Manipulating ambient pressure superconductivity in metal borocarbides through hole doping

A Ca2B4C4 ternary compound obtained by using a machine learning (ML) guided structure search is found to be metastable with a formation energy of only 18 meV/atom above the convex hull but exhibits only marginal superconducting transition temperature (Tcc). By replacing Ca with Na, the electronic density of states (DOS) at the Fermi level is significantly enhanced, increasing the predicted Tc to 21.9 K. Extending this hole-doping strategy to other Ca-B-C compounds, we found that while Na4B2C22 remains superconducting with a Tc of 4.0 K, Na substitution in Ca2B4C8 transforms it from a semiconductor to a superconductor with a Tc of 28.9 K. The flat σ bands, particularly from the 2px and 2py orbitals of carbon, and a Van Hove singularity at the Fermi level, play crucial roles in enhancing the superconductivity. This work introduces a new class of B-C-based superconductors and broadens the compositional space for high-temperature superconducting materials. .