Thermopower-conductivity relation for distinguishing transport mechanisms: Polaron hopping in CeO_2 and band conduction in SrTiO_3

The charge transport mechanism in a solid is often inferred by observing very simple features like the temperature dependency of electrical conductivity or resistivity. However, comparing complicated physical models to such simple signatures leaves much ambiguity. Because models generally have more parameters than the types of measurements available, inconsistencies can long go unrecognized until the interrelation between different measurements is closely examined. We show that a simple investigation of the thermopower-conductivity relation allows one to phenomenologically characterize transport from experiments; the phenomenologically determined transport function can be compared to physical models to distinguish transport mechanisms and straightforwardly point out inconsistencies in literature models. We highlight two example cases, ceria and strontium titanate, to show that our analysis method can clarify whether the transport mechanism is through hopping or delocalized states. We question previous suggestions about the scattering mechanism in SrTiO_3 and suggest deformation potential scattering on elongated Fermi surfaces as the origin of high-temperature T^2 resistivity.