Migdal-Eliashberg theory of multi-band high-temperature superconductivity in field-effect-doped hydrogenated (111) diamond.

We perform single- and multi-band Migdal-Eliashberg (ME) calculations with parameters exctracted from density functional theory (DFT) simulations to study superconductivity in the electric-field-induced 2-dimensional hole gas at the hydrogenated (111) diamond surface. We show that according to the Eliashberg theory it is possible to induce a high-T$_{\text{c}}$ superconducting phase when the system is field-effect doped to a surface hole concentration of $6\times10^{14}\,$cm$^{-2}$, where the Fermi level crosses three valence bands. Starting from the band-resolved electron-phonon spectral functions $\alpha^2F_{jj'}(\omega)$ computed ab initio, we iteratively solve the self-consistent isotropic Migdal-Eliashberg equations, in both the single-band and the multi-band formulations, in the approximation of a constant density of states at the Fermi level. In the single-band formulation, we find T$_{\text{c}}\approx40\,$K, which is enhanced between $4\%$ and $8\%$ when the multi-band nature of the system is taken into account. We also compute the multi-band-sensistive quasiparticle density of states to act as a guideline for future experimental works.