Self-trapped exciton defects in a charge density wave:: Electronic excitations of BaBiO3 -: art. no. 115113

In Paper I, it was shown that holes doped into BaBiO3 self-trap as small polarons and bipolarons. These point defects are energetically favorable partly because they undo locally the strain in the charge density wave (Peierls insulator) ground state. In this paper the neutral excitations of the same model are discussed. The lowest electronic excitation is predicted to be a self-trapped exciton, consisting of an electron and a hole located on adjacent Bi atoms. This excitation has been seen experimentally (but not identified as such! via the Urbach tail in optical absorption, and the multiphonon spectrum of the "breathing mode'' seen in Raman scattering. These two phenomena occur because of the Franck-Condon effect associated with oxygen displacement in the excited state.