Anomalous electron transport in epitaxial NdNiO3 films

The origin of simultaneous electronic, structural, and magnetic transitions in bulk rare-earth nickelates (RENiO₃) remains puzzling with multiple conflicting reports on the nature of these entangled phase transitions. Heterostructure engineering of these materials offers unique opportunity to decouple the metal-insulator transition (MIT) from the magnetic transition. However, the evolution of underlying electronic properties across these decoupled transitions remains largely unexplored. In order to address this, we have measured Hall effect on a series of epitaxial NdNiO3 films, spanning a variety of electronic and magnetic phases. We find that the MIT results in only a partially gapped Fermi surface, whereas the full insulating phase forms below the magnetic transition. In addition, we also find a systematic reduction of the Hall coefficient RH in the metallic phase of these films with epitaxial strain and also a surprising transition to a negative value at large compressive strain. The partially gapped, weakly insulating, paramagnetic phase is reminiscence of pseudogap behavior of high-Tc cuprates. The precursor metallic phase, which undergoes transition to the insulating phase, is a non-Fermi liquid with a temperature exponent n of resistivity of 1, whereas the exponent increases to 4/3 in the noninsulating samples. Such a nickelate phase diagram with sign reversal of RH, a pseudogap phase, and non-Fermi-liquid behavior is intriguingly similar to high-Tc cuprates, giving important guidelines to engineer unconventional superconductivity in oxide heterostructures.