Superconductivity of the two-dimensional MB2C2 (M = 3d, 4d) monolayers

CrB2C2, MoB2C2, and YB2C2 monolayers with superconducting properties are screened from a set of 20 MB2C2 (M = 3d, 4d) monolayers by evaluating their energy, dynamical, and thermodynamical stabilities, alongside solving the anisotropic Migdal-Eliashberg equation. Negative cohesive energies are exhibited by all the examined monolayers; however, only 10 are found to possess no imaginary frequencies in the first Brillouin zone, indicating dynamical stability. Among these, CrB2C2, MoB2C2, and YB2C2 monolayers are identified as having the highest superconducting transition temperatures, as derived from solving the isotropic McMillan equation. Further analysis using the anisotropic Migdal-Eliashberg equation provides more accurate transition temperatures of 25.5 K, 16.2 K, and 18.1 K, respectively. A single superconducting gap is revealed in CrB2C2 and MoB2C2 by Fermi surface analysis, while YB2C2 is found to exhibit two distinct superconducting gaps. The CrB2C2 monolayer with a large density of states and apparent Van Hove singularities near Fermi level, is recognized as a most promising candidate for superconducting materials among the MB2C2 monolayers, thus expanding the family of two-dimensional superconductors based on boron-carbon monolayers.