  Single layers of transition metal dichalcogenides (TMDCs) are excellent
candidates for electronic applications beyond the graphene platform; many of
them exhibit novel properties including charge density waves (CDWs) and
magnetic ordering. CDWs in these single layers are generally a planar
projection of the corresponding bulk CDWs because of the quasi-two-dimensional
nature of TMDCs; a different CDW symmetry is unexpected. We report herein the
successful creation of pristine single-layer VSe$_2$, which shows a ($\sqrt7
\times \sqrt3$) CDW in contrast to the (4 $\times$ 4) CDW for the layers in
bulk VSe$_2$. Angle-resolved photoemission spectroscopy (ARPES) from the single
layer shows a sizable ($\sqrt7 \times \sqrt3$) CDW gap of $\sim$100 meV at the
zone boundary, a 220 K CDW transition temperature twice the bulk value, and no
ferromagnetic exchange splitting as predicted by theory. This robust CDW with
an exotic broken symmetry as the ground state is explained via a
first-principles analysis. The results illustrate a unique CDW phenomenon in
the two-dimensional limit.
