Superconductivity in hexagonal BC monolayer-based films

A key goal in the study of superconductivity is the pursuit of $\mathrm{high}\text{\ensuremath{-}}{T}_{c}$ superconductors. Due to the metallization of $\ensuremath{\sigma}\text{\ensuremath{-}}\mathrm{bonding}$ electrons, which typically leads to strong electron-phonon coupling, two-dimensional materials based on hexagonal BC monolayers have been suggested as promising candidates for achieving $\mathrm{high}\text{\ensuremath{-}}{T}_{c}$ superconductivity. However, compared with graphene, the free-standing BC monolayer faces dynamical instability due to the absence of one $\ensuremath{\pi}$ electron. In this work, to elucidate the origin of superconductivity in the BC monolayer, we initially stabilized the system by applying feasible biaxial tensile strain, followed by exploring potential electron doping to uncover more superconducting phases, and ultimately provided a complete superconducting phase diagram for the free-standing BC monolayer through systematical ab initio anisotropic Migdal-Eliashberg theory calculations. Our findings indicate that the structural phases between relative tensile strains ${\ensuremath{\epsilon}}_{r}^{1}=(2.2+0.72{n}_{e}+1.9{n}_{e}^{2}\ensuremath{-}3.56{n}_{e}^{3}+1.57{n}_{e}^{4})%$ and ${\ensuremath{\epsilon}}_{r}^{2}=(7.1\ensuremath{-}9.6{n}_{e}+17.2{n}_{e}^{2}\ensuremath{-}17.35{n}_{e}^{3}+6.7{n}_{e}^{4})%$, with ${n}_{e}$ as electron doping density ranging from zero to one electron per unit cell, are dynamically stable and exhibit a three-gap superconducting nature with the critical temperatures ${T}_{c}$ from 76 to $122\phantom{\rule{4pt}{0ex}}\mathrm{K}$ and the maximum superconducting gaps ${\mathrm{\ensuremath{\Delta}}}_{\mathrm{Max}}$ of 19--35 meV. We demonstrated such $\mathrm{high}\text{\ensuremath{-}}{T}_{c}$ three-gap superconductivity is closely associated with the Fermi surface which exhibits an anomalously strong electron-phonon coupling and comprises two distinct $\ensuremath{\sigma}$ Fermi sheets formed by combined $s+{p}_{x,y}$ states and one $\ensuremath{\pi}$ Fermi sheet dominated by the $\mathrm{C}\text{\ensuremath{-}}{p}_{z}$ orbitals. Moreover, as an account of strong interaction with the $\ensuremath{\sigma}\text{\ensuremath{-}}\mathrm{bonding}$ electrons, the in-plane B-C stretching modes govern the strong electron-lattice coupling. Motivated by the route of emergent superconductivity in the strained and doped BC monolayer, we designed 35 two-dimensional materials based on the BC monolayer through atom adsorption, sandwich structuring, and hydrogenation of the adsorption systems, and proved that there are 17 dynamically stable superconductors, e.g., adatoms-BC systems BCLi, BCNa, BCK, BCCu, and BCAg, sandwich structures ${\mathrm{B}}_{2}{\mathrm{C}}_{2}\mathrm{Li}$, ${\mathrm{B}}_{2}{\mathrm{C}}_{2}\mathrm{Na}$, ${\mathrm{B}}_{2}{\mathrm{C}}_{2}\mathrm{K}$, and ${\mathrm{B}}_{2}{\mathrm{C}}_{2}\mathrm{Cu}$, and hydrogenated adatoms-BC systems BCLiH and BCCuH. Ab initio anisotropic Migdal-Eliashberg theory calculations demonstrate that all of the 17 newly searched two-dimensional superconductors have strong electron-phonon coupling and $\mathrm{high}\text{\ensuremath{-}}{T}_{c}$ multigap superconductivity with the ${T}_{c}$ ranging from 33.5 to $118\phantom{\rule{4pt}{0ex}}\mathrm{K}$. Further analysis shows that the decorations from adatoms, hydrogenation, or sandwich structuring provide tensile strain and electron doping simultaneously to the BC layer while adjusting interatomic forces, which, thereby drives the system into a superconducting phase. Theses results pave the way for design of two-dimensional $\mathrm{high}\text{\ensuremath{-}}{T}_{c}$ multigap superconductors.