First-principles design of ambient-pressure Mg$_x$B$_2$C$_2$ and Na$_x$BC superconductors

TL;DR

This study uses ab initio modeling to explore the potential for high-temperature superconductivity in ambient-pressure MgB₂C₂ and NaBC compounds, revealing their superconducting temperatures and structural effects on superconductivity.

Contribution

It introduces a first-principles approach to designing ambient-pressure superconductors based on MgB₂C₂ and NaBC, predicting their critical temperatures and structural influences.

Findings

Superconducting Tc between 43 K and 84 K predicted.

Hole doping via thermal deintercalation feasible in these compounds.

Buckling of honeycomb layers can suppress superconductivity.

Abstract

We employ ab initio modeling to investigate the possibility of attaining high-temperature conventional superconductivity in ambient-pressure materials based on the known MgB$_2$C$_2$ and recently proposed thermodynamically stable NaBC ternary compounds. The constructed $(T,P_M)$ phase diagrams (M = Mg or Na) indicate that these layered metal borocarbides can be hole-doped via thermal deintercalation that has been successfully used in previous experiments to produce Li$_y$BC ($y=1>x\gtrsim0.5$) samples. The relatively low temperature threshold required to trigger NaBC desodiation may help prevent the formation of defects shown recently to be detrimental to the electron-phonon coupling in the delithiated LiBC analog. According to our numerical solutions of the anisotropic full-bandwidth Migdal-Eliashberg equations, the proposed Mg$_x$B$_2$C$_2$ and Na$_x$BC materials exhibit superconducting critical temperatures between 43 K and 84 K. At the same time, we demonstrate that buckling of defect-free honeycomb BC layers, favored in heavily-doped Na$_x$BC compounds, can substantially reduce or effectively suppress the materials' potential for MgB$_2$-type superconductivity.