Coexistence ofpdσhybridization conduction and double-exchange conduction in heavily dopedLa1.85−2xSr0.15+2xCu1−xMnxO4

Abstract

Double-doped samples ${\mathrm{La}}_{1.85\ensuremath{-}2x}{\mathrm{Sr}}_{0.15+2x}{\mathrm{Cu}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}{\mathrm{O}}_{4}$ $(0.1l~xl~0.5)$ have been prepared. The possibility of Mn-O chain formation and its conduction mechanism in this system are discussed. With the increase of x, the room-temperature resistivity (\ensuremath{\rho}) is enhanced three orders, whereas the $\ensuremath{\rho}\ensuremath{-}T$ relation presents unlike forms in different ranges of x. In the scope of compositions $0.1l~xl~0.3,$ the resistivity displays the behavior of thermal activation undergoing Kondo scattering. At the doping level $0.32l~xl~0.5,$ it is surprising that the resistivity at 20 K drops three orders of magnitude compared to that of $x=0.3$ and the behavior of the susceptibility versus temperature curves is quite different in the two doping ranges. We suggest that double-exchange Mn-O-Mn chains have come into being besides Cu-O-Cu $\mathrm{pd}\ensuremath{\sigma}$ channel with doped Mn. In the range of $0.32l~xl~0.35,$ Mn-O-Mn forms clusters in the parent body. The conduction mechanism of Mn-O-Mn chains formed by clusters is variable-range hopping, while for $0.38l~xl~0.40$ the clusters are connected to Mn-O-Mn channels. The conduction mechanism of this channel is small-polaron conduction. The coexistence of two kinds of paths, the $\mathrm{pd}\ensuremath{\sigma}$ channel and Mn-O-Mn chains, dominates the conduction behavior.