Proximity-Josephson effect (PJE) evidence for triplet pairing in UBe ₁₃ (invited)

The proximity-Josephson effect (PJE) is a powerful general method of determining the symmetry of the pair wave function in exotic superconductors. The method is simple and relatively insensitive to the surface condition of the sample. A superconducting probe (S) of known pairing symmetry (typically Nb or Ta) is brought into contact with the sample (N). Observations and arguments based on the de Gennes boundary condition at the NS interface both indicate formation of a local, proximity-induced superconducting region of depth ξ in the sample (N) under the probe. The induced pairs have the same symmetry as those of S. The expected pair-phase dependence of the coupling energy between the pairs in N and S leads to a Josephson current I_c(T), which may be observed up to a junction critical temperature T* which is typically ∼0.8 of the T_c of S. When the measurement temperature falls below the bulk T_c of N (the exotic superconductor), a pair wave function of possibly different symmetry forms in the bulk of N and overlaps the induced pair wave function near the probe. Weak interactions between the induced and bulk pairs occur. In the case of UBe₁₃ contacted with a Ta tip, the interaction weakly suppresses the induced pairs [which determine I_c(T)] leading to a reduction of the Josephson current at T<T _c. This observation of a negative S-wave proximity effect in superconducting UBe₁₃, in good agreement with a Ginzburg-Landau analysis, is strong evidence for triplet pairing in this heavy fermion compound.