Electronic structure of heavy fermion uranium compounds studied by core-level photoelectron spectroscopy

High-energy-resolution core-level and valence-band photoelectron spectroscopic studies were performed for the heavy fermion uranium compounds UGe ₂, UCoGe, URhGe, URu ₂Si ₂, UNi ₂Al ₃, UPd ₂, and UPt ₃ as well as typical localized and itinerant uranium compounds to understand the relationship between the uranium valence state and their core-level spectral line shapes. In addition to the main line and high-binding energy satellite structure recognized in the core-level spectra of uranium compounds, a shoulder structure on the lower binding energy side of the main lines of localized and nearly localized uranium compounds was also found. The spectral line shapes show a systematic variation depending on the U 5f electronic structure. The core-level spectra of UGe ₂, UCoGe, URhGe, URu ₂Si ₂, and UNi ₂Al ₃ are rather similar to those of itinerant compounds, suggesting that U 5f electrons in these compounds are well hybridized with ligand states. On the other hand, the core-level spectra of UPd ₂Al ₃ and UPt ₃ show considerably different spectral line shapes from those of the itinerant compounds, suggesting that U 5f electrons in UPd ₂Al ₃ and UPt ₃ are less hybridized with ligand states, leading to the correlated nature of U 5f electrons in these compounds. The dominant final state characters in their core-level spectra suggest that the numbers of 5f electrons in UGe ₂, UCoGe, URhGe, URu ₂Si ₂, UNi ₂Al ₃, and UPd ₂Al ₃ are close to but less than three, while that of UPt ₃ is close to two rather than to three.