MoB2 under pressure: Superconducting Mo enhanced by boron

Abstract

The discovery of the first high critical temperature (Tc) transition metal diboride superconductor MgB2 structure α−MoB2 under pressure with Tc up to 32 K at 100 GPa provides new input into some unexplained aspects of electron-phonon coupling in intermetallic compounds. We establish that MoB2 is a phonon-mediated superconductor but has little in common with MgB2 (Tc=40K at zero pressure). MoB2 is a strongly metallic, three-dimensional, multi-Fermi-surface material, becoming of additional interest because it displays a frequency separation of Mo and B vibrations that mirrors that of metal superhydrides with Tc approaching room temperature. This separation, which is unusual in intermetallic compounds, allows an analysis separately for Mo and B providing, among the other parameters essential for understanding phonon coupling, the matrix elements for scattering by the individual atoms. Strong coupling (λMo=1.48) is provided by Mo (total λ=1.67). A factor of 15 weaker coupling to each B atom is compensated by that coupling being to mean high-frequency modes around 85–90 meV (maximum of 140 meV), versus 18–20 meV for Mo. As a result, B enhances Tc by 40% over the Mo-only value, to 33 K, corresponding to the experimental value. These results provide a guideline for designing higher-Tc materials from a cooperation of strong coupling from heavy atoms with weakly coupled light atoms. The new high-Tc paradigm discovered here highlights the need for studying and engineering larger ionic scattering matrix elements.

Source-reported materials — not catalogue approval

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