Letter | Published:

Strongly enhanced charge-density-wave order in monolayer NbSe2

Nature Nanotechnology volume 10, pages 765769 (2015) | Download Citation


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.

Access optionsAccess options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.


  1. 1.

    & The rise of graphene. Nature Mater. 6, 183–191 (2007).

  2. 2.

    , , , & The electronic properties of graphene. Rev. Mod. Phys. 81, 109–162 (2009).

  3. 3.

    , , , & Electronics and optoelectronics of two-dimensional transition metal dichalcogenides. Nature Nanotech. 7, 699–712 (2012).

  4. 4.

    , & Effect of dimensionality on the charge-density wave in few-layer NbSe2. Phys. Rev. B 80, 241108 (2009).

  5. 5.

    Density Waves In Solids (Westview, 2009).

  6. 6.

    On the origin of charge-density waves in select layered transition-metal dichalcogenides. J. Phys. Condens. Matter 23, 213001 (2011).

  7. 7.

    et al. Direct observation of competition between superconductivity and charge density wave order in YBa2Cu3O6.67. Nature Phys. 8, 871–876 (2012).

  8. 8.

    et al. Emergence of charge density wave domain walls above the superconducting dome in 1T-TiSe2. Nature Phys. 10, 421–425 (2014).

  9. 9.

    et al. Two-dimensional atomic crystals. Proc. Natl Acad. Sci. USA 102, 10451–10453 (2005).

  10. 10.

    , , , & Charge density waves in exfoliated films of van der Waals materials: evolution of Raman spectrum in TiSe2. Nano Lett. 12, 5941–5945 (2012).

  11. 11.

    et al. Gate-tunable phase transitions in thin flakes of 1T-TaS2. Nature Nanotech. 10, 270–276 (2015).

  12. 12.

    , & Charge-density waves and superlattices in the metallic layered transition metal dichalcogenides (Reprinted from Adv. Phys., 32, 882 (1974)). Adv. Phys. 50, 1171–1248 (2001).

  13. 13.

    et al. Electric and magnetic characterization of NbSe2 single crystals: anisotropic superconducting fluctuations above TC. Physica C 460–462, 789–790 (2007).

  14. 14.

    Landau theory of charge-density waves in transition-metal dichalcogenides. Phys. Rev. B 12, 1187–1196 (1975).

  15. 15.

    & New mechanism for a charge-density-wave instability. Phys. Rev. Lett. 35, 120–123 (1975).

  16. 16.

    et al. Charge-density-wave mechanism in 2H-NbSe2: photoemission results. Phys. Rev. Lett. 82, 4504–4507 (1999).

  17. 17.

    et al. Quasiparticle spectra, charge-density waves, superconductivity, and electron–phonon coupling in 2H-NbSe2. Phys. Rev. Lett. 92, 086401 (2004).

  18. 18.

    et al. Charge-order-maximized momentum-dependent superconductivity. Nature Phys. 3, 720–725 (2007).

  19. 19.

    et al. Extended phonon collapse and the origin of the charge-density wave in 2H-NbSe2. Phys. Rev. Lett. 107, 107403 (2011).

  20. 20.

    et al. Quasiparticle interference, quasiparticle interactions, and the origin of the charge density wave in 2H-NbSe2. Phys. Rev. Lett. 114, 037001 (2015).

  21. 21.

    et al. Gaps and kinks in the electronic structure of the superconductor 2H-NbSe2 from angle-resolved photoemission at 1 K. Phys. Rev. B 85, 224532 (2012).

  22. 22.

    , & Raman spectroscopy of soft modes at the charge-density-wave phase transition in 2H-NbSe2. Phys. Rev. Lett. 37, 1407–1410 (1976).

  23. 23.

    et al. The shear mode of multilayer graphene. Nature Mater. 11, 294–300 (2012).

  24. 24.

    Theory of two-phonon Raman scattering in transition metals and compounds. Phys. Rev. B 24, 4208–4223 (1981).

  25. 25.

    & in Proceedings of the International Conference on Lattice Dynamics, Paris, 1977 (ed. Balkanski, M.) 602 (Flammarion, 1978).

  26. 26.

    & Raman scattering by superconducting-gap excitations and their coupling to charge-density waves. Phys. Rev. Lett. 45, 660–662 (1980).

  27. 27.

    Microscopic model of charge-density waves in 2H-TaSe2. Phys. Rev. B 16, 643–650 (1977).

  28. 28.

    , & Thermal-properties of layered transition-metal dichalcogenides at charge-density-wave transitions. Phys. Rev. B 15, 2943–2951 (1977).

  29. 29.

    Superconductivity in ultrathin NbSe2 layers. Phys. Rev. Lett. 28, 299–301 (1972).

  30. 30.

    et al. Electric field effect on superconductivity in atomically thin flakes of NbSe2. Phys. Rev. B 80, 184505 (2009).

  31. 31.

    , , & Pressure effect on the charge-density-wave formation in 2H-NbSe2 and correlation between structural instabilities and superconductivity in unstable solids. Phys. Rev. B 15, 1340–1342 (1977).

  32. 32.

    , & From low to high-temperature superconductivity: a dimensional crossover phenomenon? a finite size effect? Z. Phys. B 83, 313–321 (1991).

Download references


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.

Author information


  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, Ohio 44106-7079, USA

    • Liang Zhao
    •  & Jie Shan
  3. Institute of Condensed Matter Physics, Ecole Polytechnique Fédérale de Lausanne, Lausanne 1015, Switzerland

    • Helmuth Berger
    •  & László Forró


  1. Search for Xiaoxiang Xi in:

  2. Search for Liang Zhao in:

  3. Search for Zefang Wang in:

  4. Search for Helmuth Berger in:

  5. Search for László Forró in:

  6. Search for Jie Shan in:

  7. Search for Kin Fai Mak in:


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.

Competing interests

The authors declare no competing financial interests.

Corresponding authors

Correspondence to Jie Shan or Kin Fai Mak.

Supplementary information

PDF files

  1. 1.

    Supplementary information

    Supplementary Information

About this article

Publication history






Further reading