We propose a fresh class of ${M\mathrm{B}}_3{\mathrm{C}}_3$ $(M=\mathrm{Mg}, \mathrm{Ca}, \mathrm{Sr}, \mathrm{Sc}, \text{and} \mathrm{Y})$ compounds, in which B and C atoms form $s{p}^2$-hybridized honeycomb layers with $M$ atoms serving as electron donors in the interlayers. Calculations indicate that these ambient-pressure layered structures are more stable than their reported $s{p}^3$-bonded counterparts, namely, the cubic clathrates of ${M\mathrm{B}}_3{\mathrm{C}}_3$ $(M=\mathrm{Mg}, \mathrm{Ca}, \mathrm{Sr}, \mathrm{Sc}, \text{and} \mathrm{Y})$. By solving the Migdal-Eliashberg equations, we find that the layered ${M\mathrm{B}}_3{\mathrm{C}}_3$ compounds are ambient-pressure anisotropic superconductors with critical temperatures $T_c\mathrm{s}$ of 13–58 K. Interestingly, superconducting carriers originate exclusively from the itinerant $\sigma$ electrons, with soft doubly degenerate $E$ phonon modes playing a pivotal role in the superconductivity. In particular, both ${\mathrm{ScB}}_3{\mathrm{C}}_3$ and ${\mathrm{YB}}_3{\mathrm{C}}_3$ show double-gap superconductivity, attributed to the presence of three distinct Fermi surface sheets with separate orbital features, among which only the $\sigma$-type sheets couple with phonons. Furthermore, investigation of isostructural ${\mathrm{TiB}}_3{\mathrm{C}}_3$ uncovers that, although $d$ electrons dominate the electronic states near the Fermi level, superconducting carriers are still provided by the $\sigma$ bands, whereas the $d$ orbitals make a negligible contribution to electron pairing.

Close

PrevNext

View figure in article

• Electron-phonon coupling
• Fermi surface
• Nesting
• Superconductivity
• Borides
• Eliashberg theory