Elastic Constant in Magnetic Fields and Singlet–Triplet State of Heavy Fermion Superconductor PrOs₄Sb₁₂

Abstract

We have made ultrasonic measurements on a filled skutterudite PrOs 4 Sb 12 showing a heavy fermion superconductivity at T C =1.85 K. The softening of the elastic constant with decreasing temperature has been found above T C . The ( C 11 - C 12 )/2 in applying magnetic fields along the [110] axis at 1.0 K exhibits a minimum around 8 T, which is well accounted for by the one-ion quadrupolar susceptibility reflecting the level crossing of one of an excited Γ 4 (2) triplet to a ground state Γ 1 singlet. The softening of ( C 11 - C 12 )/2 proportional to reciprocal temperature just above T C may be dominated by the tunneling of an off-center Pr-ion in an Sb-icosahedron cage.