Resonant Soft-X-Ray Emission as a Bulk Probe of Correlated Electron Behavior in Metallic ${\mathrm{Sr}}_x{\mathrm{Ca}}_{1-x}{\mathrm{VO}}_3$

The evolution of electron correlation in ${\mathrm{Sr}}_x{\mathrm{Ca}}_{1-x}{\mathrm{VO}}_3$ has been studied using a combination of bulk-sensitive resonant soft x-ray emission spectroscopy, surface-sensitive photoemission spectroscopy, and ab initio band structure calculations. We show that the effect of electron correlation is enhanced at the surface. Strong incoherent Hubbard subbands are found to lie $\sim 20\%$ closer in energy to the coherent quasiparticle features in surface-sensitive photoemission spectroscopy measurements compared with those from bulk-sensitive resonant soft x-ray emission spectroscopy, and a $\sim 10\%$ narrowing of the overall bandwidth at the surface is also observed.

Figure 1

Bulk (a) XAS and (b) RXES measurements of CVO and SCVO. The spectra of a 46 nm thin film of CVO [16] are also shown for comparison. The excitation energies used in the RXES measurements of (b) are shown by the circles in (a). The vertical line in (a) indicates separate V $L$-edge (left) and O $K$-edge (right) measurements, which have been separately scaled for clarity.

Figure 2

Comparison of PES and RXES spectra. (a) PES spectra of CVO and SCVO compared with the LDA DOS of ${\mathrm{CaVO}}_3$ and DMFT calculations of ${\mathrm{CaVO}}_3$ and ${\mathrm{SrVO}}_3$ (reproduced from Ref. 5). (b) PES spectra compared with the RXES spectra at excitation energy (f). The PES and DMFT spectra have been broadened to approximate the RXES measurement (see text), and the RXES spectra are shown rebinned to double the statistical precision.

Figure 3

O $K$-edge XES spectra of CVO and SCVO. The corresponding O and V PDOS from the LDA are shown for comparison after convolution with a Lorentzian of 0.22 eV HWHM and a Gaussian of 0.35 eV FWHM to account for lifetime and instrument broadening respectively. The $E_{\mathrm{F}}$ corresponds to that of the LDA calculations.