Thermopower of single-crystal Nd-1-x(Sr,Pb)(x)MnO3-delta

We report measurements of the thermopower of the single-crystal manganite perovskite Nd-1-x(Sr,Pb)MnO3-delta from 4.2 to 350 K at zero and finite magnetic field. The sign and magnitude of the thermopower above the magnetic transition are doping dependent. The magnitude of the thermopower drops sharply below the transition to the magnetically ordered state. At lower temperatures the thermopower changes sign from negative to positive with decreasing temperature in the range 100-150 K. Implications for the temperature-dependent electronic structure and scattering processes in this temperature range are discussed. The high-temperature behavior is interpreted within a lattice polaron model. Further analysis suggests a form of variable-range hopping conduction at high temperatures.