Low temperature transport and specific heat studies of Nd1-xPbxMnO3 single crystals

Electrical transport and specific heat properties of Nd1-xPbxMnO3 single crystals for 0.15 <= x <= 0.5 have been studied in the low temperature regime. The resistivity in the ferromagnetic insulating (FMI) phase for x <= 0.3 has an activated character. The dependence of the activation gap Delta on doping x has been determined and the critical concentration for the zero-temperature metal-insulator transition is determined as x(c)approximate to 0.33. For a metallic sample with x = 0.42, a conventional electron - electron (e - e) scattering term proportional to T-2 is found in the low temperature electrical resistivity, although the Kadowaki - Woods ratio is found to be much larger for this manganite than for a normal metal. There is a resistivity minimum observed around 60 K for a metallic sample with x = 0.5. The effect is attributed to weak localization and can be described by a negative T-1/2 weak-localization contribution to resistivity for a disordered three-dimensional electron system. The specific heat data have been fitted to contributions from free electrons (gamma), spin excitations (beta(3/2)), lattice and a Schottky-like anomaly related to the rare-earth magnetism of the Nd ions. The value of gamma is larger than for normal metals, which is ascribed to magnetic ordering effects involving Nd. Also, the Schottky-like anomaly appears broadened and weakened suggesting inhomogeneous molecular fields at the Nd-sites.