Continuous transition from weakly localized regime to strong localization regime in Nd_0.7La_0.3NiO₃ films

We report an investigation of metal-insulator transition (MIT) using conductivity and magnetoconductance (MC) measurements down to 0.3 K in Nd_0.7La_0.3NiO₃ films grown on crystalline substrates of LaAlO₃ (LAO), SrTiO₃ (STO), and NdGaO₃ (NGO) by pulsed laser deposition. The film grown on LAO experiences a compressive strain and shows metallic behavior with the onset of a weak resistivity upturn below 2 K which is linked to the onset of weak localization contribution. Films grown on STO and NGO show a cross-over from a positive temperature coefficient (PTC) resistance regime to negative temperature coefficient (NTC) resistance regime at definite temperatures. We establish that a cross-over from PTC to NTC on cooling does not necessarily constitute a MIT because the extrapolated conductivity at zero temperature though small (<10 S cm⁻¹) is finite, signaling the existence of a bad metallic state and absence of an activated transport. The value of for films grown on NGO is reduced by a factor of 40 compared to that for films grown on STO. We show that a combination of certain physical factors makes substituted nickelate (that are known to exhibit first-order Mott type transition), undergo a continuous transition as seen in systems undergoing disorder/composition driven Anderson transition. The MC measurement also supports the above observation and shows that at low temperatures, there exists a positive MC that arises from the quantum interference which co-exists with a spin-related negative MC that becomes progressively stronger as the electrons approach a strongly localized state in the film grown on NGO. (paper)