Competition between CEF Singlet and Kondo Singlet as Origin of Weak Antiferromagnetism and Resistivity Anomaly
In some uranium compounds with 5f2 configuration (U4+) the ground state in the crystalline electric field (CEF) is a nonmagnetic singlet. The singlet CEF state is also realized in the case of praseodymium compounds with 4f2 configuration (Pr3+). In such cases the spin entropy of the system goes to zero as temperature decreases even though interactions with conduction electrons or with f electrons at other sites are absent. This is in striking contrast with the case of cerium compounds with 4f1 configuration (Ce3+) where the Kramers degeneracy associated with the 4f1 configuration should lead to nonvanishing entropy if a Ce ion were isolated. It has been considered from this fact that the case of the singlet CEF ground state is much simpler because one need not consider the Kondo effect and related infrared effects which originate from interaction with conduction electrons. However, URu2Si2 for which the singlet CEF model accounts for the gross feature of the highly anisotropic susceptibility , gives clear indication of the Kondo effect in the resistivity at temperatures higher than 100 K. The Kondo-type behavior of the resistivity has also been found in other U systems with a singlet CEF ground state such as UPt2Si2 , UPd3 , UNi2Al3 , and UPd2Al3 .
KeywordsEntropy Anisotropy Uranium Cerium Praseodymium
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