Few-Hydrogen High- T c Superconductivity in (Be 4 ) 2 H Nanosuperlattice with Promising Ductility under Ambient Pressure.

The multi-hydrogen lanthanum hydride LaH 10 is well recognized as having the highest critical temperature ( T c ) of 250-260 K under unrealistically ultrahigh pressures of about 170-200 GPa. Here, we propose a novel idea for designing a new ambient-pressure high- T c superconductor by inserting a hexagonal H-monolayer into two close-packed Be monolayers to form a new and stable few-hydrogen metal-bonded layered beryllium hydride (Be 4 ) 2 H nanosuperlattice, with better ductility than multi-hydrogen, cuprate, and iron-based superconductors, completely contrary to the conventional design strategy for multi-hydrogen covalent high- T c superconductors with poor ductility at several hundred GPa. We find that (Be 4 ) 2 H is a phonon-mediated Eliashberg superconductor with a large electron-phonon coupling constant of 1.41 and a high T c of 84-72 K with Coulomb repulsion pseudopotential μ* = 0.07-0.13. Importantly, (Be 4 ) 2 H is the only new high- T c superconductor and fills the gap in the absence of ambient-pressure superconductors around the liquid-nitrogen temperature with good ductility, which is highly beneficial for practical applications.