Electrical transport properties and small polarons in (formula presented)

Temperature- and field-dependent resistivity (formula presented) and Hall effect measurements have been carried out for the (formula presented) (formula presented) 0.4, 0.6, and 0.9) compounds. The replacement of Eu with Ca invoked drastic changes in the (formula presented) although Ca and Eu are isoelectronic. While (formula presented) showed a phase transition similar to that of pure (formula presented) the (formula presented) of (formula presented) with (formula presented) and 0.6 showed a rapid increase at low temperatures (formula presented) The upturn of the (formula presented) was suppressed as the magnetic field increased, resulting in negative magnetoresistance (MR). For (formula presented) the MR changed from positive at (formula presented) to negative at (formula presented) It was found that the observed exotic (formula presented) is due to the change of the effective carrier density (formula presented) and Hall mobility (formula presented) determined from the Hall measurements. Analysis of the Hall mobility based on a small polaronic model showed that the carrier transport is dominated by hopping between the polaron sites in a nonadiabatic regime with four-site hopping for the Eu-rich side and three-site hopping for the Eu-poor side of the compounds. This polaronic scenario of the transport is consistent with the observed (formula presented) (formula presented) (formula presented) and (formula presented) variation.