Mott Electrons in an Artificial Graphenelike Crystal of Rare-Earth Nickelate

Deterministic control over the periodic geometrical arrangement of the constituent atoms is the backbone of the material properties, which, along with the interactions, define the electronic and magnetic ground state. Following this notion, a bilayer of a prototypical rare-earth nickelate, ${\mathrm{NdNiO}}_3$, combined with a dielectric spacer, ${\mathrm{LaAlO}}_3$, has been layered along the pseudocubic [111] direction. The resulting artificial graphenelike Mott crystal with magnetic $3d$ electrons has antiferromagnetic correlations. In addition, a combination of resonant X-ray linear dichroism measurements and ab initio calculations reveal the presence of an ordered orbital pattern, which is unattainable in either bulk nickelates or nickelate based heterostructures grown along the [001] direction. These findings highlight another promising venue towards designing new quantum many-body states by virtue of geometrical engineering.

Figure 1

(a) One unit cell of the perovskite structure. A RE ion, located at the center of the cube, has been omitted for clarity. (b) Bilayer of ${\mathrm{NdNiO}}_3$ sandwiched between layers of dielectric ${\mathrm{LaAlO}}_3$ along the pseudocubic [111] direction. (c) Schematic of a (111) bilayer that forms a buckled honeycomb lattice. The Ni atoms on the individual (111) plane are highlighted by thin and thick red circles, respectively. (d) HAADF-STEM image of the [$2{\mathrm{NdNiO}}_3/4{\mathrm{LaAlO}}_3$]x3 superlattice, grown on ${\mathrm{LaAlO}}_3$ (111) substrate.

Figure 2

(a) Ni $L_{3,2}$-edge XRMS spectra for the $2\mathrm{NNO}/4\mathrm{LAO}$ sample recorded in an applied magnetic field of $\pm 5\mathrm{T}$. (b) XRMS vs magnetic field. (c) XRMS vs $T$, (d) $1/\mathrm{XRMS}$ vs $T$ plot. The feature near 850 eV is a contribution from the intense La scattering signal and does not flip with $\pm H$. To eliminate it, the differences (divided by 2) of the XRMS spectra obtained for several positive and negative fields (e.g., $\pm 5\mathrm{T}$) were used for the analysis.

Figure 3

Experimental arrangement for recording the XLD spectrum for (a) flat ($\theta =0°$) and (b) wedge ($\theta =45°$) configurations. $\theta$ is the angle between the vertical sample holder and the (111) plane of the film. The orbital arrangements for the AFO state with the polarization direction for the two experimental configurations have been shown in (c), (d). The Ni XAS recorded (detection mode: total fluorescence yield) at 300 K with $V$ and $H$ polarized light and their differences are shown in (e) for both $\theta =0°$, 45°. The spectra are shifted vertically for clarity. Note, due to strong overlap of the Ni $L_3$ edge with the La $M_4$ edge, only the $L_2$ edge is shown. Because of the experimental limitation all the measurements were acquired at 45° inclination (instead of ideal 54.7°) relative to the x-ray beam.

Figure 4

Majority (blue) and minority (tangerine) band structures for (a) ferromagnetic ($\uparrow \uparrow$) and (c) antiferromagnetic ($\uparrow \downarrow$) order for the ($\sqrt{3}\times \sqrt{3}$) supercell. (e) Band structure for ($\uparrow \downarrow$) order on the ($1\times 1$) supercell. The corresponding spin-density distributions are shown in (b), (d), and (f), respectively, and also in the Supplemental Material [51].