HUGE LATTICE SOFTENING IN CERU2 WITHOUT STRUCTURAL TRANSITION

Symmetrized elastic moduli of high-quality single-crystalline CeRu2 ha ve been measured between 2 and 300 K by means of an ultrasonic techniq ue. Both transverse moduli (C-11 - C-12)/2 and C-44 soften markedly fr om 300 to similar to 20 K without structural transition. In contrast, the bulk modulus C-B shows a normal temperature dependence without sof tening. These results indicate that there exists a marked structural f luctuation corresponding to a pure transverse strain in the normal con ducting state of CeRu2. Our analysis, taking account of the deformatio n potential coupling, reveals that a narrow band with relatively high density of states at E(F) is responsible for the huge lattice softenin g.