Cobalt-dimer Nitrides -- a Potential Novel Family of High Temperature Superconductors

We predict that the square lattice layer formed by [Co$_2$N$_2$]$^{2-}$ diamond-like units can host high temperature superconductivity. The layer appears in the stable ternary cobalt nitride, BaCo$_2$N$_2$. The electronic physics of the material stems from Co$_2$N$_2$ layers where the dimerized Co pairs form a square lattice. The low energy physics near Fermi energy can be described by an effective two orbital model. Without considering interlayer couplings, the two orbitals are effectively decoupled. This electronic structure satisfies the "gene" character proposed for unconventional high temperature superconductors. We predict that the leading superconducting pairing instability is driven from an extended $s$-wave (s$^\pm$) to a $d$-wave by hole doping, for example in Ba$_{1-x}$K$_x$Co$_2$N$_2$. This study provides a new platform to establish the superconducting mechanism of unconventional high temperature superconductivity.