LOW-FIELD DIAMAGNETIC RESPONSE OF THE SUPERCONDUCTING PHASES IN UPT3

By measuring the gradual disappearance of diamagnetism in a single crystal of UPt3 upon warming through the superconducting transition in a constant magnetic field, we have investigated the temperature dependence of a characteristic 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.