P. Vavassori

Evidence for correlation shifts in the photoemission spectrum of late 4d transition metal compounds

Abstract Photoemission spectra of the Rh 4d contribution to the valence band in diluted Rh-rare earth compounds (with Ce and Lu) deviate substantially from standard local density approximation (LDA) predictions, at variance with the results on Rh-rich phases. We attribute these deviations to incomplete screening of the final 4d hole due to the narrowing of the 4d band, related to the change in structure. We propose a scheme that improves considerably the coincidence between LDA and experiment by taking into account final state relaxation effects.

A high efficiency photon detector for parallel acquisition of UV inverse photoemission spectroscopy

We describe an ultraviolet photon detector realized for parallel acquisition of eight inverse photoemission spectra in the isochromatic mode, at photon energies from 10 to 25 eV. This simple and reliable device is obtained by combining an array of eight channeltrons corresponding to the various photon energies of the spectral and a single Csl-coated photocathode.

Surface electronic structure of CeCo2, CeRh2 and CeRh3 probed by valence band resonant photoemission spectroscopy

Abstract The 4f surface versus bulk electronic structure of three strongly hybridized Ce intermetallic compounds (CeCo 2 , CeRh 2 and CeRh 3 ) is investigated by exploiting the 4f photoemission resonant enhancement at the Ce 3d–4f and Ce 4d–4f thresholds. The much higher surface sensitivity at the shallow 4d threshold allows to extract the surface and bulk 4f electronic structure. Our results show a decreasing of the hybridization strength at the surface as compared to the bulk. The analysis of the whole set of results highlights the effect on the 4f spectral function obtained by varying the Ce metallic partner in the same crystal structure (CeCo 2 versus CeRh 2 ) and by changing the stoichiometry in the Ceue5f8Rh phase diagram (CeRh 2 versus CeRh 3 ).

Sensitivity of the valence states upon rare earth substitution in intermetallic compounds

Abstract The d valence states of some rare earth intermetallic Laves phases (i.e. CeCo2, LuCo2, CeRh2 and LuRh2) have been studied both experimentally and theoretically. This joint analysis shows that the experimental trends are satisfactorily reproduced by LDA calculations even if a straighforward comparison leads to sizable discrepancies which do not seem to be related to correlation effects. The analysis allowed us to discuss also the effects of the Ce vs Lu substitution in these isostructural compounds, assessing the limit of applicability of such an approach for a d states modeling.

Surface hybridization shift of the empty f spectral function in Ce compounds

Abstract Surface effects are detected between the f spectral functions obtained from UV and X-ray inverse photoemission spectra of a number of Ce compounds. They are: a variation of the I( f 1 ) I( f 2 ) intensity ratio, which indicates the presence of a less hybridized surface layer compared to the bulk, and a surface-to-bulk shift of the f2 component, which is larger than the expected surface core level shift as measured on isostructural Lu compounds. The extra shift (up to 0.3 eV) is interpreted as a hybridization effect in agreement with theoretical calculations. Some consequences on the on-site f-f correlation energy at the surface are drawn.

Valence band resonant photoemission spectroscopy on Ce compounds at Ce 3d-4f and 4d-4f thresholds: surface vs. bulk electronic structure

Abstract Electron correlation in 4f orbitals strongly influences the low energy properties of Ce and its inter-metallic compounds. By exploiting the different surface sensitivity of valence band resonant photoemission (VBRP) at Ce 3d-4f and Ce 4d-4f thresholds, a lowering of the 4f hybridization has been recently observed on going from the bulk to the surface layers. We report an extensive VBRP work at both the thresholds (3d and 4d) on three Ce compounds (Ce7Rh3, CeRh2, and CeCo2). In addition to surface effects, we found different resonance behaviours for f0 and f1 configurations. These differences should not be disregarded when extracting the 4f bulk and surface contributions from the photoemission spectra.

Effects of Ce vs. Lu substitution on the electronic structure of rare earth-transition metal compounds

Abstract We report the results of a combined UV photoemission and inverse photoemission spectroscopy investigation of some isostructural rare earth (R) intermetallic compounds, i.e. CeCo 2 , LuCo 2 , CeRh 2 , LuRh 2 , Ce 7 Rh 3 and Lu 7 Rh 3 , aiming to highlight the effects, on both the occupied and unoccupied d-derived partial density of states, of R substitution on different transition metal (M) hosts. These effects cannot be detected by X-ray spectroscopies because of the sizeable f-related contribution while, for cross-section reasons, UV spectroscopies may allow direct access to the d-like states. The results are discussed in terms of the different spatial extents of Ce and Lu 5d wavefunctions and of the degree of intermixing with the M partner d valence states.

Determination of the surface and bulk 4f spectral functions in CeIr compounds

Abstract The surface electronic structure of two Ceue5f8Ir compounds, CeIr5 and Ce7Ir3, is investigated by means of valence band resonant photoemission at the Ce 3d–4f and Ce 4d–4f thresholds. By taking advantage of the different surface sensitivity at the two thresholds, the 4f spectral weight has been decomposed into a surface and a bulk component. For both compounds, an increased localization of the 4f orbitals in the surface layers is observed, accompanied by a decreased hybridization with the valence band states. From the comparison between the two compounds, the different distribution of the 4f spectral weight between the 4f0 and 4f1 final states is interpreted by means of the difference in stoichiometry and crystal structure, which results in a different degree of f-d hybridization. A surface core level shift in the 4f0 final state is moreover observed for Ce7Ir3.

4f binding energies and surface core-level shifts of Lu compounds

Abstract A He II ( hv = 40.8 eV) investigation of the 4f emission in a number of Lu compounds (LuCo 2 , LuRh 2 , LuRh, Lu 7 Rh 3 ) is reported. The closed 4f shell and the large (> 0.7 eV) surface core-level shifts allow the spectral bulk and surface components to be accurately disentangled. The observed binding energy shifts of the bulk and surface 4f levels relative to the pure Lu are within 0.4 and 0.26 eV respectively. The results are discussed in terms of the Johansson and Martensson fully screened core-hole model, evaluating the compounds heat of formation within the Miedema semiempirical scheme.

4f surface components in Lu compounds by UV photoemission

Abstract We present a Hell ( hv = 40.8 eV) investigation on Lu and on a variety of Lu compounds (LuCo 2 , LuRh 2 , LuRh and Lu 7 Rh 3 ). Due to the closed 4f shell and the large (≥ 0.7 eV) surface core shifts (SCS) the bulk and surface components in the spectra can be disentangled accurately. In elemental Lu the SCS is 0.72 eV, a value slightly smaller than that estimated from less surface sensitive data [1]. The SCS increases with dilution (0.95 eV in LuCo 2 ) with a strong enhancement of the surface component (a factor of ∼ 4.4 greater in LuRh 2 than in Lu). The results are discussed in terms of cohesion energy and atomic coordination.