Phase coexistence near the metal-insulator transition in a compressively strained $\mathrm{NdNi}{\mathrm{O}}_3$ film grown on $\mathrm{LaAl}{\mathrm{O}}_3$: 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.

Figure 1

(a) Cross-sectional SEM image of the film. (b) X-ray diffraction rocking curve of the $\mathrm{NdNi}{\mathrm{O}}_3$ film, where the solid curve is fitted to identify the peaks. (c) STM topography image of the film, where the red lines are guides to the eye, showing the underlying terrace pattern in which the film was grown.

Figure 2

Resistivity of $\mathrm{NdNi}{\mathrm{O}}_3$ film as a function of temperature during the heating and cooling cycle.

Figure 3

Local tunneling conductance map at (a) 290 K and (b) 130 K taken in the heating cycle. The data are plotted as normalized conductance $\frac{g}{g_{\mathrm{rms}}}$. The red and black arrows show the highest and lowest $g$ regions, respectively.

Figure 4

(a) The bias dependence of the local tunneling conductance $g=\frac{dI}{dV}$ taken at two locations marked by arrows in Fig. 3 (solid lines show the fit to equation $g=\alpha +\beta V^2$). (b) Normalized tunneling conductance $g_n$ as defined in the text for the region with low conductivity at 130 K.

Figure 5

(a) Normalized spectral power density of fluctuation as a function of $f$ at different temperatures taken during the heating cycle where the solid lines represent fit to $\frac{1}{f^{\alpha }}$ behavior. (b) Variation of the exponent α with T. (c) Normalized mean square resistance fluctuation shown as a function of T. The inset shows the expanded part of the marked region.

Figure 6

Normalized second spectra $S^2\left(f_2\right)$ calculated at different temperatures.

Figure 7

Normalized variance $\left({\mathrm{\Sigma }}^2\right)$ of the second spectra and dρ/dT as a function of temperature. The arrows mark the region of phase coexistence.

Figure 8

Capacitance $C_{\mathrm{C}}$ as a function of $T/T_{\mathrm{MI}}$.