Disorder-induced crossover of Mott insulator to weak Anderson localized regime in an argon-irradiated NdNiO3 film

Abstract

We show that an introduction of disorder in a controlled way using 1 MeV argon (Ar) ion irradiation, suppresses the correlation driven metal-insulator transition (MIT) in $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_{3}$ films. The films make a crossover to a heavily disordered conductor governed by weak localization (WL) and at even higher disorder, an Anderson localized state. The disorder (atomic displacement up to 2% of the total atoms) in the $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_{3}$ films was created using 1 MeV ${\mathrm{Ar}}^{4+}$ ion irradiation. We show that the pristine films of $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_{3}$ exhibit an MIT with the conduction process being governed by variable range hopping (VRH). For disorder up to 1% of the displaced atoms or lower, the insulating state arising from a gap in the density of states (DOS) at the Fermi level (${E}_{F}$) as in a Mott insulator is suppressed and the conduction in the film shows a WL behavior with finite conductivity at temperature $T\ensuremath{\rightarrow}0$. This behavior is expected in a disordered conductor that does not have a gap in DOS at ${E}_{F}$. At higher fluences the conductivity reduces substantially but the electrical conduction shows a power-law temperature dependence with a small but finite zero temperature conductivity $\ensuremath{\sigma}(T=0)$ which is expected in a solid with electrons that are Anderson localized. A similar experiment was performed on the La substituted $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_{3}$ films (${\text{Nd}}_{1\ensuremath{-}x}{\text{La}}_{x}{\text{NiO}}_{3}$) with $x=0.3$ that are grown in the same way. La substitution in $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_{3}$ suppresses the temperature driven transition and leads to a metallic state with critical composition at $x\ensuremath{\approx}0.3$. The pristine as well as films irradiated with lowest fluence shows metallic or marginally metallic behavior grown on $\mathrm{La}\mathrm{Al}{\mathrm{O}}_{3}$ and $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_{3}$ substrates, respectively. However, at higher fluences they too exhibit a convergence in electronic transport and $\ensuremath{\sigma}$ shows a power-law temperature dependence at low $T$ with $\ensuremath{\sigma}(T=0)\ensuremath{\ne}0$. Evidence of suppression of correlated behavior can also be seen in the irradiated films where the non-Gaussian nature of resistance fluctuation at $T\ensuremath{\approx}{T}_{\text{MI}}$, a signature of correlated electron systems, is suppressed on irradiation that leads to collapse of the MIT. Evidence for progressing disordering of the films on irradiation were observed in Raman spectroscopy as well as x-ray studies that show the basic integrity of the ${\text{NiO}}_{6}$ octahedra is preserved and the structure retains its crystallinity.