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arXiv · 1507.05595

Strongly enhanced charge-density-wave order in monolayer NbSe$_2$

Xiaoxiang Xi·Liang Zhao·Zefang Wang·Helmuth Berger·László Forró·Jie Shan·Kin Fai Mak
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Abstract

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.

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Xiaoxiang Xi, Liang Zhao, Zefang Wang, Helmuth Berger, László Forró, Jie Shan, Kin Fai Mak. 2015-07-20. Strongly enhanced charge-density-wave order in monolayer NbSe$_2$. https://doi.org/10.1038/nnano.2015.143

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