Disorder-induced crossover of Mott insulator to weak Anderson localized regime in an argon-irradiated ${\mathrm{NdNiO}}_3$ film

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 $T\to 0$. 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 $\sigma (T=0)$ 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 (${\text{Nd}}_{1-x}{\text{La}}_x{\text{NiO}}_3$) 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 $x\approx 0.3$. 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 $\sigma$ shows a power-law temperature dependence at low $T$ with $\sigma (T=0)\ne 0$. Evidence of suppression of correlated behavior can also be seen in the irradiated films where the non-Gaussian nature of resistance fluctuation at $T\approx T_{\text{MI}}$, 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.

Figure 1

Schematic of the phase diagram with correlation and disorder: MI: Mott insulator, AI: Anderson insulator, WL: weak localization regime. The vertical arrows show the schematic path of disordering by 1 MeV Ar ions for (a) $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_3$ (NNO) film and (b) ${\text{Nd}}_{0.7}{\text{La}}_{0.3}{\text{NiO}}_3$ film. Shading around the arrows show expected variations arising from the strain for films grown on different substrates. The phase diagram has been adapted from [13].

Figure 2

X-ray diffraction around indices (002) in pristine and irradiated films of NNO grown on (a) STO and (b) LAO. (Full range data are in Fig. S3 [32]).

Figure 3

X-ray diffraction around indices (002) in pristine and irradiated films of NLNO grown on (a) STO and (b) LAO. (Full range data are in Fig. S4 [32]).

Figure 4

Raman spectra on pristine and irradiated films on LAO substrate for (a) NNO and (b) NLNO. Progressive smearing of the NNO peaks on disordering can be seen.

Figure 5

Resistivity ($\rho$) as a function of temperature ($T$) for pristine and irradiated films of NNO grown on (a) STO and (b) LAO substrates.

Figure 6

(a) Evolution of room temperature (300 K) and low temperature (3 K) conductivity $\left(\sigma \right)$ as a function of atomic displacement in NNO/STO film on Ar ion irradiation. (b)–(d) Low temperature power-law dependence of $\sigma$ on $T$.

Figure 7

(a) Evolution of room temperature (300 K) and low temperature (3 K) conductivity $\left(\sigma \right)$ as a function of atomic displacement in NNO/LAO film on Ar ion irradiation. (b) and (c) Low temperature power-law dependence of $\sigma$ on $T$.

Figure 8

Resistivity as a function of temperature ($T$) for pristine and irradiated films of NLNO grown on (a) STO and (b) LAO substrates.

Figure 9

Dependence on conductivity $\sigma$ on atomic displacement (%) (fraction of atoms knocked out by heavy ions) at 300 and 3 K for NLNO on STO. Power-law dependence of low temperature conductivity as shown in Fig. S7 of the Supplemental Material [32].

Figure 10

Dependence on conductivity $\sigma$ on atomic displacement (%) (fraction of atoms knocked out by heavy ions) at 300 and 3 K for NLNO on LAO. Power -law dependence of low temperature conductivity as shown in Fig. S8 of the Supplemental Material [32].

Figure 11

(a) Normalized mean square resistance fluctuation $\frac{\langle \mathrm{\Delta }{R}^2\rangle }{R^2}$ and (b) the normalized second spectra ${\mathrm{\Gamma }}^2$ of fluctuation as a function of temperature in the pristine and irradiated films of NNO/STO. The metal insulator transition temperature $T_{\text{MI}}$ is marked. The data for pristine film has been taken from our past work [31].