  The discovery of archetypal two-dimensional (2D) materials provides enormous
opportunities in both fundamental breakthroughs and device applications, as
evident by the research booming in graphene, atomically thin transition-metal
chalcogenides, and few-layer black phosphorous in the past decade. Here, we
report a new, large family of semiconducting dialkali-metal monochalcogenides
(DMMCs) with an inherent A$_{2}$X monolayer structure, in which two alkali
sub-monolayers form hexagonal close packing and sandwich the triangular
chalcogen atomic plane. Such unique lattice structure leads to extraordinary
physical properties, such as good dynamical and thermal stability, visible to
near-infrared light energy gap, high electron mobility (e.g. $1.87\times10^{4}$
cm$^{2}$V$^{-1}$S$^{-1}$ in K$_{2}$O). Most strikingly, DMMC monolayers (MLs)
host extended van Hove singularities near the valence band (VB) edge, which can
be readily accessed by moderate hole doping of $\sim1.0\times10^{13}$
cm$^{-2}$. Once the critical points are reached, DMMC MLs undergo spontaneous
ferromagnetic transition when the top VBs become fully spin-polarized by strong
exchange interactions. Such gate tunable magnetism in DMMC MLs are promising
for exploring novel device concepts in spintronics, electronics and
optoelectronics.
