Theoretical prediction of superconductivity in two-dimensional hydrogenated metal diboride: M2B2H(M=Al , Mg, Mo, W)

In recent years, superconductivity in two-dimensional (2D) layered metal borides has aroused much interest. Here, based on first-principles calculations, we theoretically report four 2D hydrogenated metal diborides: ${M}_{2}{\mathrm{B}}_{2}\mathrm{H}\phantom{\rule{4pt}{0ex}}(M=\mathrm{Al}$, Mg, Mo, W), and investigate their geometrical structures, electronic structures, phonon dispersions, thermal stability, dynamic stability, electron-phonon coupling (EPC), superconducting properties, and so on. Results reveal that the introduction of hydrogen atoms expands the frequency range of the phonon spectrum of monolayer ${M}_{2}{\mathrm{B}}_{2}$, and significantly increases the EPC. We systematically analyze the specific origins of superconductivity in these hydrogenated low-dimensional systems. The obtained EPC constants $\ensuremath{\lambda}$ of ${M}_{2}{\mathrm{B}}_{2}\mathrm{H}\phantom{\rule{4pt}{0ex}}(M=\mathrm{Al}$, Mg, Mo, W) are 2.93, 0.86, 1.09, and 1.40, and the corresponding superconducting transition temperatures $({T}_{c})$ are 52.6, 23.2, 21.5, and 18.6 K, respectively. By further applying electron/hole doping or biaxial tensile strain, the ${T}_{c}$ can be further increased, with the highest ${T}_{c}$ of 60.2 K in ${\mathrm{Al}}_{2}{\mathrm{B}}_{2}\mathrm{H}$ under 0.4% biaxial tensile strain. The predicted ${M}_{2}{\mathrm{B}}_{2}\mathrm{H}$ provides a new platform for 2D superconductivity and may have potential applications in 2D nanodevices.