Pressure-induced f-electron delocalization in the U-based strongly correlated compounds UPd_3 and UPd_2Al_3: Resonant inelastic x-ray scattering and first-principles calculations

摘要

We have measured the uranium L_3 absorption and resonant emission spectra in the localized magnetic compound UPd_3 and heavy fermion UPd_2Al_3 as a function of pressure. The spectral line shape of the absorption edge is found to vary rapidly in UPd_2Al_3 with a notable broadening of the white line above the structural transition around 25 GPa while it shows a more gradual variation in UPd_3 over the considered pressure range (0-40 GPa), indicating different responses of the f-d electrons to the compressed lattice in the two compounds. The U L_3 absorption spectra in both UPd_3 and UPd_2Al_3 and their pressure dependence were further simulated via first-principles band calculations within the linear muffin-tin orbital approach. The calculations reproduce the main features of the experimental absorption edges. The calculated pressure dependence of the f charge reveals a stronger localization of the f electrons in UPd_3 which shows a remarkably stable valency under pressure, close the nominal value of 4. On the contrary, our results point to a mixed valent U~(4-δ) ground state in UPd_2Al_3 at ambient conditions, evolving into a U~(4+) (or possibly U~(4+δ)) configuration at high pressure. The f-electron delocalization could be responsible for the known structural transition in UPd_2Al_3.