Electron Localization in Mixed-Valence Manganites

Mixed-valence manganites $({{A}^{3+}}_{0.7}{{B}^{2+}}_{0.3})\mathrm{Mn}{\mathrm{O}}_{3}$ exhibit a resistivity peak with giant negative magnetoresistance below the Curie point. Residual resistivity ranges from 5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}8}$ to 8\ifmmode\times\else\texttimes\fi{}${10}^{3}$ \ensuremath{\Omega} m, but the electronic heat capacity when $A=\mathrm{Y}\mathrm{or}\mathrm{La}$ is that of a normal $d$-band metal; $\ensuremath{\gamma}\ensuremath{\approx}6$ mJ ${\mathrm{mole}}^{\ensuremath{-}1}$${\mathrm{K}}^{\ensuremath{-}2}$. Spatial fluctuations in the Coulomb and spin-dependent potentials localize the ${e}_{g}\ensuremath{\uparrow}({\ensuremath{\sigma}}^{*})$ electrons in wave packets that are large on the scale of the Mn-Mn separation. Transport in the magnetically localized state involves zero-point-energy assisted hopping or tunneling of electrons from one weakly localized wave packet to another.