Lower and upper critical fields in the heavy electron superconductorUPt3

We report the first comprehensive measurements of the lower critical field, &, for both orientations H∥a axis and H∥c axis, of the heavy electron superconductor ${\mathrm{UPt}}_3$. An accurate & is obtained in a LC oscillator whose resonance frequency changes on flux entry into the superconductor. The & values obtained with this technique indicate a ‘‘kink’’ for H∥a axis only. With this same technique we also measure the upper &. The results obtained are compared with & and & in anisotropic superconductors.