Evolution of the metallic state in ${\mathrm{LaNiO}}_3/{\mathrm{LaAlO}}_3$ superlattices measured by ${}^8\mathrm{Li}\beta$-detected NMR

Using ion-implanted ${}^8\mathrm{Li} \beta$-detected NMR, we study the evolution of the correlated metallic state of ${\mathrm{LaNiO}}_3$ in a series of ${\mathrm{LaNiO}}_3/{\mathrm{LaAlO}}_3$ superlattices as a function of bilayer thickness. Spin-lattice relaxation measurements in an applied field of $6.55\mathrm{T}$ reveal two equal amplitude components: one with metallic ($T$ linear) $1/T_1$ and a second with a more complex $T$ dependence. The metallic character of the slow relaxing component is only weakly affected by the ${\mathrm{LaNiO}}_3$ thickness, while the fast component is much more sensitive, exhibiting the opposite temperature dependence (increasing toward low $T$) in the thinnest, most magnetic samples. The origin of this bipartite relaxation is discussed in terms of electronic phase separation.

Figure 1

Examples of ${}^8\mathrm{Li}^{+}$ SLR data in ${\left[{\left({\mathrm{LaNiO}}_3\right)}_n{\left({\mathrm{LaAlO}}_3\right)}_2\right]}_{20}$ with $B_0=6.55\mathrm{T}$ at two temperatures for $n=2,4$ and 10. The lines are fits to Eq. (1) convoluted with the 4 s beam pulse indicated by the shaded area. The data has been normalized and for clarity, offset vertically. The colored $y$-axis ticks on the left (right) represent the full (zero) asymmetry for each SL.

Figure 2

The slow [(a), (c)] and fast [(b), (d)] ${}^8\mathrm{Li}$ SLR rates and ${\left(T_{1}T\right)}^{-1}$ as a function of temperature at $B_0=6.55\mathrm{T}$ in ${\left[{\left({\mathrm{LaNiO}}_3\right)}_n{\left({\mathrm{LaAlO}}_3\right)}_2\right]}_{20}$ for $n=2,4$, and 10. Below $\sim 200\mathrm{K}, {(1/T_1)}_s$ appears linear with a nonzero intercept (fits shown with solid lines). The vertical arrow at $\sim 200\mathrm{K}$ indicates the sublinear deviation. The shaded region and the vertical line illustrate the reported $T_{\mathrm{MI}}$ and $T_N$ in an $n=2$ SL on ${\mathrm{SrTiO}}_3$ [3]. For reference, the bulk Korringa slope is shown as the solid purple line [14]. The inset of panel (d) illustrates the low $T$ upturn for $n=2$, which spans more than an order of magnitude. The data for the $n=3$ SL has been omitted to avoid clutter; however, the trend is very similar to $n=2$.

Figure 3

(a) Slopes from the linear region of the slow component as a function of the ${\mathrm{LaNiO}}_3$ thickness $n$. The bulk slope is shown as the solid purple line. The $n=4$ and 10 values are scattered around the bulk, while the thinner SLs ($n=2$ and 3) are significantly enhanced. (b) The $T\to 0$ intercepts appear independent of $n$, with average value shown by the dashed gray line.