DIFFERENT ELECTRONIC GROUND-STATES IN THE HEXAGONAL AND CUBIC PHASES OF UAU2SN

For cubic and hexagonal UAu2Sn, the specific heat and elastic modulus have been measured down to 1.3 K in the magnetic field up to 14 T. We have clarified that the 5f electrons in the cubic and hexagonal phases are localized above 120 K and the both phases are characterized as a nonmagnetic heavy-fermion system and an antiferromagnetic heavy-fermio n system, respectively. In the cubic phase, the 5f ground state in cry stal-electric-field is determined to be a singlet, and the first excit ed state a triplet. In the hexagonal phase, both of the ground and fir st excited states are doublets and the ground doublet has an electroni c multipolar degeneracy. The difference between the ground states of t he two phases is discussed in terms of an electric multipole moment in addition to the competition between the Kondo effect and the RKKY int eraction.