Strong vortex pinning in the low-temperature superconducting phase of (U₁-xThₓ)Be₁3

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We have found a sharp transition at Tc2=350 mK in the vortex creep rate of a single crystal of (U₁-xThₓ)Be₁3 with Tc=523 mK (x=0.0275). For T<<Tc2, no creep of vortices is observed in a time scale of 10⁵ s, while for Tc2<T<Tc, vortices creep at very high rates (30 % of decay from a metastable configuration in the first 10⁵ s at T=400 mK). The sharp transition occurs at the same temperature at which the second jump in the specific heat appears in these samples. Similar low levels of creep rates have been reported by us in the low-T superconducting phase of UPt₃.