Partial valence-band spectra of Fe in Cd1-xFexSe.

Abstract

We have studied the partial valence-band spectrum of Fe in ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Fe}}_{\mathit{x}}$Se by exploiting the resonant enhancement of the Fe 3d-derived features at the 3p-3d excitation threshold. Two resonance energies of 53.6 and 55.5 eV were detected by performing constant-initial-state measurements. The resonances correspond to transitions from the 3${\mathit{p}}^{6}$3${\mathit{d}}^{6}$ $^{5}$D ground state of the ${\mathrm{Fe}}^{2+}$ ion to 3${\mathit{p}}^{5}$3${\mathit{d}}^{7}$ final states with ${(}^{5}$P${,}^{5}$F) and $^{5}$D symmetry, respectively. By taking difference spectra between energy-distribution curves measured on and off resonance, we obtained partial Fe 3d-derived valence-band spectra that turned out to be quite different for the two resonance energies. In order to understand this difference and to derive ground-state properties from the valence-band spectra of the highly correlated Fe 3d system, a full-fledged configuration-interaction calculation for a [${\mathrm{FeSe}}_{4}$${]}^{6\mathrm{\ensuremath{-}}}$ cluster was performed.Taking the differences in coupling strength of the direct photoemission channels and the two core-excitation resonances into account, these model calculations reproduce the salient features of the partial valence-band spectra quite well. From this model calculation we obtain, among other things, a value for the Fe 3d correlation energy of about 7 eV. The ground state of Fe in ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Fe}}_{\mathit{x}}$Se turns out to be describable in terms of 83% of the \ensuremath{\Vert}${\mathit{d}}^{6}$〉 configuration and 17% of the \ensuremath{\Vert}${\mathit{d}}^{7}$L〉 configuration where an electron has been transferred from the ligand (L) to the 3d shell. The effective number of electrons on the Fe atoms is thus calculated to be 6.16. To obtain an independent estimate of the correlation energy we also measured the Fe 2p core lines and modeled the spectrum with a calculation using again the configuration-interaction scheme in the cluster approximation. The best agreement between model and spectrum holds for a correlation energy of 5.5 eV, which is significantly smaller than the value derived from the valence-band spectra.