Theory of bipolarons and bipolaronic bands

It is shown that in narrow-band crystals with sufficiently strong electron-lattice interaction a new energy band occurs. The tunneling motion of localized electron pairs (bipolarons), which is responsible for this band, is caused by virtual transitions of bipolarons to the polaron state. The electronic excitation spectrum of a bipolaronic crystal is examined. It is shown that in the low bipolaron-density limit the excitation spectrum is superfluidlike, so that bipolarons might be superconducting. In case of high density of bipolarons and their strong repulsion, a charge-density wave is predicted.