Josephson effect between superconductors in possibly different spin-pairing states

The Josephson current between two weakly coupled superconductors of which one has pairs in a spin-triplet state is considered. It is shown that under quite general conditions there is no Josephson effect up to second order in the transition-matrix elements between a superconductor with spin-triplet pairs and one with spin-singlet pairs. This offers a possibility to investigate experimentally whether a particular superconductor has spin-triplet pairs by coupling it weakly to a well-known spin-singlet pairing superconductor. Some superconducting materials which have been suggested earlier as possibly having spin-triplet pairs to account for their measured properties are investigated by forming junctions with a niobium point and looking at the Josephson effect. It turns out that all these junctions behave normally as far as the ac Josephson effect is concerned. We therefore provide strong experimental evidence that the investigated materials, viz., U₆Fe, Th₇{Co}₃$, ${Th}₇Fe₃CeRu₂, Ce_1-xGdₓRu₂, Zr₂Co, and Zr₂Ni, are singlet superconductors.