Energy versus wave vector for the conduction band of La-doped SrTiO_3 interpreted by mixed-polaron theory

Abstract

An unusual published energy versus wave vector curve extending to the zone edge determined by ARPES in 5% La-doped SrTiO3 is interpreted using mixed-polaron theory. After modification of the theory to permit tight-binding bare energy bands and anisotropy, a fit is made to the 24 points nearest to the Fermi energy, adding constraints (a) that the Fermi volume is approximately equal to that determined from the carrier concentration, and (b) that the transport mass from experiments is approximately twice the bare mass. In order to fit the remaining five of 29 observed points we empirically add the effect of a kink. The fit to all points is good. From the parameters of the fit to the 24 points it appears that: (a) The energy overlap integral for the narrow band in the heavy direction has the opposite sign to that for the wide band, implying that it is energetically favourable for the nearly small polarons to be centred in between two sites; and (b) The narrower energy band in the theory may be nearly-small-polaron like in the heavy direction but large-polaron like in the other two directions. A simpler energy versus wave vector curve for a 1% La-doped crystal is discussed more briefly.