Possible Superconductivity for Layered Metal Boride Carbide Compounds MB₂C₂ (M = Alkali, Alkaline-Earth, or Rare-Earth Metals)

The possible emergence of superconductivity in layered metal boride carbide compounds MB₂C₂ (M = Sc, Y, Be, Ca) was investigated using density functional theory calculations upon the topology of a boron-carbon network and the nature of the metal. ScB₂C₂ and YB₂C₂ show metallic and superconductive properties with low critical temperatures (T_cs). The semiconducting BeB₂C₂ compound may show superconductivity upon carrier doping with a high T of 47.8 K by hole doping─comparable to the structurally related MgB₂ superconductor─but with a low T_c by electron doping. In contrast, the semiconducting CaB₂C₂ compound is predicted to be a superconductor by hole and electron doping but with low T_cs. These differences arise from the spatial distribution of electrons at the Fermi level. For compounds with low T_cs, electrons at the Fermi level are localized primarily on B and C π states perpendicular to the BC layers, experiencing minimal influence from atomic oscillations and resulting in weak electron-phonon interactions. Conversely, for a high T_c, electrons are found in σ-bonding states, leading to strong electron-phonon interactions. Electrons at the Fermi level in boron-carbon σ-bonding states seem to be a prerequisite to expect high T_c superconductivity in this kind of compound.