Unconventional vortex dynamics in superconducting states with broken time-reversal symmetry

We report vortex dynamics in the unconventional superconductors Sr2RuO4, thoriated UBe13 and compare it with previous data on UPt3 [A. Amann, A. C. Mota, M. B. Maple, and H.v. Lohneysen, Phys. Rev. B 57, 3640 (1998)]. In all three systems, a pinning mechanism, which is very distinct from the standard pinning by defects, can be associated with the appearance of broken time-reversal symmetry in the superconducting state. The pinning mechanism is so strong that no vortex creep is observed in a time scale of several hours. Our observations could be explained by the presence of domain walls, separating different degenerate superconducting states, as proposed by Sigrist and Agterberg [Prog. Theor. Phys. 102, 965 (1999)]. A conventional vortex approaching such a domain wall can decay into vortices with fractional flux quanta. Domain walls occupied with strongly pinned fractional vortices, represent efficient barriers for vortex motion and thus prevent relaxation towards equilibrium. In the case of UPt3 and U0.9725Th0.0275Be13, two consecutive phase transitions are known to occur at H=0, of which the low temperature one leads to a superconducting phase with broken time-reversal symmetry. In both systems, one observes a sharp drop of initial creep rates by more than three orders of magnitude to undetectabely low levels at their second superconducting transition. In Sr2RuO4 time-reversal symmetry is reported to occur right below T-c. However, we do not observe unconventional pinning immediately below the superconducting transition, but zero creep sets in only much below T-c. While in U0.9725Th0.0275Be13 and UPt3, the drop in creep rates at the lower superconducting transition temperature is very sudden and strong, in Sr2RuO4 it looks more like a crossover.