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Strongly enhanced charge-density-wave order in monolayer NbSe2

Nature Nanotechnology volume 10, pages 765–769 (2015)Cite this article

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Abstract

Two-dimensional materials possess very different properties from their bulk counterparts. While changes in single-particle electronic properties have been investigated extensively1,2,3, modifications in the many-body collective phenomena in the exact two-dimensional limit remain relatively unexplored. Here, we report a combined optical and electrical transport study on the many-body collective-order phase diagram of NbSe2 down to a thickness of one monolayer. Both the charge density wave and the superconducting phase have been observed down to the monolayer limit. The superconducting transition temperature decreases on lowering the layer thickness, but the newly observed charge-density-wave transition temperature increases from 33 K in the bulk to 145 K in the monolayer. Such highly unusual enhancement of charge density waves in atomically thin samples can be understood to be a result of significantly enhanced electron–phonon interactions in two-dimensional NbSe2 (ref. 4) and is supported by the large blueshift of the collective amplitude vibration observed in our experiment. Our results open up a new window for search and control of collective phases of two-dimensional matter, as well as expanding the functionalities of these materials for electronic applications.

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Figure 1: Characterizations of atomically thin NbSe2 samples.
Figure 2: Temperature dependence of Raman spectra of bulk and two-dimensional NbSe2.
Figure 3: Many-body phase diagram of two-dimensional NbSe2.
Figure 4: Thickness dependence of the amplitude mode frequency and electron–phonon coupling in NbSe2.

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Acknowledgements

The authors acknowledge support from the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering (award no. DESC0012635), for the development of two-dimensional NbSe2 samples and devices, and from the National Science Foundation (NSF, awards nos. DMR-1106225 and DMR-1410407) for the development of the low-temperature terahertz Raman spectrometer. The authors also acknowledge support from the NSF MRSEC (award no. DMR-1420451, Z.W.) and the MRI-2D Center at Penn State University (X.X.). The work in Lausanne was supported by the Swiss National Science Foundation.

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Authors and Affiliations

  1. Department of Physics and Center for 2-Dimensional and Layered Materials, The Pennsylvania State University, University Park, Pennsylvania, 16802-6300, USA

    Xiaoxiang Xi, Zefang Wang, Jie Shan & Kin Fai Mak

  2. Department of Physics, Case Western Reserve University, Cleveland, 44106-7079, Ohio, USA

    Liang Zhao & Jie Shan

  3. Institute of Condensed Matter Physics, Ecole Polytechnique Fédérale de Lausanne, Lausanne, 1015, Switzerland

    Helmuth Berger & László Forró

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  1. Xiaoxiang Xi
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Contributions

X.X., J.S. and K.F.M. conceived and designed the experiments, analysed the data and co-wrote the paper. X.X., L.Z., Z.W. and K.F.M. performed the experiments. H.B. and L.F. contributed bulk NbSe2 crystals. All authors discussed the results and commented on the manuscript.

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Correspondence to Jie Shan or Kin Fai Mak.

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Xi, X., Zhao, L., Wang, Z. et al. Strongly enhanced charge-density-wave order in monolayer NbSe2. Nature Nanotech 10, 765–769 (2015). https://doi.org/10.1038/nnano.2015.143

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