Enhanced electron correlations at the ${\mathrm{Sr}}_x{\mathrm{Ca}}_{1-x}{\mathrm{VO}}_3$ surface

We report hard x-ray photoemission spectroscopy measurements of the electronic structure of the prototypical correlated oxide ${\mathrm{Sr}}_x{\mathrm{Ca}}_{1-x}{\mathrm{VO}}_3$. By comparing spectra recorded at different excitation energies, we show that 2.2 keV photoelectrons contain a substantial surface component, whereas 4.2 keV photoelectrons originate essentially from the bulk of the sample. Bulk-sensitive measurements of the O $2p$ valence band are found to be in good agreement with ab initio calculations of the electronic structure, with some modest adjustments to the orbital-dependent photoionization cross sections. The evolution of the O $2p$ electronic structure as a function of the Sr content is dominated by $A$-site hybridization. Near the Fermi level, the correlated V $3d$ Hubbard bands are found to evolve in both binding energy and spectral weight as a function of distance from the vacuum interface, revealing higher correlation at the surface than in the bulk.

Figure 1

Core level HAXPES spectra at 4.2 keV of ${\mathrm{CaVO}}_3$ and ${\mathrm{Sr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{VO}}_3$.

Figure 2

(a) HAXPES measurements at 4.2 keV of ${\mathrm{CaVO}}_3$ and ${\mathrm{Sr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{VO}}_3$, illustrating the Shirley-type background contribution (dashed line). The broadened total DOS (TDOS) is shown at the bottom of the figure for ${\mathrm{CaVO}}_3$ and ${\mathrm{SrVO}}_3$. (b) Broadened partial densities of states of ${\mathrm{CaVO}}_3$ and ${\mathrm{SrVO}}_3$, showing the contribution from each orbital component, and their respective weights. Each curve has been offset vertically by 2 ${\mathrm{eV}}^{-1}$ f.u.${}^{-1}$ for clarity. (c) HAXPES measurements after subtraction of the Shirley-type background, compared with the results of fitting the PDOS to the data (fit). Also shown is the theoretical cross-section corrected PDOS (c-PDOS).

Figure 3

High-statistics HAXPES spectra of the V $3d$ states of (a) ${\mathrm{CaVO}}_3$ and (b) ${\mathrm{Sr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{VO}}_3$ at 4.2 and 2.2 keV incident photon energies (vertically offset for clarity). The dashed (green) lines indicate the contributions from the tails of the O $2p$ band, and the black solid lines represent fits of the experimental spectra (see text). Finally, the dotted lines near $E_{\mathrm{F}}$ are the experimental spectra divided by the Fermi function (determined from Ag foil).

Figure 4

Results of fitting the HAXPES spectra to Gaussian components. (a) Fitted spectra (excluding the O $2p$ tails) shown alongside the UPS spectra of Ref. [3]. All spectra have been broadened to have the same effective instrument resolution function, corresponding to $\sigma =220$ meV, and normalized to the intensity of the LHB. (b) Binding energy of the LHB for the two compounds, shown as a function of the estimated IMFP, $\lambda$. The horizontal dashed lines indicate the separation between the QP and LHB from bulk-sensitive RXES [3]. The error bars represent estimated statistical errors associated with the fitting.

Figure 5

Comparison between various probes of different depth sensitivities of the correlated electronic structure of ${\mathrm{CaVO}}_3$ and ${\mathrm{Sr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{VO}}_3$. The approximate depth sensitivity of each technique is illustrated on the left, with reference to the SrO-terminated surface of cubic ${\mathrm{SrVO}}_3$(100). The HAXPES spectra are those shown in Fig. 3 after subtraction of the O $2p$ tails. Corresponding Ag reference spectra recorded at the same energies are also shown. The UPS and RXES results are from Ref. [3].