In operando adjustable orbital polarization in $\mathrm{LaNi}{\mathrm{O}}_3$ thin films

The different occupancy of electronic orbitals, the so-called orbital polarization, is a key parameter determining electric and magnetic properties of materials. Here we report on the demonstration of in operando voltage-controlled tuning of the orbital occupation in $\mathrm{LaNi}{\mathrm{O}}_3$ epitaxial thin films grown on piezoelectric substrates. The different static contributions to the orbital occupation are disentangled, namely the epitaxial strain and the surface symmetry breaking, and the superimposed electric-field controlled orbital polarization are determined by x-ray linear dichroism at the Ni $L_{2,3}$ edges. The voltage-controlled orbital polarization allows changing the orbital polarization by about an additional 50%.

Figure 1

Straining the $\mathrm{Ni}{\mathrm{O}}_6$ octahedron by applied electric field causes degeneracy breaking at $3d{e}_g$ orbitals. The ON (closed) and OFF (open) switch state of the battery refers to strained and unstrained states of the octahedron, respectively.

Figure 2

(a), (b) Ni $L_{2,3}$ XAS (for $E\left|\right|c$ and $E\left|\right|ab$ polarizations) of 14-nm LNO thin films on STO and LAO, respectively. The spectra are plotted after subtracting the $\mathrm{La}{M}_4$ and normalizing to their average $L_{3\text{−}}\mathrm{peak}$ intensity $\left(E_0\right)$. (c) The Ni $L_{2,3}$ XLD (i.e., $E\left|\right|ab-E\left|\right|c$) of LNO(14 nm) films grown on different (001)-oriented single-crystalline substrates indicates that the STO and LAO (tensile and compressive strained) exhibit opposite signs corresponding to the $x^2-y^2$ and $3z^2-r^2$ orbital occupancy, respectively, while the LSAT remains intermediate. (d) The Ni $L_{2,3}$ XLD of 2, 5, and 14-nm LNO on STO indicate that the sign and magnitude of XLD changes, indicating a larger $3z^2-r^2$ orbital occupancy when reducing thickness. (e) The O $K\text{−}\mathrm{edge}$ XAS for different thicknesses of LNO films on STO substrates. (f) Thickness variation of the O $K$ XLD for the LNO films on LAO, LSAT, and STO substrates.

Figure 3

(a) Hole ratio ($X$) (left axis) and electron orbital polarization ($P$) (right axis) as a function of substrate-induced epitaxial in-plane strain $\left(\varepsilon \right)$ for the LNO films (2, 5, and 14 nm) on different strains (tensile for STO and LSAT, compressive for LAO). Dashed oblique line passing through the origin $(\varepsilon =0,X=1)$ indicates the apparent orbital polarization if strain-only phenomena were considered. However, the yellow arrows emphasized the observed downward shift in $X$ due to the symmetry breaking at surface-induced ($3z^2-r^2$) orbital polarization. (b) The $X$ and $P$ values of LNO film in LNO(10 nm)/STO(5 nm)//PMN-PT heterostructure as a function of the voltage ($V_{\mathrm{bias}}$) applied across the PMN-PT substrate (bottom axis). Top axis show the maximal possible in-plane (tensile) strain caused by the STO//PMN-PT at some selected $\pm V_{\mathrm{bias}}$ values, assuming a fully elastic response of LNO to PMN-PT deformation.

Figure 4

(a) Sketches illustrating the orbital polarization definitions. (b), (c) Orbital polarization induced by compressive and tensile strain, respectively. (e) Surface-induced orbital polarization acting on a degenerate $e_g$ orbital manifold (d).

Figure 5

(a) Sketch of the experimental arrangement to measure the XAS absorption in a voltage-biased LNO/STO//PMN-PT heterostructure. (b) The raw TEY signal at Ni $L_{2,3}$ edges collected with $V_{\mathrm{bias}}=-200\mathrm{V}$, 0 V, and +200 V. (c)–(e) The Ni $L_{2,3}$ XAS recorded at $V_{\mathrm{bias}}=-200\mathrm{V}$, 0 V, and + 200 V, respectively, using polarizations ${\mathbf{E}}_{\mathrm{ab}}$ and ${\mathbf{E}}_{\mathrm{c}}$ as indicated, after subtracting $\mathrm{La}\text{−}{M}_4$. Insets in (c)–(e) are zooms of the $L_{3\text{−}}\mathrm{peak}$ data recorded with ${\mathbf{E}}_{\mathrm{ab}}$ and ${\mathbf{E}}_{\mathrm{c}}$, where dichroism is more evident. Integrated XAS values were taken from the light cyan-colored region after removing the step function.

Figure 6

(a) The Ni $L_{2,3}$ XLD of an LNO/STO//PMN-PT film at 0 V and ±200 V indicates that $V_{\mathrm{bias}}$ changes the XLD (by magnitude) and so the orbital occupancy (towards the $3z^2-r^2$ orbitals). The spectral features of the unbiased sample resembles those of 14-nm LNO/STO (in gray color) from the substrate-strain experiment except slight differences in the orbital polarization values. (b) Hole occupancy ratio ($X$, left axis) and orbital polarization ($P$, right axis) show an irreversible piezoresponse with the $V_{\mathrm{bias}}$.