Phase coexistence near the metal-insulator transition in a compressively strained NdNiO3 film grown on LaAlO3 : Scanning tunneling, noise, and impedance spectroscopy studies

We report an observation of phase coexistence near the metal-insulator transition (MIT) in a film of $\mathrm{NdNi}{\mathrm{O}}_{3}$ grown on crystalline substrate $\mathrm{LaAl}{\mathrm{O}}_{3}$. This was established through a combination of three techniques, namely, scanning tunneling spectroscopy, $1/f$ noise spectroscopy, and impedance spectroscopy experiments. The spatially resolved scanning tunneling spectroscopy showed that the two coexisting phases have different types of density of states (DOS) at the Fermi level. One phase showed a depleted DOS close to ${E}_{\mathrm{F}}$ with a small yet finite correlation gap, while the other coexisting phase showed a metal-like DOS that had no depletion. The existence of the phase separation leads to a jump in the resistance fluctuation (as seen through $1/f$ noise spectroscopy) at the transition, and, notably, the fluctuation becomes non-Gaussian when there is a phase separation even in the metallic phase. This was corroborated by the impedance spectroscopy, which showed a broad hump in capacitance at the transition region as a signature of the existence of two phases that have widely different electrical conductivities. The phase separation starts well within the metallic phase much above the transition temperature and makes the sample electronically ``inhomogeneous'' in nanoscopic scale close to the transition. We discuss certain scenarios that lead to such a phase separation in the general context of strongly correlated oxides.