Connecting bulk symmetry and orbital polarization in strained RNiO${}_3$ ultrathin films

We examine the structural and electronic properties of LaNiO${}_3$ and NdNiO${}_3$ epitaxial thin films grown on cubic (001) SrTiO${}_3$ from the viewpoint of bulk crystal symmetry and misfit strain. X-ray scattering and polarization-dependent x-ray absorption spectroscopy measurements are performed to determine the crystal symmetry and extract the local Ni $3d$ orbital response, respectively, to understand the strain-induced distortions of the bulk structure. A strain-induced orbital polarization is found in NdNiO${}_3$ films, but is absent in LaNiO${}_3$ films. The difference in electronic structure is attributed to the bulk thermodynamic phases through group theoretical methods, which reveals that thin film perovskites retain a “memory” of their preferred electronic and structural configurations.

Figure 1

Schematic of the bulk LaNiO${}_3$ [(a),(b)] and NdNiO${}_3$ [(c)] structures showing both rhombohedral ($R\overline{3}c$) unit cell (dashed line) and orthorhombic ($Pbmn$) unit cell (dashed line) and the relationship to the pseudocubic unit cell (solid line). Part of the oxygen atoms and octahedra are not shown in (c) for clarity. La atoms shown in green; Nd atoms shown in orange; Ni atoms shown in blue at center of oxygen (red) octahedra.

Figure 2

X-ray reflectivity data (dots) and fit (solid line) for scattering along the (00L) specular CTR through the $Q_z=3.217$ Å${}^{-1}$ (002) Bragg peaks of the SrTiO${}_3$ substrate and (a) LaNiO${}_3$ and (b) NdNiO${}_3$ film. Reciprocal space map around the (113) Bragg reflection showing the films are coherent with the STO substrate (inset).

Figure 3

Half-order Bragg reflections through $L$ at various $H$ and $K$ points. (a) A schematic illustration of the octahedral rotation axes. (b) $H=K=1/2;$ (c),(d) $H=3/2$ and $K=1/2$.

Figure 4

Polarization averaged XAS (average of out-of-plane and in-plane polarization absorption) measured in fluorescence yield (FY) mode at Ni L edge.

Figure 5

Polarization dependent x-ray absorption and x-ray linear dichroism (XLD) at the Ni L${}_2$ edge for (a) NdNiO${}_3$ and (b) LaNiO${}_3$ on SrTiO${}_3$. On the right, schematic orbital level diagram of NdNiO${}_3$ on SrTiO${}_3$ with the anticipated strain-induced orbital polarization effect on the e${}_g$ doublet and LaNiO${}_3$ on SrTiO${}_3$ without splitting of e${}_g$ orbitals. [Due to the overlap of M${}_4$ edge of La (853 eV) with the Ni L${}_3$ edge (852.7 eV), the spectra of films at the Ni L${}_3$ edge is strongly distorted. Thus, the XLD was focused on the Ni L${}_2$ edge (870 eV) where the line shape is free from distortions.]

Figure 6

Distortion-mode analysis of (a) LNO and (b) NNO structures in both thin film and bulk phases with bond elongation modes producing orbital polarizations shown schematically to the right.