Theory of the two-peak photoemission spectra in cerium pnictides

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Abstract
Cerium and many of its compounds exhibit a double-peaked photoemission spectrum arising from the 4f states. Considerable conjecture as to the origin of this phenomenon has given rise to numerous models for the electronic structure of these materials. In this paper we show that this phenomenon arises from the nature of the excitation process. The initial state including the f states are assumed to be adequately described by a band calculation. The possible final states are then simulated. Two primary channels occur for the f states which are characterized as the fully-screened–poorly-screened peaks: In the fully screened channel, the f hole is screened by f states; in the poorly screened channel, the f hole is screened by d states. As a first approximation, the fully screened final state is described with use of the ground-state density of states. The poorly screened final state is simulated by creating an f hole at a central atom in a large unit cell—a supercell technique which periodically repeats the screened hole at an adequate separation to minimize interaction of the holes. The separation of the two peaks in the photoemission spectra is then determined with use of Slater transition-state arguments. Peak separations are found to be 2.71 eV for CeSb and 2.47 eV for CeP which compare favorably with the experimental values of 2.5 and 2.4 eV, respectively. No stable d-screened f hole is found in CeN, in agreement with the fact that it is not observed.