Evolution of spin dynamics across the discontinuous metal-insulator transition in a deformable lattice

We performed electron spin-resonance (ESR), magnetic, and optical absorption studies of a model system used to investigate the metal-insulator transition in a deformable lattice. The ESR and magnetic studies revealed that the small polarons have no correlation to the conductivity below or above the mobility edge. The conducting transition is marked by the appearance of an asymmetric ESR signal simultaneously with a significant suppression of the small bipolaron optical absorption band indicating that the extended states result from an abrupt dissociation of the small bipolarons. Immediately above the mobility edge competing phases of extended states, small polarons, and small bipolarons coexist. We propose a phase diagram for the charge and spin dynamics of an interacting gas of small (bi)polarons across the discontinuous conducting transition that has implications for understanding the long-standing conundrum of Mooij correlations.