Continuous transition from weakly localized regime to strong localization regime in Nd_{0.7}La_{0.3}NiO_{3} films

TL;DR

This study investigates the metal-insulator transition in Nd_{0.7}La_{0.3}NiO_{3} films, revealing a continuous transition from weak localization to strong localization regimes influenced by strain and disorder, with detailed conductivity and magnetoconductance analysis.

Contribution

It demonstrates that the transition in these nickelate films is continuous and driven by disorder, contrasting with the first-order Mott transition typically observed in similar materials.

Findings

Films on LAO show weak localization at low temperatures.

Films on STO and NGO exhibit a crossover from PTC to NTC resistance regimes.

Magnetoconductance measurements support the continuous transition and localization effects.

Abstract

We report an investigation of Metal Insulator Transition (MIT) using conductivity and magnetoconductance (MC) measurements down to 0.3 K in Nd_{0.7}La_{0.3}NiO_{3} films grown on crystalline substrates of LaAlO_{3} (LAO), SrTiO_{3} (STO), and NdGaO_{3}(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 crossover 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 \sigma_{0} though small (<10 S/cm) is finite, signalling the existence of a bad metallic state and absence of an activated transport. The value of \sigma_{0} 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 support the above observation and show that at low temperature 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.