Unconventional strong pinning in the low-temperature phase of U0.9725Th0.0275Be13

We investigated low-held vortex dynamics in a single crystal of U-0.9725 Th -0.0275 Be-13.We found a sharp transition in the vortex creep rate at the l ower transition temperature T-c2, coincident with the second jump in the sp ecific heat. In the high-temperature phase, rather strong creep rates are o bserved. In the low-temperature phase, the rates drop to undetectably low l evels. This behaviour indicates that a very strong pinning mechanism is pre sent in the low-temperature phase of U0.9725Th0.0275Be13, Which could be ex plained by the existence of domain walls, separating discreetly degenerate states of a superconductor, that can sustain fractional vortices and thus a ct as very strong pinning centers. (C) 2000 Elsevier Science B.V. All right s reserved.