Conventional s-wave superconductivity in the non-centrosymmetric compounds Re3W, Re3W0.5Nb0.5, and Re3W0.5Ta0.5

Abstract

We carried out an investigation of the structural and superconducting properties of Re3W1−xMx alloys, which can crystallize in both a centrosymmetric (CS) hexagonal, and a non-centrosymmetric (NCS) cubic α-Mn phase. The superconducting transition temperatures (Tc) of the CS and NCS phases in pure Re3W are ∼ 9.1 and ∼ 7.7 K, respectively. While boules fast cooled from the melt have a mixture of CS phases, the NCS phase can be stabilized upon annealing at very high temperatures (∼ 1700 °C), or upon the partial substitution of the transition metals M = Ta, or Nb for W. X-ray diffraction and wavelength dispersive X-ray spectroscopy (WDS) data suggest that the optimal composition for the formation of the α-Mn phase directly from the melt in Re3W1−xMx is x ≈ 0.5. A detailed study of the superconducting properties of the Re3W1−xMx solid solutions was carried out by means of measurement of magnetization, ac magnetic susceptibility, electrical transport, and low temperature specific heat. As the M content is increased, the superconducting critical temperature Tc progressively decreases, though the overall superconducting properties remain similar to those of the parent NCS Re3W compound. The partial substitution at the W site does not seem to affect the pairing mechanism. Given the exponentially vanishing character of the electronic specific heat Ce(T) at low temperatures, the superconducting behavior of the NCS alloys is consistent with a nodeless, s-wave superconducting gap symmetry in all NCS samples. These findings add the pseudo-binary Re-M-W systems to the small roll of NCS materials where the pairing symmetry of the superconducting state can be explored.