Investigation of superconducting gap structure and multi-vortex phase crossover in PbMo6S8 and PbMo6Se8

Despite significant interest in its multigap superconductivity and unusual positions on the Uemura plot, the multi-gap Chevrel phase superconductor PbMo6S8 still lacks knowledge about its superconducting gap structures. It has the highest upper critical field and superconducting transition temperature in this family of compounds. In contrast, in the sister compound PbMo6Se8, the upper critical field and the superconducting temperature are reduced by 15 and 4 times respectively. Our initial analysis indicates that the Pauli paramagnetic field dominates over the orbital limiting field in the cooper pair breaking mechanism in PbMo6S8, stipulating a probable triplet ground state, whereas in PbMo6Se8, the orbital limiting field dominates in that mechanism, stipulating a singlet ground state. Both compounds display an unconventional vortex state, evidenced by the fishtail effect and multi-vortex crossovers. Unfortunately, the earlier MuSR study [PRB 48, 16589 (1993)] doesn’t deal with the superconducting gap structure, local magnetic fields, and the multi-vortex crossovers in them. Further, the observation of multigap superconductivity [PRL 106, 017003 (2011)]] and the theoretical prediction of the spin-triplet ground state [PRM 2, 1 (2018)] is comparatively recent compared to the existing MuSR study of 1993. This demands a thorough reinvestigation of the MuSR on the samples to answer the existing questions and address the new findings of unconventional vortex states.