LONDON FIELD PENETRATION IN HEAVY FERMION SUPERCONDUCTORS

We present results of a series of high resolution, low field d c-magnetization measurements on the heavy fermion superconductors UPt3, UBe13, U1-xTh(x)Be13 and CeCu2Si2, from which values of the magnetic penetration depth lambda can be extracted. A study of the temperature variation reveals a T2 power law in all cases. This can not be reconciled with a BCS-like isotropic energy gap but may be explained by the presence of low energy quasiparticle states inside the gap. In the case of very pure superconductors, one such possibility is the assumption of point-nodes in the gap function. We argue, however, that an interpretation in terms of resonant impurity scattering in various anisotropic superconducting states is more likely to explain a broad range of experimental data. The results on differently oriented single crystals of UBe13 and UPt3 reveal no pronounced anisotropy related to the crystal lattice. Absolute values of lambda at T = 0 are deduced by a novel method in which the field is first screened out from the sample by means of an evaporated superconducting film (of lower T(c)). Above this lower T(c) the sudden penetration of field into the sample can then be measured absolutely.