ELASTIC-CONSTANT ANOMALIES AND THE SUPERCONDUCTING B-T PHASE-DIAGRAMS OF UPT3 AND URU2SI2

We present elastic constant measurements for the heavy fermion superconductors UPt3 and URu2Si2. The temperature and field dependence of elastic anomalies is studied and used to construct superconducting B-T phase diagrams for these compounds. Ultrasonic attenuation and AC susceptibility measurements are also discussed. The results are interpreted within a phenomenological model assuming unconventional doubly degenerate E1g(UPt3) and E(g) (URu2Si2) superconducting order parameters. Both compounds exhibit a spin density wave (SDW) already far above the superconducting transition. The coupling of SDW to the superconducting order parameters, its consequences and problems are discussed. The elastic constant anomalies are investigated within a phenomenological model for the strain-order parameter coupling. It is found that similar to the normal state behaviour Gruneisen parameter coupling to longitudinal modes is the dominant mechanism. Estimates of coupling parameters are given and compared to results from the pressure dependence of specific heat results. The double transition observed in both materials is intrinsic only for UPt3. In URu2Si2 it is not allowed by symmetry but is due to phase inhomogeneities.