Orbital engineering in ${\mathrm{YVO}}_3-\mathrm{La}\mathrm{Al}{\mathrm{O}}_3$ superlattices

Oxide heterostructures provide unique opportunities to modify the properties of quantum materials through a targeted manipulation of spin, charge, and orbital states. Here, we use resonant x-ray reflectometry to probe the electronic structure of thin slabs of ${\mathrm{YVO}}_3$ embedded in a superlattice with $\mathrm{La}\mathrm{Al}{\mathrm{O}}_3$. We extend the previously established methods of reflectometry analysis to a general form applicable to $t_{2g}$ electron systems and extract quantitative depth-dependent x-ray linear dichroism profiles. Our data reveal an artificial, layered orbital polarization, where the average occupation of $xz$ and $yz$ orbitals in the interface planes next to $\mathrm{La}\mathrm{Al}{\mathrm{O}}_3$ is inverted compared to the central part of the ${\mathrm{YVO}}_3$ slab. This phase is stable down to 30 K and the bulklike orbital ordering transitions are absent. We identify the key mechanism for the electronic reconstruction to be a combination of epitaxial strain and spatial confinement by the $\mathrm{La}\mathrm{Al}{\mathrm{O}}_3$ layers, in good agreement with predictions from ab initio theory.

Figure 1

(a) Orbital occupation of ${\mathrm{V}}^{3+}$ ion in $R{\mathrm{VO}}_3$. Since the configuration mixing of the lowest triplet states (${}^{3}T_1$) is small in the $d^2$ system, the use of the single electron basis ($t_{2g}^2$) is justified and we will refer to it throughout this Letter. (b) Sketch of the 8/4, 6/6, and 4/8 superlattice structures. For all three, the YVO-LAO bilayers are repeated six times. (c) Orientation of the orthorhombic YVO unit cell, with lattice parameters $a, b$, and $c$

Figure 2

Characterization of the 8/4 sample. (a) HAADF image showing YVO and LAO layers. (b) STEM-EELS displaying elemental compositions of V, La, Y, and Al in the SL. (c) XAS of the V-$L_{2,3}$ and O-$K$ edge. (d) STEM-EELS V-$L_{2,3}$ edge line spectra at different positions across the YVO slab (see inset).

Figure 3

Resonant reflectivity of the (4/8) SL at room temperature (a) at fixed energy of 516.7 eV as a function of $Q$, and (b) as a function of energy across the V-$L_{2,3}$ edge at the fixed momentum transfer of superlattice reflections: $Q_{\left(001\right)}=0.147{\AA }^{-1}, Q_{\left(002\right)}=0.285{\AA }^{-1}$, and $Q_{\left(003\right)}=0.430{\AA }^{-1}$. Experimental and simulated spectra shown for $\sigma$ and $\pi$ polarizations. (c) The constant-$Q$ dichroism defined as $(I_{\sigma }-I_{\pi })/(I_{\sigma }+I_{\pi })$ shown for simulated and measured spectra for different temperatures of 30, 140, and 290 K. (d) Scattering geometry with respect to the YVO unit cell and frame of reference of the orbitals.

Figure 4

(a) Experimental (circles) and fitted (lines) linear dichroism profiles between two pairs of polarizations for the central (C) layers of all three stackings (top) and the interfacial (IF) layers of 8/4, 6/6, and 4/8 SLs. (b) Schematic showing the orbital polarization for bulk, C, and IF layers of the SLs with normalized orbital polarizations $P_1=(n_{xz}-n_{yz})/(n_{xz}+n_{yz})$ and $P_2=[n_{xy}-(n_{xz}+n_{yz})/2]/[n_{xy}+(n_{xz}+n_{yz})/2]$ in $\%$, both within an error of $\pm 2\%$. (c) Layer-resolved, site-averaged, and orbital-projected density of states of a representative 4/4 YVO-LAO SL obtained from $\mathrm{DFT}+U$. The green and blue curves correspond to V $yz$ and $xz$ orbitals, respectively, and reveal a distinct lifting of the orbital degeneracy in C vs IF layers.