Breakdown of the lattice polaron picture inLa0.7Ca0.3MnO3single crystals

When heated through the magnetic transition at $T_C,$ ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ changes from a band metal to a polaronic insulator. The Hall constant $R_H,$ through its activated behavior and sign anomaly, provides key evidence for polaronic behavior. We use $R_H$ and the Hall mobility to demonstrate the breakdown of the polaron phase. Above ${1.4T}_C,$ the polaron picture holds in detail, while below, the activation energies of both $R_H$ and the mobility deviate strongly from their polaronic values. These changes reflect the presence of metallic, ferromagnetic fluctuations, in the volume of which the Hall effect develops additional contributions tied to quantal phases.