Lattice Instability and Elastic Response in the Heavy Electron System URu_<b>2</b>Si_<b>2</b>

Abstract

We have performed thermal expansion and elastic constant measurements of the heavy electron system URu 2 Si 2 , focusing attention on the Γ 3 ( x 2 - y 2 ) and Γ 4 ( x y ) symmetry of the tetragonal group D 4 h . It is reconfirmed that there is no sizable uniform spontaneous distortion for these two types of symmetry through the puzzling phase transition at T 0 =17.5 K. On the other hand, a weak but significant tendency of softening are found below about 70 K in the transverse ( c 11 - c 12 )/2 mode. The results strongly suggest the presence of a lattice instability in the Γ 3 symmetry. From the results, proposed crystalline-electric-field models are also discussed.