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ARPES has played a crucial role in the investigation of 2D strongly correlated system over the last then years, as it offers a direct method to probe the character and dynamics of the low-lying electronic states responsible for superconductivity other many-body phenomena in solids. However, ARPES in the conventional UV range shows some limitations due to its surface sensitivity. For example, only a few reliable ARPES studies are available for YBCO-123 due to the controversy on the "surface state" and the influence of the cleaved surface on the CuO2 plane doping, which results in coexistence of metallicity and superconductivity in adjacent bilayers [1]. In case of 3D strongly correlated systems, moreover, ARPES encounters difficulties due to the final state effects, matrix element effects and broadening of the surface-perpendicular dispersions [2]. Even in 3D materials conventionally viewed as free-electron-like systems, the ARPES experiments [3] reveal clear discrepancies with the free-electron-like description of the final states. Pushing ARPES into the soft-X-ray range is essential to overcome the above shortcomings. One of the advantages of this energy range is enhancement of the photoelectron escape depth allowing to probe the electronic structure at different sub-surface layers. In addition, the concomitant improvement of the intrinsic perpendicular momentum resolution, truly free-electron character of the final states and simplified matrix element effects allow reliable extraction of 3D dispersions. The new soft-X-ray ARPES facility constructed at the ADvanced RESonant Spectroscopies (ADRESS) beamline [4] at the Swiss Light Source (SLS) delivers soft-X-ray radiation in the energy range 400 - 1600 eV with variable polarization, high energy resolution up to 35 meV at 1 KeV photon energy, and flux more than one order of magnitude better than the soft-X-ray ARPES beamlines existent worldwide. Such advanced parameters of the beamline set up new standards in ARPES studies of strongly correlated materials including 3D systems. [1] V.B. Zabolothyy et al., cond-mat/0608295 [2] V.N. Strocov, Journal of Electron Spectroscopy and Related Phenomena, 130, 65-78 (2003) [3] Ph. Hofmann et al. Phys. Rev. B 66, 245422 (2002) [4] http://sls.web.psi.ch/view.php/beamlines/adress/index.html |
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