Excitonic Polarons in Molecular Crystals

Band narrowing and band shifts due to excitonic-polaron formation are considered for an electronically polarizable medium. Experimental evidence of such effects in molecular crystals is cited. The central result is that the electron-exciton interaction in the strong-coupling regime leads not only to an indirect attractive electron-electron interaction, but also because of small-polaron formation leads to severe band narrowing. This band narrowing tends to favor the insulating state. The implications of the excitonic polaron for the metal-insulator transition, organic semiconductors, and the question of the possible existence of superconductivity in organic molecular crystals are discussed.