density and local interactions are important parameters that determine the balance among competing states in a many-polaron system.