Ambient-pressure topological superconductivity in Ag4H up to 63 K by metallization of hydrogen

Topological superconductors (TSCs) are an ideal platform for realizing Majorana fermions to implement fault-tolerant topological quantum computation. However, the low transition temperature (${T}_{c}$) of TSCs hinders experimental measurements and practical applications. Here, we propose that metal-bonded perovskite ${\mathrm{Ag}}_{4}\mathrm{H}$ is a TSC, characterized by nontrivial topological surface states, a bulk $s$-wave superconducting gap, and a ${T}_{c}$ reaching up to 63 K at ambient pressure. The structural stability, synthesis routes, band topology, and superconductivity are well investigated by first-principles calculations, Wannier interpolation method, effective k $\ifmmode\cdot\else\textperiodcentered\fi{}$ p model, and Migdal-Eliashberg theory. The ambient-pressure phase ${\mathrm{Ag}}_{4}\mathrm{H}$ can be realized through kinetic processes of cooling and depressurization from its pressure state. Topological nodal lines and Dirac points with nontrivial ${Z}_{2}$ index are identified in ${\mathrm{Ag}}_{4}\mathrm{H}$. Our in-depth analysis reveals an unconventional band inversion mechanism, in which the usually low-lying H-$1s$ band is inverted partially to the Fermi level by intermediate metallization of hydrogen resulting from the metallic bonding of the H sublattice with the Ag matrix. Gap anisotropy is related to Fermi surface nesting and $\mathbf{q}$-dependent electron-phonon coupling. The large H interstitial space, more $s/p$ orbital electrons at the Fermi level, and high H concentration promote high-temperature superconductivity under ambient pressure. Last but not least, ${\mathrm{Ag}}_{4}\mathrm{H}$ is the first few-hydrogen metal-bonded intrinsic Eliashberg TSC, exhibiting high ${T}_{c}$ near the liquid-nitrogen temperature region. This work may pave a different way to realize topological superconductivity at higher temperature under ambient pressure.