Urbach-Martienseen Rule and Exciton Trapped Momentarily by Lattice Vibrations

摘要: Theory for the Urbach-Martienssen (U-M) rule on low energy tail of fundamental absorption edge insulators is proposed. The exciton propagator solved an adiabatic lattice averaged vibrations at finite temperature, and self-energy obtained. It describes two characters in lattice; one mobile nature undeformed lattice, other localized trapped momentarily by deformation due to thermal vibrations. interplay natures results U-M below peak. result can be interpreted terms Franck-Condon principle where incorporated giant oscillator strength exciton. Emission from this state discussed connection with rule.