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 _1.95Al_1.95Si_2.05O_8.00; 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 . Despite the increasing number density of small (bi)polarons, the Na-system remains a stubborn (bi)polaronic insulator due to strong , while the other three systems show closing of the respective mobility gaps giving way to conducting phases with differing properties. These interesting evolutions and dynamical properties are compared and discussed. We conjecture that the manifestation of anomalous electronic transport properties and Mooij correlations near the MIT or superconductor-insulator transition in systems with non-negligible electron-lattice coupling effects may be linked to the coexistence of competing polaronic phases and the dynamical intertwining of the deformable lattice and the random electronic potential. (paper)