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V.N. Strocov and L. Patthey
Most of our knowledge about the electronic structure with resolution in the k‑space has been achieved by angle-resolved photoemission spectroscopy (ARPES). This technique has however a fundamental flaw in that due to the involvement of the surface the control over full 3-dimensional k requires knowledge of the final-state dispersions and lifetimes to recover the surface-perpendicular wavevector k^. At the low-energy limit of the final-state energies typically employed in the ARPES experiment (below ~ 40 eV) this information can be unveiled by Very-Low-Energy Electron Diffraction (VLEED). The energies of the VLEED spectral structures give energies of characteristic points in the final-state dispersions, and their broadenings reflect the lifetimes. A vast body of the VLEED experimental data demonstrates that for many materials the final states, contrary to the conventional free-electron-like picture, can feature dramatic deviations from that such as non-parabolic dispersions and multiband composition (often referred to as umklapp bands). Combining ARPES with the VLEED derived final states allows complete resolution of the electronic structure in 3-dimensional k‑space under control over the intrinsic resolution of the ARPES experiment due to broadening in k^. This innovative approach is illustrated on some weakly and strongly correlated metals (Cu, Ni), and quasi-2-dimensional materials (graphite, TiTe2, Bi2Sr2CaCu2O8). Analysis of the existing ARPES data demonstrates that the non-free-electron effects in the final states, in particular the multiband composition, can persist up to energies around 400 eV. The alternative approach is therefore to push the ARPES experiment its high-energy limit in the soft-X-ray range, where the final states become truly free-electron-like. Additional virtues of the soft-X-ray ARPES include increasing photoelectron mean free path and thus intrinsic resolution in k^, and simplified matrix element effects. |
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