Strong electron-phonon coupling superconductivity in compressed α−MoB2 induced by double Van Hove singularities

Abstract

A recent experiment of ${\mathrm{MgB}}_{2}$-type structure $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{MoB}}_{2}$ has realized $\ensuremath{\sim}32$ K superconductivity (SC) at 90 GPa, exhibiting the highest superconducting transition temperature (${T}_{\mathrm{c}}$) among transition-metal diborides. Although the SC was characterized by the electron-phonon coupling (EPC), the microscopic mechanism of how the large EPC constant and high ${T}_{\mathrm{c}}$ are attained is unclear. Here, based on first-principles calculations, we found that in contrast to ${\mathrm{MgB}}_{2}$, B atoms contribute most to electronic states near Fermi level ($E{}_{\mathrm{F}}$), Mo ${d}_{{z}^{2}}$ orbital is more dominant component in ${\mathrm{MoB}}_{2}$ and provides two impressive peaks in density of states near ${E}_{\mathrm{F}}$ associated with emergent double Van Hove singularities (VHS). The EPC analysis reveals that the electronic sates around double VHS could strongly interact with the softened acoustic modes of Mo out-of-plane vibration, giving rise to a large single gap with the ${T}_{\mathrm{c}}$ up to $\ensuremath{\sim}37$ K, which distinctly differs from the superconducting feature of ${\mathrm{MgB}}_{2}$. Furthermore, by electron doping into ${\mathrm{MoB}}_{2}$, the VHS is tuned to be aligned with the ${E}_{\mathrm{F}}$ and ${T}_{\mathrm{c}}$ can be increased to $\ensuremath{\sim}43$ K. Our findings not only elucidate the microscopic mechanism of observed high ${T}_{\mathrm{c}}$ in ${\mathrm{MoB}}_{2}$, but also demonstrate that ${\mathrm{MoB}}_{2}$ provides an ideal platform to explore the role of the VHS in emergent strong EPC SC.