High-resolution UV-photoemission of solids: success, limitations and the future

Numerous studies have demonstrated that a gratifying agreement is found bet ween UV-photoemission data and single-particle band calculations. However, many-body mechanisms can markedly modify this simple analysis and require a n interpretation in terms of quasi-particles. This aspect will be illustrat ed by photoemission data deeply influenced by two types of many-body intera ction: the very strong 4f electron correlation in heavy fermion systems and the important electron-phonon interaction modifying the surface state of B e(0001). Moreover, a complete breakdown of the quasi-particle concept occur s in one-dimensional (1D) metals and spectral functions calculations within the Tomonaga-Luttinger liquid formalism are required. Photoemission studie s devoted to quasi-1D metals confirm the predicted absence of Fermi step bu t systematically fail to reveal the collective spin and charge excitations (spinons and holons). We have tackled this problem on single-atom metallic Au chains prepared on a vicinal surface of Si(lll) and we observe an explic it hint of separated spin and charge excitations. These different examples illustrate the fact that the photoemission process is not setting any limit to the observation of very low-energy excitations but that the crucial pro blems arise from the quality and stability of the surfaces probed at low te mperature, (C) 2001 Elsevier Science B.V. All rights reserved.