POLARON AND BIPOLARON FORMATION IN A CUBIC PEROVSKITE LATTICE

The Rice-Sneddon model for BaBiO3 offers a starting point for discussi ng polarons and bipolarons in a three-dimensional oxide crystal. We us e exact diagonalization methods on finite samples to study the stabili ty and properties of polarons and bipolarons. Because polarons, when t hey form, turn out to be very well localized, we are able to converge accurately our calculations for two-electron bipolaron wave functions, accounting for the Coulomb interaction without approximation. Some of our results are compared with and interpreted by reference to the var iational method of Landau and Pekar. We calculate both electronic acid vibrational excitations of the small polaron solutions, finding a sin gle vibrational state localized with the full. symmetry of the polaron , which has its energy significantly decreased at the onset of polaron formation. Both on-site (Hubbard) and long-range Coulomb repulsion ar e included in the bipolaron calculation, but due to the high degree of localization, the long-range part has only a small influence. For a r easonable on-site repulsion U equal to two times the band width W, bip olaron formation is significantly suppressed; there is a large window of electron-phonon coupling where the polaron is stable but the bipola ron decays into two polarons.