Exciton versus Interband Absorption in Peierls Insulators

The effect of long-range Coulomb interaction between an electron and a hole on the optical absorption spectrum of a one-dimensional Peierls insulator is studied theoretically. Energy levels and oscillator strengths of exciton states in the Peierls gap and the line shape of the continuous spectrum (interband transitions) above the gap are calculated by taking the interatomic nature of the optical transition matrix element into account. The effect turns out to be of great significance, in the sense that even a very weak interaction induces an anomalously large oscillator strength of the lowest exciton ( n =0) state.