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E. Lundgren, J.N. Andersen, R. Nyholm, X. Torrelles, J. Rius, A. Delin, A. Grechnev, O. Eriksson, C. Konvicka, M. Schmid, P. Varga:
"Geometry of the valence transition induced surface reconstruction of Sm(0001)";
Physical Review Letters, 88 (2002), 13; 1361021 - 1361024.

We present a structural determination of the surface reconstruction of the Sm(0001) surface using surface x-ray diffraction, scanning tunneling
microscopy and ab-initio calculations. The reconstruction is associated with a very large (22 %) expansion of the atomic radius for the top
monolayer surface Sm atoms. The mechanism driving the surface reconstruction in Sm is unique among all elements and is closely connected to
the strong correlations of the 4f electrons in Sm and the intermediate valence observed in certain Sm compounds. The atoms constituting the top
monolayer of Sm(0001) have vastly different chemical properties compared to the layer underneath and behave as if they were an adsorbate of a
different chemical species.

http://www.iap.tuwien.ac.at/www/SURFACE/abstracts/A2002E06.HTML