Conventional high-temperature superconductivity in σ-band driven metallized two-dimensional metal borocarbides

The superconductivity driven by the strong coupling of $\ensuremath{\sigma}$-bonding electrons with interlayer covalent bond-stretching modes presents a promising pathway for achieving high-${\mathit{T}}_{c}$ superconductivity under atmospheric conditions. Herein, we conduct high-throughput first-principles calculations to comprehensively investigate potential superconductivity in two-dimensional (2D) metal borocarbides ($M{\mathrm{B}}_{2}{\mathrm{C}}_{2}$, $M$ = metal). Our analysis of 41 $M{\mathrm{B}}_{2}{\mathrm{C}}_{2}$ films reveals that 23 demonstrate dynamical stability and 17 of them are potential superconductors. Moreover, we identify six promising candidates within this subset with critical temperatures ${T}_{c}\phantom{\rule{4pt}{0ex}}\ensuremath{\ge}$ 75 K, i.e., ${\mathrm{LiB}}_{2}{\mathrm{C}}_{2}$ (75 K), ${\mathrm{NaB}}_{2}{\mathrm{C}}_{2}$ (81 K), ${\mathrm{KB}}_{2}{\mathrm{C}}_{2}$ (102 K), ${\mathrm{CaB}}_{2}{\mathrm{C}}_{2}$ (88 K), ${\mathrm{SrB}}_{2}{\mathrm{C}}_{2}$ (91 K), and ${\mathrm{LaB}}_{2}{\mathrm{C}}_{2}$ (94 K). Based on comprehensive analysis of the electronic and vibrational properties, we propose two key descriptors for filtering and diagnosing high-${T}_{c}\phantom{\rule{4pt}{0ex}}M{\mathrm{B}}_{2}{\mathrm{C}}_{2}$ films: one involving $\ensuremath{\sigma}$ electrons of covalent B-C bonds driven metallic and the other focusing on large deformation potentials for $\ensuremath{\sigma}$ bands near the Fermi level originating from frozen-in in-plane B-C bond-stretching modes at the zone center. Utilizing two descriptors, we expediently identify the ${\mathrm{KLaB}}_{4}{\mathrm{C}}_{4}$ film as a potential high-${\mathit{T}}_{c}$ candidate (70 K) among 15 bimetal borocarbides. These findings provide valuable insights into the electronic and vibrational properties that contribute to enhanced superconducting behavior, aiding in the design and discovery of new 2D materials with the potential to break the ${T}_{c}$ record for conventional superconductivity.