Electronic states of EuCu2Ge2 and EuCu2Si2 studied by soft x-ray photoemission spectroscopy

We have carried out angle-integrated photoemission spectroscopy (AIPES) and angle-resolved photoemission spectroscopy (ARPES) experiments using soft x rays on single crystals of ${\mathrm{EuCu}}₂{\mathrm{Ge}}₂$ and ${\mathrm{EuCu}}₂{\mathrm{Si}}₂$ grown by the Bridgman method to investigate their electronic structures. The AIPES results showed that the Eu ions in ${\mathrm{EuCu}}₂{\mathrm{Ge}}₂$ and ${\mathrm{EuCu}}₂{\mathrm{Si}}₂$ are in a divalent state and a nearly trivalent state, respectively, in accordance with the previously reported magnetic properties. The three-dimensional band structures and shapes of the Fermi surfaces of ${\mathrm{EuCu}}₂{\mathrm{Ge}}₂$ and ${\mathrm{EuCu}}₂{\mathrm{Si}}₂$ were studied by ARPES measurements. We found that the band structures near the Fermi level and Fermi surfaces of ${\mathrm{EuCu}}₂{\mathrm{Ge}}₂$ and ${\mathrm{EuCu}}₂{\mathrm{Si}}₂$ are very different from each other and are well reproduced by the band structure calculations based on density-functional theory for ${\mathrm{SrCu}}₂{\mathrm{Ge}}₂$ and ${\mathrm{YCu}}₂{\mathrm{Si}}₂$. This suggests that a charge transfer from the localized $4f$ states into the valence bands is responsible for the difference in the electronic states between ${\mathrm{EuCu}}₂{\mathrm{Ge}}₂$ and ${\mathrm{EuCu}}₂{\mathrm{Si}}₂$.