Orbital occupancy and hybridization in strained $\mathrm{SrV}{\mathrm{O}}_3$ epitaxial films

Oxygen packaging in transition metal oxides determines the metal-oxygen hybridization and electronic occupation at metal orbitals. Strontium vanadate $\left(\mathrm{SrV}{\mathrm{O}}_3\right)$, having a single electron in a $3d$ orbital, is thought to be the simplest example of strongly correlated metallic oxides. Here, we determine the effects of epitaxial strain on the electronic properties of $\mathrm{SrV}{\mathrm{O}}_3$ thin films, where the metal-oxide sublattice is corner connected. Using x-ray absorption and x-ray linear dichroism at the $\mathrm{V}{L}_{2,3}$ and O $K$ edges, it is observed that tensile or compressive epitaxial strain change the hierarchy of orbitals within the $t_{2g}$ and $e_g$ manifolds. Data show a remarkable $2p-3d$ hybridization, as well as a strain-induced reordering of the $\mathrm{V}3d\left(t_{2g},e_g\right)$ orbitals. The latter is itself accompanied by a consequent change of hybridization that modulates the hybrid ${\pi }^{*}$ and ${\sigma }^{*}$ orbitals and the carrier population at the metal ions, challenging a rigid band picture.

Figure 1

(a) Top panel: θ-2θ scans of SVO films of $t=10\mathrm{nm}$, on STO, NGO, and LAO substrates. The continuous red lines are the results of the optimal simulation used to extract the $c$ axis and the film thickness. Bottom panel: The corresponding reciprocal space maps measured around the (-103) reflection. The SVO reflection is circled. (b) Experimental $c/a$ ratio for SVO films of various thicknesses (10, 20, and 70 nm), grown on LAO, NGO, and STO substrates. The dashed line indicates the expected $c/a$ values for fully strained films ($a=a_{\mathrm{S}}$), and $c$ is calculated using the Poisson equation (with Poisson ratio $\nu =0.28$). (c) Measured unit cell volume $V_{\mathrm{uc}}$ as a function of structural mismatch $f$. The dashed line indicates the unit cell volume calculated using the indicated Poisson ratio. Dotted lines in (b) and (c) indicate the $c/a$ ratio and unit cell volume of cubic bulk SVO, respectively.

Figure 2

(a) Temperature dependence of the resistivity of SVO (10 nm) films grown on various substrates. (b) Dependence of the room-temperature resistivity of films of 10, 20, and 70 nm grown on substrates having different lattice mismatch ($f$) as indicated. (c) Carrier concentration ($n$) per unit volume of film ($\mathrm{c}{\mathrm{m}}^{-3}$; left axis) and per unit cell (right axis). Error bars (shown only for the 10 nm series for the sake of clarity) are calculated assuming a maximum error of 5% in thickness determination and neglecting any possible contribution from the substrate.

Figure 3

X-ray absorption spectroscopy (XAS) of $\mathrm{V}{L}_{2,3}$ and O $K$ absorption edges recorded at room temperature of SVO films grown on STO, NGO, and LAO, and for thicknesses of (a) 10 nm and (b) 70 nm. The energy ranges of $\mathrm{V}{L}_{2,3}$ and O $K$ edges are indicated along the $x$ axis. Arrows of $M_1$ and $M_2$ at O $K$ edge prepeak doublet represent the main absorption lines between O $1s$ and O $2p$ states, hybridized with $3d\text{−}{t}_{2g}$ and $3d\text{−}{e}_g$ states. Inset in (a): Illustrative O $K$ XAS data for different $\mathrm{V}{\mathrm{O}}_x$ oxides (experimental data adapted from Hébert et al. [38]).

Figure 4

(a) Experimental arrangement for x-ray absorption spectroscopy (XAS) and x-ray linear dichroism (XLD) measurements. (b) and (c) Illustrative $H$- and $V$-polarized XAS spectra (collected at room temperature and grazing incidence $\theta ={30}^{\circ }$) of 10 and 70 nm SVO//STO film, respectively. Bottom spectra of the panels represent the XLD spectra [i.e., $I\left({\mathbf{E}}_{\mathbf{c}}\right)-I\left({\mathbf{E}}_{\mathbf{ab}}\right)$] for each substrate (STO, NGO, and LAO). Some reference energies, XLD-1 and XLD-2, for maxima of positive and negative dichroism, respectively, are indicated by vertical dashed lines. (d) Summary of the XLD maxima values of SVO films for various thicknesses (10, 20, and 70 nm) grown on STO, NGO, and LAO substrates.

Figure 5

O $K$ edge x-ray absorption spectroscopy (XAS) prepeak of 10 nm SVO films grown on (a) STO, (b) NGO, and (c) LAO. A significantly larger dichroism at $t_{2g}$ peak for STO than for NGO/LAO and the reverse at $e_g$ peak reflect changes of hybridization with strain. Data were recorded at room temperature.

Figure 6

Left axis: X-ray linear dichroism (XLD) at O $K$ edge for SVO films (10 nm thick) on substrates imposing different tetragonality ratios ($c/a$). Square symbols indicate the XLD at ${\pi }^{*}\left(t_{2g}\right)$ and ${\sigma }^{*}\left(e_g\right)$ absorption peaks, as indicated. Right axis: $I\left[{\pi }^{*}\left(t_{2g}\right)\right]/I\left[{\sigma }^{*}\left(e_g\right)\right]$ intensity ratios. Data were recorded at room temperature.

Figure 7

Central panel: Energy-band diagram for $\mathrm{SrV}{\mathrm{O}}_3$ thin films. The hybridized $(\sigma ,{\sigma }^{*})$ and $\left(\pi ,{\pi }^{*}\right)$ orbitals and their parentage are indicated. The symmetry broken hybridized orbitals under the effect of (a) tensile strain and (b) compressive strain.

Figure 8

(a) Sum of the integrated partial density of states (IDOS) of in-plane (left axis) and out-of-plane (right axis) $3d$ orbitals. (b) Similar data for in-plane (left axis) and out-of-plane (right axis) $2p$ orbitals. (c) Ratio between the total density of states of $2p$ and $3d$ character IDOS($2p$)/IDOS($3d$) in-plane (left) and out-of-plane (right).