Valence-band structures of layered oxychalcogenides, LaCuOCh (Ch=S, Se, and Te), studied by ultraviolet photoemission spectroscopy and energy-band calculations

摘要

To examine the electronic structure of the valence band, ultraviolet photoemission spectra of a series of layered oxychalcogenides, LaCuOCh (Ch=S, Se, and Te), were measured. The measurements were conducted using He II, He I, and Ne I excitation lines to observe the excitation energy dependence of the spectral shape. Energy-band calculations based on a full-potential linearized augmented plain-wave method were performed. The calculated density of states and partial density of states were compared to the observed photoemission spectra. Five bands were observed in the valence band of LaCuOCh, and Ne I radiation remarkably enhanced two of them. The energy dependence of the photoionization cross section of atomic orbitals indicated that the two enhanced bands were due to the Ch p states. Energy calculations were used to assign the remaining bands. The electronic structure of LaCuOCh was further discussed using molecular-orbital diagrams to visualize the (La2O2)2+ and (Cu2Ch2)2– layers as large donor-acceptor pairs. The energy-band calculation and molecular-orbital diagram analyses suggested that the main difference among the valence-band structures of LaCuOCh (Ch=S, Se, and Te) originates from the variations in the energy position of the Ch p bands. The observed spectra are consistent with the results of the band calculations and clearly show the energy variations in the Ch p bands with respect to spectral shape and excitation energy dependence.