Orbital reflectometry of ${\text{PrNiO}}_3/{\text{PrAlO}}_3$ superlattices

We present an x-ray orbital reflectometry and linear dichroism study of ${\text{PrNiO}}_3{\text{-PrAlO}}_3$ superlattices grown on substrates that induce either compressive or tensile strain. For superlattices under tensile strain, we observe a pronounced change of the local nickel electronic structure and a decrease of orbital polarization below the metal-insulator transition temperature, in qualitative agreement with theoretical scenarios for charge disproportionation. In contrast, the x-ray absorption spectra of the superlattice under compressive strain with suppressed metal-insulator transition show no temperature dependence at the magnetic transition, consistent with a spin density wave transition driven by epitaxial strain and spatial confinement of the conduction electrons. The layer-resolved orbital occupations indicate a linear orbital-strain coupling previously found in ${\text{LaNiO}}_3$-based superlattices and suggest that, when present, the charge order encompasses the entire ${\text{PrNiO}}_3$ layer stack.

Figure 1

(a) Temperature dependence of the dc resistance of ${\text{PrNiO}}_3{\text{-PrAlO}}_3$ superlattices grown on LSAO (compressive) as well as STO and LSAT (tensile strain) substrate. (Right) X-ray absorption spectra with $E\parallel x$ polarization of PNO-PAO superlattices grown on (b) LSAT and (c) LSAO substrate, respectively.

Figure 2

Temperature and polarization dependent x-ray absorption spectra of PNO-PAO superlattices: (a) and (b) under tensile strain (LSAT substrate) and (c) and (d) under compressive strain (LSAO substrate). The insets show enlarged views near the Ni $L_3$ edge. The bottom panels (green lines) in (a) and (d) show the normalized difference spectra $[I_x\left(E\right)-I_z\left(E\right)]/\left[\frac{1}{3}(2I_x+I_z)\right]$. ${\text{P}}_{\mathrm{av}}$ denotes the averaged orbital polarization (2).

Figure 3

(a)–(c) XAS spectra measured with linearly polarized light for PNO-PAO (4 u.c.//4 u.c.)$\times 8$ superlattices grown on LSAO, STO, and LSAT substrates. In the bottom panels, the corresponding normalized difference spectra [$I_x\left(E\right)\text{−}{I}_z\left(E\right)]/\left[\frac{1}{3}(2I_x+I_z)\right]$ are shown. (d)–(f) Reflectivity curves as a function of ${\text{q}}_z$ for PNO-PAO superlattices grown on LSAO, STO, and LSAT substrates. Top, middle, and bottom panels show the reflectivity curves at $E=8047.7eV$ (Cu ${\text{K}}_{\alpha }$), and at the Ni $L_3$ and Ni $L_2$ energies, respectively. All data were normalized to unity at ${\text{q}}_z=0$. The solid lines show the fit results based on Parratt's recursive approach. The fitting parameters are summarized in Table 1. (g)–(i) Experimental and simulated ${\text{constant-q}}_z$ energy scans at the superlattice (002) reflection. The experimental curves are shifted for clarify. The corresponding normalized difference curves $\left[I_{\sigma }\left(E\right)\text{−}{I}_{\pi }\left(E\right)\right]/[I_{\sigma }\left(E\right)+I_{\pi }\left(E\right)]$ are shown directly below together with the difference in modulation denoted as $\alpha$.

Figure 4

Averaged $\left({\text{P}}_{\mathrm{av}}\right)$ and layer-resolved $({\text{P}}_{\mathrm{A}},{\text{P}}_{\mathrm{B}}$) orbital polarizations obtained from the combined analysis of XAS and resonant reflectivity as a function of the in-plane lattice constant ${\text{a}}_{\mathrm{SL}}$ measured by x-ray diffraction for PNO-based superlattices. The orange line represents the result of a linear fit of the ${\text{P}}_{\mathrm{A}}\text{-vs-}{a}_{\mathrm{SL}}$ relationship of LNO-based superlattices, which is reproduced from Ref. [30]. The purple line represents the corresponding result for the PNO layer stacks. The grey-shaded area indicates the influence of interfaces on the orbital polarization. The literature values of the in-plane lattice constant of bulk PNO and LNO are marked by the dashed lines with ${\text{a}}_{\mathrm{PNO}\mathrm{bulk}}=3.815\text{Å}$ and ${\text{a}}_{\mathrm{LNO}\mathrm{bulk}}=3.838\text{Å}$ [47]. Note that PNO-PAO superlattices grown on LSAT and STO substrate are partially relaxed (see Appendix for details).

Figure 5

Temperature dependent resonant x-ray reflectivity of the PNO-PAO superlattices on LSAT substrate. (a) Resonant x-ray reflectivity curves measured with $\sigma$ polarized light. (b) and (d) show the energy and polarization dependent normalized reflectivity spectra with fixed ${\text{q}}_z=0.4148\text{Å}$ measured at $T=300$ and $85\text{K}$, respectively. (c) compares the normalized different spectra $[{\text{I}}_{\sigma }\left(\text{E}\right)-{\text{I}}_{\pi }\left(\text{E}\right)]/[{\text{I}}_{\sigma }\left(\text{E}\right)+{\text{I}}_{\pi }\left(\text{E}\right)]$ of both phases.

Figure 6

(a) X-ray diffraction spectra for PNO-PAO (4 u.c.//4 u.c.)$\times 8$ on LSAO, LSAT and STO substrates along the specular (00l) direction measured with Cu ${\text{K}}_{\alpha 1}$ x rays. (b)–(d) show the corresponding reciprocal space maps around the (pseudo-)cubic (103) reflection.