Revisiting the Yb electronic structure by low energy photoemission

Characterization of the electronic states of correlated materials is a challenging issue, insofar the details of the electronic structure in the Fermi region, which determine the wondrous phenomena they exhibit like phase transitions and superconductivity, differ at surface with respect to bulk. For their investigation, low-energy photoemission (LEPES) is experiencing a revival, due to the unprecedented energy resolution and to its claimed bulk sensitivity [1]. In our synchrotron based LEPES experiments on Yb thin films, grown on polycrystal W, an information depth at photon energy less than 10eV is found to be significantly smaller than predicted by the "universal curve" [2] and expected on the basis of the material dependence of the electron attenuation length [3]. Valence band profiles are found to be strongly dependent on the photon energy; in particular, huge density of electronic states at the Fermi level is measured by LEPES, its spectral weight being much larger than the 4f shallow levels which dominates PES at commonly used excitation energy (higher than 10eV). Photon energy dependence is used to discern their orbital character and surface vs bulk origin, while a Kondo effect can be excluded on the basis of temperature-dependent measurements. A prominent angle dispersing structure centered around 0.7 eV binding energy is related to the film thickness. A robust interpretation of all these findings claim for further theoretical effort. [1] J.D. Koralek et al., Phys. Rev. Lett. 96 (2006 ) 017005; T. Shimojima et al., Phys. Rev. Lett. 99 (2007) 117003 [2] M. P. Seah and W. A. Dench, Surf. Interface Anal. 1 (1979) 2 [3] F. Offi, S. Iacobucci, P. Vilmercati, A. Rizzo, A. Goldoni, M. Sacchi, and G. Panaccione, Phys. Rev. B, 77, (2008) 201101 (R) and ref therein (abstract n. 512)