Strain tuning of interorbital correlations in ${\mathrm{LaVO}}_3$ thin films

$3d^2$ vanadate perovskites exhibit an unusual phase diagram where the low temperature orbital and spin orders are strongly dependent upon the rare-earth cation size. Here, we demonstrate the delicate role of strain in epitaxial ${\mathrm{LaVO}}_3$ films by performing low-energy muon spin spectroscopy and ellipsometry measurements. The electronic structure is revealed by the measurement of the multipeak structure of the charge excitations. The analysis highlights the independent roles of the $d_{xy}$ band confinement and the $d_{zx},d_{zy}$ doublet polarization for the orbital ordering to establish.

Figure 1

(a) The multiplet representation of the four first charge excitations for a cubic $3d^2$ transition metal oxide with $t_{2g}$ orbitals [23]. The $xy$ (black), $zy$ (blue), $zx$ orbitals (red), and spin (gray arrows) are represented. The transition to the high-spin state (${}^{4}A_2$) has the lowest excitation energy $U-3J_h$. The low-spin transition with electrons in different orbitals ${}^{2}E$ has an excitation energy $U$. The low-spin transitions with the electrons on the same orbitals (${}^{2}T_1$ and ${}^{2}T_2$) have excitations energies of $U$ and $U+2J_h$, respectively. (b) Illustration of the two allowed antiferromagnetic (AFM) SO assuming the $G$-type OO: The ground state observed for ${\mathrm{LaVO}}_3$ is the $C$-type SO (ferromagnetic chains along $z$ with the lower energy excitation energy $U-3J_h$). (c) Assuming the $C$-type OO, only the $G$-type SO is allowed. (b) and (c) The excitations involved per spin/orbital bond are marked by the black arrows. In the 1D orbital fluctuation regime, the spin/orbital bonds fluctuate between the three possible arrangements; as the temperature decreases, the $G$-type OO/$C$-type SO ground state sets in for ${\mathrm{LaVO}}_3$.

Figure 2

Typical asymmetry spectra measured at various temperatures using an implantation energy of 7 keV: In (a)–(c) are shown the data of ${\mathrm{LaVO}}_3/\mathrm{Dy}\mathrm{Sc}{\mathrm{O}}_3$ and in (d)–(f) the ones of ${\mathrm{LaVO}}_3/\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ films. The lines indicate the best fit using the formula presented in the text.

Figure 3

Plot of the temperature dependent optical conductivity vs energy for (a) a ${\mathrm{LaVO}}_3$ layer on top of $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$, where the incident beam is polarized perpendicularly to both $c_o$ and ${\left(ab\right)}_o$ axes, and (b) a ${\mathrm{LaVO}}_3$ layer on top of $\mathrm{Dy}\mathrm{Sc}{\mathrm{O}}_3$, where $E$ is perpendicular to $c_o$ axis. In both plots, the temperature of the antiferromagnetic order $T_{\text{SO}}$ is marked by the dashed black line.

Figure 4

(a) Black curve: SW of peak $A_1,A_2$ and their sum ($A$) for the compressive ${\mathrm{LaVO}}_3$ layer. In red: The muon initial spin polarization revealing the occurrence of the antiferromagnetic order ($T_N$) at 125 K. (b) Black curve: SW of peak $A$ and peak $B$ for the ${\mathrm{LaVO}}_3$ layer under tensile strain. In red: The muon initial spin polarization drops when the antiferromagnetic order sets in at 155 K.

Figure 5

Schematic representation of the epitaxial strain effect on the CF splitting and bandwidth of the V $t_{2g}$ orbitals. In the zero-strain state, the $xy$, $yz$, and $xz$ orbitals are split by an absolute value of $\sim 0.1$ eV [32]. Left side: A compressive epitaxial strain is applied to the $zx$ plane, which increases the original CF splitting between $xz$ and $yz$ orbitals. Right side: A tensile epitaxial strain is applied to the $xy$ plane, increasing the original CF splitting between the $xy$ and the $yz/xz$ orbital states. The strain-induced energy levels shifting ($\sim \pm 0.06$ eV) are accompanied by a decrease or increase of the first neighbor hopping amplitudes ($\sim \pm 0.25$ eV), which manifests in a bandwidth decrease or increase [32].