Onset of metallic behavior in strained ${\left({\text{LaNiO}}_3\right)}_n/{\left({\text{SrMnO}}_3\right)}_2$ superlattices

${\left({\text{LaNiO}}_3\right)}_n/{\left({\text{SrMnO}}_3\right)}_2$ superlattices were grown using ozone-assisted molecular beam epitaxy. In situ reflection high-energy electron diffraction and x-ray scattering has been used to characterize the structural properties of the superlattices, which are strained to the ${\text{SrTiO}}_3$ substrates. The superlattices exhibit excellent crystallinity and interfacial roughness of less than 1 unit cell. A metal-insulator transition is observed as $n$ is decreased from 4 to 1. Analysis of the transport data suggests an evolution from gapped insulator $\left(n=1\right)$ to hopping conductor $\left(n=2\right)$ to metal $\left(n=4\right)$ with increasing ${\text{LaNiO}}_3$ concentration.

Figure 1

(Color online) RHEED patterns (a) before and (b) after growth of a ${\left[{\left(\text{LNO}\right)}_2/{\left(\text{SMO}\right)}_2\right]}_{14}$ superlattice along with corresponding line profiles. The short black lines indicate where each line profile was obtained. The arrows in (b) highlight the spectral intensity believed to arise from a surface reconstruction of the ${\text{NiO}}_2$-terminated sample. The specular spot intensity measured during deposition of the fourth through seventh periods of the superlattice is given in (c).

Figure 2

(Color online) (a) X-ray reflectivity of ${\left(\text{LNO}\right)}_n/{\left(\text{SMO}\right)}_2$ superlattices. The notation above the Bragg reflections refers to the number of LNO $\left(L\right)$ and SMO $\left(S\right)$ unit cells in each superlattice period. The fit obtained for the $n=2$ superlattice is shown in (b).

Figure 3

(a) $2\theta \text{−}\theta$ scan around the (0 0 2) peak of the ${\left[{\left(\text{LNO}\right)}_1/{\left(\text{SMO}\right)}_2\right]}_{18}$ superlattice. A $\theta$ scan measured in the same superlattice at the (0 0 2) peak center is shown in (b). The solid line is the Gaussian fit. The average $c$-axis parameter of the superlattices is given in (c) as a function of the number of LNO unit cells divided by the total number of unit cells in each sample. A $\varphi$ scan measured at the (1 0 1) peak of a ${\left[{\left(\text{LNO}\right)}_2/{\left(\text{SMO}\right)}_2\right]}_{14}$ superlattice is shown in the top panel of (d), while the corresponding scan of the STO is shown in the bottom panel.

Figure 4

(Color online) Resistivity in the superlattices and single material films.

Figure 5

(a) High $T$ activated behavior and (b) low $T$ 3D variable range hopping in SMO (90–200 K) and the $n=1$ superlattice (80–150 K).

Figure 6

(Color online) (a) Resistivity of the ${\left[{\left(\text{LNO}\right)}_2/{\left(\text{SMO}\right)}_2\right]}_{14}$ superlattice fit to activated behavior from 5–25 K. The inset of (a) shows $\rho$ as a function of $1/T$ over the range of 5–325 K. The fits to 2D and 3D VRH are shown in (b) from 5–325 K.

Figure 7

(Color online) Resistivity of the ${\left[{\left(\text{LNO}\right)}_4/{\left(\text{SMO}\right)}_2\right]}_9$ superlattice and LNO film. Also shown (blue) is the resistivity of the superlattice normalized by the number of $\text{LaO}/{\text{NiO}}_2/\text{LaO}$ layers in the superlattice period (3/6). The solid line shows the $T^{1.5}$ fit to the LNO resistivity.