Resonant valence-band satellites and polar fluctuations in nickel and its compounds

The photon energy dependences of the valence-band photoemission spectra of the Mott insulator NiO, the Pauli paramagnetic metal NiTe, the diamagnetic metal NiSb, and the semiconductor ${\mathrm{Cu}}_2$O have been measured in the vicinity of the transition-metal $3p$ threshold. Resonant satellites similar to those of Ni and Cu metal were found in all of these compounds. The resonance behaviors of the satellites and the main $3d$ emissions are also similar to those of the metals. From these data it is concluded that (1) the satellites and the resonance behaviors are in the first approximation of an atomic origin, (2) the Fano line shapes in the constant initial-state spectra of Ni and its compounds arise from the super-Coster-Kronig interaction between a discrete state $3p^{5}3d^{n+1\mathrm{}}$ and continuum states $3p^{6}3d^{n-1\mathrm{}}\epsilon l$ following the $3p\to 3d$ photoabsorption, (3) initial-state configuration mixing contributes significantly to the Ni "6-eV" satellite, and (4) the Ni metal main $3d$ band resonates strongly due to a $3d^{8}4s$ final state. Different interpretations of the Ni "6-eV" satellite are critically assessed in the light of these findings, and it is argued that these conclusions imply an appreciable amount of polar fluctuation to the $3d^{8}4s^2$ configuration in the Ni metal many-body ground state. The importance of Hund's-rule coupling for the ferromagnetism of Ni is also discussed in view of different interpretations of the Ni satellite.