Electronic structure of Ni-based superconducting quaternary compounds: YNi2B2X (X=B, C, N, and O).

Abstract

In order to explore a correlation between superconductivity and the electronic structure of Ni-based superconducting quaternary compounds, a systematic investigation of the electronic structures for ${\mathrm{YNi}}_{2}$${\mathrm{B}}_{2}$X (X=B, C, N, and O) is carried out, by employing the linearized muffin-tin orbital band method. It is found that the Ni 3d density of states (DOS) in ${\mathrm{YNi}}_{2}$${\mathrm{B}}_{2}$C is broader than in fcc Ni metal, and so N(${\mathit{E}}_{\mathit{F}}$) becomes small enough to make the system nonferromagnetic and then superconducting. A rigid-band-like shift of the Fermi level is observed as atom X varies. In the case of ${\mathrm{YNi}}_{2}$${\mathrm{B}}_{2}$C, the Fermi level is located right at the van Hove--like singular DOS peak, which originates from saddle-point extremal band crossing \ensuremath{\Gamma}. This singular DOS peak at ${\mathit{E}}_{\mathit{F}}$ is expected to be related to the superconductivity observed in ${\mathrm{YNi}}_{2}$${\mathrm{B}}_{2}$C. A crude estimate within the framework of the simple rigid-ion approximation indicates that the superconductivity in ${\mathrm{YNi}}_{2}$${\mathrm{B}}_{2}$C can be properly described by the conventional phonon mechanism.