Scanning tunneling microscopy and spectroscopy on nickelate thin films

We present a scanning tunnelling microscopy study of the metal insulator (MI) phase transition in NdNiO3 thin films grown on NdGaO3. Although topographic resolution is limited to grain structures and step edges, we did achieve high resolution and reproducible spectroscopic mapping of the thin film surface. The spectra we measure are either metallic or insulating, and mapping the local gap as a function of position shows the emergence of well-defined insulating domains in a conducting background upon cooling the film through the MI transition. When warming the film through the MI transition, the opposite is observed, with metallic domains appearing in the insulating background. There is no gradual shift in the measured insulating gap amplitude at the phase transition. The measured proportion of metallic surface area as a function of temperature follows the resistive hysteresis loop and allows a direct comparison with a percolation model for the MI phase transition.