Electronic structure of semiconducting $\mathrm{Ce}{\mathrm{Fe}}_4{\mathrm{P}}_{12}$: Strong hybridization and relevance of single-impurity Anderson model

Semiconducting skutterudite $\mathrm{Ce}{\mathrm{Fe}}_4{\mathrm{P}}_{12}$ is investigated by synchrotron x-ray photoemission spectroscopy (PES) and x-ray absorption spectroscopy (XAS). $\mathrm{Ce}3d$ core-level PES and $3d\text{−}4f$ XAS, in combination with single-impurity Anderson model (SIAM) calculations, confirm features due to $f^0$, $f^1$, and $f^2$ configurations. The $\mathrm{Ce}3d\text{−}4f$ resonant-PES spectra provide the $\mathrm{Ce}4f$ density of states (DOS), which indicates the absence of a Kondo resonance at the Fermi level, but can still be explained by SIAM with a small gap in non-$f$ DOS. While $\mathrm{Ce}4f$ partial DOS from band structure calculations is consistent with the main $\mathrm{Ce}4f$ DOS, the importance of SIAM for core-level PES and XAS spectra quantifies the mixed valence for semiconducting $\mathrm{Ce}{\mathrm{Fe}}_4{\mathrm{P}}_{12}$, derived from strong hybridization between non-$f$ conduction and $\mathrm{Ce}4f$ DOS.

Figure 1

(Color online) (a) $\mathrm{Ce}3d$ core-level photoemission spectra for $\mathrm{Ce}{\mathrm{Fe}}_4{\mathrm{P}}_{12}$ measured at several excitation energies. (b) An expanded scale for the $f^1$ and $f^2$ parts in $\mathrm{Ce}3d_{5∕2}$ region.

Figure 2

(Color online) Comparison of (a) $\mathrm{Ce}3d$ core-level photoemission spectrum and (b) $\mathrm{Ce}3d\text{−}4f$ x-ray absorption spectrum with corresponding spectra calculated by the SIAM. Solid triangles mark energies at which off- and on-resonant photoemission spectra in Fig. 3 were measured.

Figure 3

(Color online) $\mathrm{Ce}3d\text{−}4f$ resonant photoemission spectra for $\mathrm{Ce}{\mathrm{Fe}}_4{\mathrm{P}}_{12}$ in comparison with the LDA band structure calculation ($\mathrm{Ce}4f$ pDOS) and SIAM. Off-resonant spectrum is compared to LDA $\mathrm{Fe}3d$ pDOS.

Figure 4

Schematic for the ground state configurations in the SIAM with an energy gap at the Fermi level. Dashed lines: normal metal DOS (no energy gap).