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 Δ on doping x has been determined and the critical concentration for the zero-temperature metal–insulator transition is determined as xc≈0.33. For a metallic sample with x = 0.42, a conventional electron–electron (e–e) scattering term 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 T1/2 weak-localization contribution to resistivity for a disordered three-dimensional electron system. The specific heat data have been fitted to contributions from free electrons (γ), spin excitations (β3/2), lattice and a Schottky-like anomaly related to the rare-earth magnetism of the Nd ions. The value of γ 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.