Magnetic properties ofUPt3in the superconducting state

The unconventional phase diagram of superconducting ${\mathrm{UPt}}_3$ has been explored using measurements of magnetic properties at low external fields. We investigated the relaxation of vortices from metastable configurations as well as the low-field ac magnetic susceptibility $\chi$. The data yield a clear distinction between the behavior of vortices in the two low-field phases A and B. In the high-temperature A phase, vortices trapped in the bulk of the specimen after cycling it in a magnetic field creep out as expected with a rate which increases with increasing temperature. Bulk vortices in the B phase, on the other hand, remain so strongly pinned that no creep could be detected for those vortices. This behavior indicates that an intrinsic and very effective pinning mechanism exists in the low-temperature superconducting phase of ${\mathrm{UPt}}_3$. Furthermore, a rather small dissipation peak at the lower transition temperature $T_c^{-}$ could be detected in the out-of phase component of the magnetic ac susceptibility. Both these effects can be well explained with the notion that the low-temperature, low-field B phase of ${\mathrm{UPt}}_3$ is characterized by a superconducting order parameter which breaks time-reversal symmetry.