Breakdown of the lattice polaron picture in La0.7Ca0.3MnO3 single crystals

When heated through the magnetic transition at T-C, La0.7Ca0.3MnO3 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 polaroni c behavior. We use R-H and the Hall mobility to demonstrate the breakdown o f the polaron phase. Above 1.4T(C), the polaron picture holds in detail, wh ile below, the activation energies of both R-H and the mobility deviate str ongly from their polaronic values. These changes reflect the presence of me tallic, ferromagnetic fluctuations, in the volume of which the Hall effect develops additional contributions tied to quantal phases.