Strong vortex pinning in the low-temperature superconducting phase of (U1-xThx)Be-13

We have found a sharp transition at T-c2 = 350 mK in the vortex creep rate of a single crystal of (U1-xThx)Be-13 with T-c = 523 mK (x = 0.0275). For T much less than T-c2, no creep of vortices is observed in a time scale of 1 0(5) s, while for T-c2 < T < T-c, vortices creep at very high rates (30% of decay from a metastable configuration in the first 10(5) s at T = 400 mK). The sharp transition occurs at the same temperature at which the second ju mp in the specific heat appears in these samples. Similar low levels of cre ep rates have been reported by us in the low-T superconducting phase of UPt 3.(1)