Bulk and surface electronic structures of CePdX „ X Ä As,Sb… studied by 3 d -4 f resonance photoemission T. Iwasaki, S. Suga, S. Imada, A. Sekiyama, K. Matsuda, M. Kotsugi, K.-S. An,* T. Muro, ² S. Ueda, and T. Matsushita ² Department of Material Physics, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka 560-8531, Japan Y. Saitoh and T. Nakatani Japan Atomic Energy Research Institute, SPring-8, Mikazuki, Hyogo 679-5198, Japan H. Ishii and O. Sakai Department of Physics, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan R. Takayama ‡ and T. Suzuki Department of Physics, Tohoku University, Sendai 980-8578, Japan T. Oguchi Department of Physics, Hiroshima University, Higashi-Hiroshima 739-8526, Japan K. Katoh Department of Mathematics and Physics, The National Defense Academy, Yokosuka 239-8686, Japan A. Ochiai Department of Material Science and Technology, Niigata University, Niigata 950-2181, Japan ~Received 8 September 1999! We have performed Ce 3 d -4 f resonance photoemission spectroscopy ~RPES! for CePdAs and CePdSb and compared the results with those of Ce 4 d -4 f RPES. The 3 d -4 f RPES spectra are remarkably different from the 4 d -4 f RPES spectra, showing the smaller contribution of the surface electronic structures in the 3 d -4 f RPES. On the other hand, the 3 d -4 f and 4 d -4 f resonance-minimum spectra for CePdSb are well described by the band-structure calculations for LaPdSb by taking the photoionization cross sections into account. This indicates that the surface effect is negligible in the resonance-minimum spectra. The theoretical calculation based on the single-impurity Anderson model well reproduces the surface- and bulk-sensitive Ce 4 f spectra of both compounds, revealing that the difference between the surface and bulk electronic states originates mainly from the surface core-level shift of the bare 4 f level. The spectral difference between the two compounds is explained by the different energy dependence of the hybridization strength. I. INTRODUCTION Many cerium compounds have been extensively studied by various experimental and theoretical methods because of their fascinating physical properties, for example, the Kondo effect, heavy-fermion phenomena, and valence fluctuation. They originate from peculiar behaviors of strongly correlated Ce 4 f electrons near the Fermi level ( E F ). The Ce 4 f elec- trons show itinerant characters through the hybridization be- tween the Ce 4 f and other valence electron states. In order to probe the Ce 4 f electron states, photoemission spectroscopy is a very useful method. 1 In particular, resonance photoemis- sion spectroscopy ~RPES! using synchrotron radiation is very effective in revealing the electronic states such as Ce 4 f orbit. 2 A widely used Ce 4 d -4 f RPES has an advantage of good energy resolution, which enables one to observe such fine structures as the Ce 4 f spin-orbit splitting of the order of 300 meV. 3,4 However, the small mean free path of the pho- toelectron is a weak point of the 4 d -4 f RPES for probing bulk electronic states. On the other hand, Ce 3 d -4 f RPES measurements provide relatively bulk-sensitive information of electronic states. It has been reported that the Ce 3 d -4 f RPES spectra show Ce 4 f spectral line shapes remarkably different from those of the 4 d -4 f RPES, 5–11 indicating that the Ce 4 d -4 f RPES spectra strongly reflect contributions from the surface region due to the shorter mean-free path of the excited electron. However, the energy resolution of the 3 d -4 f RPES has so far been not sufficient to resolve such fine structures near E F , making it difficult to quantitatively discuss the difference between the surface and bulk elec- tronic states. Very recent development in high brilliance syn- chrotron radiation source and instrumentation enable us to do the high-energy excitation RPES with sufficient energy reso- lution. Thus we can compare the details of the bulk-sensitive 3 d -4 f RPES results with the surface-sensitive 4 d -4 f RPES spectra. CePdAs and CePdSb are two dimensional layered com- pounds, which are composed of the Ce layer and the Pd-X layer piled up along the c axis. The crystal structure of CeP- dAs is the hexagonal ZrBeSi type with the Pd and As atoms in a plane while CePdSb has the hexagonal LiGaGe-type crystal structure, where the Pd and Sb atoms form puckered PHYSICAL REVIEW B 15 FEBRUARY 2000-I VOLUME 61, NUMBER 7 PRB 61 0163-1829/2000/61~7!/4621~8!/$15.00 4621 ©2000 The American Physical Society