Anisotropic superconductivity in metal-intercalated layered borocarbides induced by σ electrons coupling with soft phonon modes

Abstract

We propose a fresh class of MB3C3 (M=Mg, Ca, Sr, Sc, and Y) compounds, in which B and C atoms form sp2-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 sp3-bonded counterparts, namely, the cubic clathrates of MB3C3 (M=Mg, Ca, Sr, Sc, and Y). By solving the Migdal-Eliashberg equations, we find that the layered MB3C3 compounds are ambient-pressure anisotropic superconductors with critical temperatures Tcs of 13–58 K. Interestingly, superconducting carriers originate exclusively from the itinerant σ electrons, with soft doubly degenerate E phonon modes playing a pivotal role in the superconductivity. In particular, both ScB3C3 and YB3C3 show double-gap superconductivity, attributed to the presence of three distinct Fermi surface sheets with separate orbital features, among which only the σ-type sheets couple with phonons. Furthermore, investigation of isostructural TiB3C3 uncovers that, although d electrons dominate the electronic states near the Fermi level, superconducting carriers are still provided by the σ bands, whereas the d orbitals make a negligible contribution to electron pairing.

Source-reported materials — not catalogue approval

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