Stability and superconductivity of freestanding two-dimensional transition metal boridene: M_4/3B₂

The small atomic mass of boron indicates strong electron-phonon coupling (EPC), so it may have a brilliant performance in superconductivity. Recently, a new 2D boride sheet with ordered metal vacancies and surface terminals (Mo_4/3B_2-x) was realized in experiments (Zhouet al2021Science373801). Here, the 2D monolayer freestanding Mo_4/3B₂is evidenced to be thermodynamically stable. Through electronic structure, phonon spectrum and EPC, monolayer Mo_4/3B₂is found to be an intrinsic phonon-mediated superconductor. The superconducting transition temperature (T_c) is determined to be 4.06 K by the McMillian-Allen-Dynes formula. Remarkably, theT_cof monolayer Mo_4/3B₂can be increased to 6.78 K with an appropriate biaxial tensile strain (+5%). Moreover, we predict that other transition metal replacing Mo atoms is also stable and retaining the superconductivity. Such as monolayer W_4/3B₂is also a superconductor with theT_cof 2.37 K. Our research results enrich the database of 2D monolayer superconductors and boron-related formed materials science.