Understanding and control of the metallic state in epitaxial NdNiO3

This thesis reports a systematic analysis on epitaxial nickelate thin films. As oxides with strong electron-lattice coupling, the electrical properties of nickelate thin films can be tuned by many factors that are often inter-twinned. By separating the contributions of these different factors, we unveil that in pristine nickelate films, the resistivity follows a linear temperature dependence, suggesting Planckian dissipation in a wide temperature range (100- 500 K) with no signatures of bad metal behavior, a characteristic that has been reported in some nickelates. By comparing with other electron correlated metals, we show that the nickelates belong to a class of systems with strength of electron interactions intermediate between those normal and strongly correlated metals, explaining the difficulties in their classification. We also show that, both the increase of strain and disorder exert significant effects on their electrical performance, including the metal-insulator transition and resistivity-temperature scaling. These tunable properties impact greatly their applications. Therefore, the study of the underlying physics of these materials should be accompanied by a detailed characterization of the microstructure.