A New Analysis of Strong Electron-Phonon Coupling in Compressed Metal Hydrides

The experimental discovery that H3S becomes superconducting at 200 K under 160 GPa pressure has re-invigorated interest in metal hydrides under compression. Understanding the role of hydrogen relative to that of metal atom and the crystal structure is essential for designing materials with even higher Tc. Importantly for H3S and other hydrides, various physical quantities including the phonon spectra, the coupling strength λ, and matrix elements can be clearly identified as contributions from either hydrogen or the metal atom. We systematically study a few classes of binary hydrides, viz. H3S, CaH6, MgH6 and recently reported lanthanum hydride to carry out this separation and gain a unique type of insight into the origin of strong electron-phonon coupling and high Tc in hydrides.