Two-gap superconductivity in a Janus MoSH monolayer

Hydrides under ultrahigh pressure, such as ${\mathrm{H}}_{3}\mathrm{S}$ and ${\mathrm{LaH}}_{10}$, can achieve coveted superconducting critical temperatures via the conventional electron-phonon coupling mechanism. In this work, we report first-principles investigation of high-temperature phonon-mediated superconductivity in a two-dimensional metal hydride, namely, the Janus MoSH monolayer. The MoSH sheet is a recently synthesized intermediate in realizing Janus transition metal dichalcogenides by involving stripping the top-layer S of ${\mathrm{MoS}}_{2}$ with H atoms [X. Wan et al., ACS Nano 15, 20319 (2021); A. Y. Lu et al., Nat. Nanotechnol. 12, 744 (2017)]. We find coupling of electrons from $\mathrm{Mo}\text{\ensuremath{-}}d$ orbitals around the Fermi level to ultrahigh frequency H- and S-derived phonons, analogous to superconducting hydrides of ${\mathrm{H}}_{3}\mathrm{S}$ and ${\mathrm{LaH}}_{10}$. This leads to strong coupling two-gap superconductivity with the calculated critical temperature ${T}_{c}$ being about 28.58 K at atmosphere pressure. The presence of soft phonon bands from the Mo in-plane vibrations, in cooperation with the electron susceptibility, accounts for the strong electron-phonon coupling of the MoSH monolayer. By further aligning the Fermi level via doping, the ${T}_{c}$ can be boosted to 37.31 K, close to the McMillan limit (39 K). Thus, our work points out a real metal hydride for the realization of two-dimensional high-temperature superconductivity at atmosphere pressure and facilitates further studies on new families of Janus transition-metal sulfhydrates.