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The rare earths (also called lanthanides) are a group of 15 elements in the periodic table,
characterized by the gradual filling of 14 electrons in the 4f-shell. The rare earths form a
large number of metallic compounds in chemical combination with the other elements of
the periodic table. Their intermetallic compounds have proved useful in the study of local
moment magnetism. Some compounds of mostly Ce(4f 1 ) and Yb(4f 13 ) and to a lesser extent
Sm(4f 6 ) and Eu(4f 7 ), and 5f-U exhibit a variety of strongly correlated electron behavior known
as valence fluctuations, Kondo lattices, heavy fermions, Kondo insulators, unconventional
superconductivity, quantum criticality and non-Fermi liquid behavior. I shall present a brief
overview of these topics, which have been extensively studied during the last four to five
decades. |