Electronic structure of EuCu ₂Ge ₂ studied by resonant photoemission and x-ray absorption spectroscopy

The electronic structure of EuCu ₂Ge ₂ single crystal has been investigated by the valence band photoemission across the Eu 4d-4f resonance and Eu L ₃ x-ray absorption edge spectroscopy. Signatures of both the Eu2 ⁺ and Eu3 ⁺ states are observed in the resonant photoemission near the 4d threshold of Eu at 140 and 143 eV, respectively. The constant initial state spectra for Eu2 ⁺ and Eu3 ⁺ states exhibit Fano-type resonance profile. The experimental spectra have been interpreted with the help of first-principles density functional calculations within the generalized gradient approximations taking into account the strong intra-atomic (onsite) interaction Hubbard U term. The Eu 4f related resonant features are found to be hybridized with the Cu and Ge states. Supporting evidence for the existence of Eu in Eu2 ⁺ (92%) and Eu3 ⁺ (8%) states are also obtained from the x-ray absorption edge spectra, which probe the bulk response. Both the resonant photoemission and Eu L ₃ x-ray absorption edge spectroscopy gives the evidence that Eu is present in the inhomogeneous and static mixed valence state in EuCu ₂Ge ₂.