Elastic Anomalies of Single Crystalline Rh17S15 Associated with Structural Fluctuation

We have studied elastic properties by ultrasonic measurements for a single crystal of Rh₁₇S₁₅ with the superconducting transition temperature T_C = 5.4 K. A pronounced elastic softening toward the T_c under zero magnetic field was observed in the temperature dependence of the shear elastic constant C_E = (C₁₁ - C₁₂)/2, presumably due to the high density of states of d bands at the Fermi level E_F. In the T dependence under several applied magnetic fields (H), the softening is gradually suppressed and the minimum shifts to the lower temperature side with increasing the H. From the results of various T and H scans, we summarized them in the magnetic phase diagram. We discuss the elastic properties and the mechanism of the elastic softening in terms of the electronic structure of Rh₁₇S₁₅ in the vicinity of E_F.