Unconventional strong pinning in the low-temperature phase of U 0.9725Th 0.0275Be 13

Abstract

We investigated low-field vortex dynamics in a single crystal of U 0.9725Th 0.0275Be 13.We found a sharp transition in the vortex creep rate at the lower transition temperature T c2, coincident with the second jump in the specific heat. In the high-temperature phase, rather strong creep rates are observed. In the low-temperature phase, the rates drop to undetectably low levels. This behaviour indicates that a very strong pinning mechanism is present in the low-temperature phase of U 0.9725Th 0.0275Be 13, which could be explained by the existence of domain walls, separating discreetly degenerate states of a superconductor, that can sustain fractional vortices and thus act as very strong pinning centers.