Electrical transport in the ferromagnetic state of manganites: Small-polaron metallic conduction at low temperatures

We report measurements of the resistivity in the ferromagnetic state of epi taxial thin films of La1-xCaxMnO3 and the low-temperature specific heat of a polycrystalline La0.8Ca0.2MnO3. The resistivity below 100 K can be well f itted by rho - rho(0) = E omega(s)/sinh(2)(h omega(s)/2k(B)T) with h omega( s)/k(B) similar or equal to 80 K and E being a constant. Such behavior is c onsistent with small-polaron coherent motion which involves a relaxation du e to a soft optical phonon mode that is strongly coupled to the carriers. T he specific-heat data also suggest the existence of such a phonon mode. The present results thus provide evidence for small-polaron metallic conductio n in the ferromagnetic state of manganites.