Electronic structures of antiperovskite superconductors MgXNi₃ (X=B, C, and N)

We have investigated electronic structures of a newly discovered antiperovskite superconductor MgCNi₃ and related compounds MgBNi₃ and MgNNi₃. In MgCNi₃, a peak of very narrow and high density of states is located just below the Fermi level, which corresponds to the π^* antibonding state of Ni 3d and C 2p but with the predominant Ni 3d character. The prominent nesting feature is observed in the Γ-centered electron Fermi surface of an octahedron-cage-like shape that originates from the 19th band. The estimated superconducting parameters are in reasonable agreement with experiment, suggesting that the superconductivity in MgCNi₃ is described properly by the conventional BCS phonon mechanism.