Mixed-valence manganites $\left({A^{3+}}_{0.7}{B^{2+}}_{0.3}\right)\mathrm{Mn}{\mathrm{O}}_3$ exhibit a resistivity peak with giant negative magnetoresistance below the Curie point. Residual resistivity ranges from 5×${10}^{-8\mathrm{}}$ to 8×${10}^3$ Ω m, but the electronic heat capacity when $A=\mathrm{Y}\mathrm{or}\mathrm{La}$ is that of a normal $d$-band metal; $\gamma \approx 6$ mJ ${\mathrm{mole}}^{-1\mathrm{}}$${\mathrm{K}}^{-2\mathrm{}}$. Spatial fluctuations in the Coulomb and spin-dependent potentials localize the $e_g\uparrow \left({\sigma }^{*}\right)$ 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.

• Received 6 July 1995

DOI:https://doi.org/10.1103/PhysRevLett.75.3910