Colloquium: Room temperature superconductivity: The roles of theory and materials design

Abstract

For half a century after the discovery of superconductivity, materials exploration for better superconductors proceeded without knowledge of the underlying mechanism. The 1957 BCS theory cleared that up. The superconducting state occurs due to strong correlation in the electronic system: pairing of electrons over the Fermi surface. Over the following half century a higher critical temperature Tc was achieved only serendipitously as new materials were synthesized. Meanwhile, the formal theory of phonon-coupled superconductivity at the material-dependent level became progressively more highly developed: by 2000, given a known compound, its value of Tc, the corresponding superconducting gap function, and several other properties of the superconducting state became available independent of further experimental input. In this century, density-functional-theory-based computational materials design has progressed to a predictive level; new materials can be predicted from free energy functionals on the basis of various numerical algorithms. Taken together these capabilities enable theoretical predictions for new superconductors, justified by applications to superconductors ranging from weak to strong coupling. Limitations of the current procedures are discussed; most of them can be handled with additional procedures. Here the process that has resulted in the three new highest temperature superconductors is recounted, with compressed structures predicted computationally and values of Tc obtained numerically that have subsequently been confirmed experimentally: the designed superconductors SH3, LaH10, and YH9. These hydrides have Tc in the 200–260 K range at megabar pressures; the experimental results and confirmations are discussed. While the small mass of hydrogen provides the anticipated strong coupling at high frequency, it is shown that it also enables identification of the atom-specific contributions to coupling, in a manner that was previously possible only for elemental superconductors. The following challenge is posed: that progress in understanding of higher Tc is limited by the lack of understanding of screening of H displacements. Ongoing activities are mentioned and current challenges are suggested, together with regularities that are observed in compressed hydrides that may be useful to guide further exploration.

Source-reported materials — not catalogue approval

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