Optical investigations of the (dis)continuous metal-insulator transitions in strongly-coupled electron-lattice systems.

Optical properties of four model systems (Na-, K-, Rb- or Cs-doped quasi-two-dimensional X AlSiO; X  =  Na, K, Rb, or Cs) used to study the metal-insulator transition (MIT) in a deformable lattice are investigated. The doping evolution of the optical absorption band(s) originating from small bipolarons show strong variations depending on the electron-lattice coupling strength [Formula: see text]. Despite the increasing number density of small (bi)polarons, the Na-system remains a stubborn (bi)polaronic insulator due to strong [Formula: see text], while the other three systems show closing of the respective mobility gaps giving way to conducting phases with differing properties.