MoB2 under pressure: Superconducting Mo enhanced by boron

Abstract

The discovery of the first high critical temperature (T-c) transition metal diboride superconductor MgB2 structure alpha-MoB2 under pressure with T-c 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 (T-c = 40 K 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 T-c 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 (lambda(Mo) = 1.48) is provided by Mo (total lambda = 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 T-c by 40% over the Mo-only value, to 33 K, corresponding to the experimental value. These results provide a guideline for designing higher-T-c materials from a cooperation of strong coupling from heavy atoms with weakly coupled light atoms. The new high-T-c paradigm discovered here highlights the need for studying and engineering larger ionic scattering matrix elements.