Hidden Ordering and Heavy Mass in URu2Si2and Its Alloys

Various unusual properties in the anomalous superconductor URu 2 Si 2 in the normal state have been studied based on the standpoint of pair Jahn-Teller distortion. The pair distortion occurs well above the ordering temperature and thus the transition for the pair ordering is weak. Due to an inherent characteristic of pair ordering, the ordering is not perfect but includes unpaired U atoms. This is the origin of the weak antiferromagnetic ordering with the moment of 0.01–0.04 µ B /U and a heavy fermionlike behavior. No Kondo effect is observed. To confirm the above model, dilute alloy systems have also been studied and it is shown that all of them can be explained by a common model consistent with that of URu 2 Si 2 . The proposed two-channel Kondo state for dilute U atoms in the ThRu 2 Si 2 system was shown to be unacceptable.