Non-centrosymmetric superconductivity in Re-based solid solutions

Rhenium based superconducting compounds Re₃M (M = W, Mo, Nb, Ta) crystallize in the non-centrosymmetric cubic α-Mn structure and have critical temperatures T_C ranging from ~ 4 to 9 K. Polycrystalline samples of Re₃M and its solid solutions were synthesized through an arc-melting furnace and subsequently heat treated at 1700 °C. Atomic substitutions of W by Ta or Nb in Re₃W lead to single phase materials even in as-cast samples, a feature not observed in the parent compound. From the superconducting point of view, the low temperature heat capacity c_p(T) data suggest conventional s-wave pairing, which is also supported by magnetization M(T), electrical resistivity ρ(T), and thermal conductivity κ(T) measurements. The estimated upper critical fields H_c2(0), however, are sizeable in comparison with the Pauli limiting field, and may be related to either a possible triplet channel or even disorder. Electron tunneling measurements of the superconducting density of states of Re₃M materials are also presented to provide further insight into the superconducting pairing mechanism. The results are discussed within the context of the emergence of superconductivity in non-centrosymmetric materials.