Why Compressed Metal Hydrides are Near-room-temperature Superconductors

Abstract

This contribution provides a partial response to the titular statement since, it will be claimed,the ``why'' is not yet understood, but there is a pathway for achieving a more complete understanding. The sense of the community has been that, given a prospective metal hydride and pressure, the energy landscape can be surveyed computationally for thermodynamic and dynamic stability, the Eliashberg spectral function with its required input (energy bands, phonon modes, coupling matrix elements) can be calculated, and the critical temperature T$_c$ obtained. Satisfyingly large values of the electron-phonon coupling strength $λ$=2-3 at high mean frequency are obtained, giving very reasonable agreement with existing high T$_c$ hydrides. Typically 80-85\% of $λ$ is attributable to high frequency H vibrations. This much was envisioned by Ashcroft two decades ago, so why should there be any angst? This paper addresses more specifically the question {\it why hydrogen?} Light mass is indeed a factor, but with possibilities not yet explored. This paper provides a concise overview of related formal developments occurring sporadically over several decades that, when implemented, could resolve the question of {\it why hydrogen, why so high T$_c$.} The dearth of success of numerous high throughput searches proposing higher T$_c$ materials, especially hydrides, is touched on briefly. Based on as yet unapplied developments in simplifying effects of atomic displacement, it is proposed that there is a straightforward path toward a deeper understanding of ``metallic hydrogen superconductivity" in conjunction with added computational efficiency, and that some human-learning should assist in focusing the search for higher T$_c$ superconductors.