Stability and superconductivity of freestanding two-dimensional transition metal boridene: M4/3B2

Abstract 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 4/3 B 2− x ) was realized in experiments (Zhou et al 2021 Science 373 801). Here, the 2D monolayer freestanding Mo 4/3 B 2 is evidenced to be thermodynamically stable. Through electronic structure, phonon spectrum and EPC, monolayer Mo 4/3 B 2 is 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, the T c of monolayer Mo 4/3 B 2 can 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 4/3 B 2 is also a superconductor with the T c of 2.37 K. Our research results enrich the database of 2D monolayer superconductors and boron-related formed materials science.