  Two-dimensional (2D) atomic materials possess very different properties from
their bulk counterparts. While changes in the single-particle electronic
properties have been extensively investigated, modifications in the many-body
collective phenomena in the exact 2D limit, where interaction effects are
strongly enhanced, remain mysterious. Here we report a combined optical and
electrical transport study on the many-body collective-order phase diagram of
2D NbSe$_2$. Both the charge density wave (CDW) and the superconducting phase
have been observed down to the monolayer limit. While the superconducting
transition temperature ($T_C$) decreases with lowering the layer thickness, the
newly observed CDW transition temperature ($T_{\mathrm{CDW}}$) increases
drastically from 33 K in the bulk to 145 K in the monolayers. Such highly
unusual enhancement of CDWs in atomically thin samples can be understood as a
result of significantly enhanced electron-phonon interactions in 2D NbSe$_2$,
which cause a crossover from the weak coupling to the strong coupling limit.
This is supported by the large blueshift of the collective amplitude vibrations
observed in our experiment.
