M-edge x-ray absorption spectroscopy of 4f instabilities in rare-earth systems (invited)

Abstract

Soft x‐ray absorption spectroscopy in the M‐edge region is explored as a tool for studying 4f instabilities. The quasiatomic M_IV,V spectra, recorded by total‐electron yield under UHV conditions, carry information on the initial‐state 4f occupancy. For compounds of the heavy rare earths, even at high dilution (e.g., Tm_0.05Y_0.95Se), mean valences can be derived, which agree with those from lattice‐constant systematics (e.g., for TmSe and Sm_0.3Y_0.7S). A moderate surface sensitivity allows the observation of surface‐induced valence changes on EuPd₃, TmS, and SmAl₂. The M_IV,V spectra studied for 10 Ce compounds fall in two categories: pure 3d⁹4f¹ final‐state multiplets for γ‐like compounds, and additional peaks at about 5 eV higher energies for α‐like and intermediate systems. With increasing intensity of the satellites, which are assigned to 3d⁹4f¹ final states, the 3d⁹4f² multiplet structure gets washed out. An interpretation of the spectra with the recent Anderson‐impurity theory of Gunnarsson and Schönhammer leads to 4f occupancies greater than 0.7, even in extreme α‐like systems. In addition, M_III‐edge spectra exhibit one peak in γ‐like and two peaks in α‐like and intermediate Ce systems, which are assigned to 3p⁵4f¹ and 3p⁵4f⁰ final corestates. With increasing α‐like character, the 4f¹ peak is found to move to higher energies presumably due to a decrease in 4f screening with increasing 4f hybridization.