Muon Knight shift and zero-field relaxation in (U,Th)Be13

We report μ+ zero-field relaxation and Knight shift studies of the heavy-fermion superconductors U1-xThxBe13, x=0 and 0.033. The Knight shift in UBe13 shows a strong decrease as the temperature is reduced in the superconducting state, unlike U0.967Th0.033Be13 in which the shift remains at about the normal state value. If the superconducting state in UBe13 has odd-parity, the decrease of Kμ below Tc suggests that the order parameter is pinned to the lattice. Either spin-orbit scattering or the existence of two distinct superconducting states with different spin susceptibilities in the (U,Th)Be13 system would explain the differences observed in the Th-doped and pure UBe13 materials. The latter hypothesis would exclude conventional BCS superconductivity. No evidence for magnetic order is seen in the zero-field relaxation rate for either material down to 0.3 K. © 1986 J.C. Baltzer A.G., Scientific Publishing Company