Noncentrosymmetric superconductor with a bulk three-dimensional Dirac cone gapped by strong spin-orbit coupling

The layered, noncentrosymmetric heavy element PbTaSe₂ is found to be superconducting. We report its electronic properties accompanied by electronic-structure calculations. Specific heat, electrical resistivity, and magnetic-susceptibility measurements indicate that PbTaSe₂ is a moderately coupled, type-II BCS superconductor (Tc=3.72 K, Ginzburg--Landau parameter κ=17) with an electron-phonon coupling constant of λₑₚ=0.74. Electronic-structure calculations reveal a single bulk three-dimensional Dirac cone at the K point of the Brillouin zone derived exclusively from its hexagonal Pb layer; it is similar to the feature found in graphene except there is a 0.8 eV gap opened by spin-orbit coupling. The combination of large spin-orbit coupling and lack of inversion symmetry also results in large Rashba splitting on the order of tenths of an eV.