Low-field diamagnetic response of the superconducting phases in UPt ₃

By measuring the gradual disappearance of diamagnetism in a single crystal of UPt₃ upon warming through the superconducting transition in a constant magnetic field, the authors have investigated the temperature dependence of a characterstic field H_c1*, closely related to the superconducting lower critical field. H_c1. The amplitude of the applied field ranged from 1 to 60 Oe, for directions along the b- and the c-axis of the hexagonal structure. A distinct change in the slope of H _c1*(T) is observed for both directions, at a temperature which coincides well with the lower transition detected by specific heat within the superconducting regime. It is further found that the initial slope of H _c1* is roughly isotropic, in contrast with the marked anisotropy of dH_c2/dT, and that the Meissner effect is extremely small despite the long electronic mean free path.