Ultrahigh-resolution angle-resolved photoemission spectroscopy of La and Ce monochalcogenides

High quality single crystals of CeX_c and LaX_c with X_c = S, Se, or Te were grown, and a systematic study of the energy band structure was carried out by ultrahigh-resolution angle-resolved photoemission spectroscopy. In LaX_c, the bottom of the conduction d bands shifts toward higher binding energy with decreasing atomic weight of the chalcogen, i.e., with decreasing lattice constant in the same way as that caused by the physical pressure, whereas the top of the valence p bands more drastically moves to higher binding energy in the opposite direction of that caused by the physical pressure. Accordingly, the energy gap between the conduction and valence bands increases from LaTe to LaS. On the other hand, the electronic structure of CeX_c is well represented by that of the corresponding LaX_c plus the localized 4f state except for CeTe, in which a new band related to the 4f state is observed. Considering the systematic change of the electronic structure from CeS to CeTe, this band is likely to originate from the hybridization of the valence p-band and 4f state. The role of the p-f hybridization for their magnetic properties is discussed.