Enhanced superconductivity in CuH2 monolayers

Achieving superconductivity in hydrides at lower pressures is a long-standing scientific challenge. Here, we propose that reducing their effective dimensionality might be a possible route to achieve this goal. First-principles structural search calculations identify several two-dimensional transition metal hydrides (TMHs) with phonon-mediated superconductivity. Among them, the two ${\mathrm{CuH}}_{2}$ phases with $P\text{\ensuremath{-}}3m1$ and $P\text{\ensuremath{-}}6m2$ symmetries are predicted to have the highest ${T}_{\mathrm{c}}$ values up to 44.4 and 47.8 K, comparable to the well-known ${\mathrm{MgB}}_{2}$ superconductor. Their superconductivity mainly originates from the coupling of Cu $3{d}_{xz}/{d}_{yz}$ and $\mathrm{H}\text{\ensuremath{-}}1s$ electrons with in-plane H-vibrational phonons. Interestingly, both ${\mathrm{CuH}}_{2}$ structures show a unique superconducting energy gap by solving the anisotropic Eliashberg equations. Furthermore, the superconductivity of TMHs is closely related to the polarity of the TM-H bond. Our paper paves the way for finding hydride superconductors in low-dimensional materials.