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 NbSe2. Both the charge density wave (CDW) and the superconducting phase have been observed down to the monolayer limit. While the superconducting transition temperature (TC) decreases with lowering the layer thickness, the newly observed CDW transition temperature (TCDW) 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 NbSe2, 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.
@article{arxiv.1507.05595,
title = {Strongly enhanced charge-density-wave order in monolayer NbSe$_2$},
author = {Xiaoxiang Xi and Liang Zhao and Zefang Wang and Helmuth Berger and László Forró and Jie Shan and Kin Fai Mak},
journal= {arXiv preprint arXiv:1507.05595},
year = {2015}
}