Spin-orbit-stable type-II nodal line band crossing in n -doped monolayer MoX2 ( X=S , Se,Te)

$1H$-phase monolayer transition-metal dichalcogenides have spin-split electronic band structures near the $K({K}^{\ensuremath{'}})$ point due to strong spin-orbit coupling and intrinsic inversion symmetry breaking. In the case of the monolayer ${\mathrm{Mo}X}_{2}$ ($X=\text{S}$, Se, Te), the spin-split conduction bands cross near the $K({K}^{\ensuremath{'}})$ point. We show that the band crossing occurs not only along the high-symmetry directions, but also along arbitrary directions due to a mirror reflection symmetry of the monolayer. As a result, $n$-doped monolayer ${\mathrm{Mo}X}_{2}$ is a two-dimensional type-II nodal-line material.