Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

Characterization of collective ground states in single-layer NbSe2


Layered transition metal dichalcogenides are ideal systems for exploring the effects of dimensionality on correlated electronic phases such as charge density wave (CDW) order and superconductivity. In bulk NbSe2 a CDW sets in at TCDW = 33 K and superconductivity sets in at Tc = 7.2 K. Below Tc these electronic states coexist but their microscopic formation mechanisms remain controversial. Here we present an electronic characterization study of a single two-dimensional (2D) layer of NbSe2 by means of low-temperature scanning tunnelling microscopy/spectroscopy (STM/STS), angle-resolved photoemission spectroscopy (ARPES), and electrical transport measurements. We demonstrate that 3 × 3 CDW order in NbSe2 remains intact in two dimensions. Superconductivity also still remains in the 2D limit, but its onset temperature is depressed to 1.9 K. Our STS measurements at 5 K reveal a CDW gap of Δ = 4 meV at the Fermi energy, which is accessible by means of STS owing to the removal of bands crossing the Fermi level for a single layer. Our observations are consistent with the simplified (compared to bulk) electronic structure of single-layer NbSe2, thus providing insight into CDW formation and superconductivity in this model strongly correlated system.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Figure 1: Structure of single-layer NbSe2 on bilayer graphene.
Figure 2: Superconductivity in single-layer NbSe2 on bilayer graphene.
Figure 3: Electronic structure of single-layer NbSe2 on bilayer graphene.
Figure 4: CDW gap of single-layer NbSe2.
Figure 5: Spatially and energetically resolved CDW phase in single-layer NbSe2.


  1. 1

    Frohlich, H. Electrons in lattice fields. Adv. Phys. 3, 325–361 (1954).

    ADS  Article  Google Scholar 

  2. 2

    Peierls, R. E. Quantum Theory of Solids (Clarendon, 1955).

    MATH  Google Scholar 

  3. 3

    Guo, Y. et al. Superconductivity modulated by quantum size effects. Science 306, 1915–1917 (2004).

    ADS  Article  Google Scholar 

  4. 4

    Qin, S. Y., Kim, J., Niu, Q. & Shih, C. K. Superconductivity at the two-dimensional limit. Science 324, 1314–1317 (2009).

    ADS  Article  Google Scholar 

  5. 5

    Bose, S. et al. Observation of shell effects in superconducting nanoparticles of Sn. Nature Mater. 9, 550–554 (2010).

    ADS  Article  Google Scholar 

  6. 6

    Calandra, M., Mazin, I. I. & Mauri, F. Effect of dimensionality on the charge-density wave in few-layer 2H- NbSe2 . Phys. Rev. B 80, 241108 (2009).

    ADS  Article  Google Scholar 

  7. 7

    Lebegue, S. & Eriksson, O. Electronic structure of two-dimensional crystals from ab initio theory. Phys. Rev. B 79, 115409 (2009).

    ADS  Article  Google Scholar 

  8. 8

    Darancet, P., Millis, A. J. & Marianetti, C. A. Three-dimensional metallic and two-dimensional insulating behaviour in octahedral tantalum dichalcogenides. Phys. Rev. B 90, 045134 (2014).

    ADS  Article  Google Scholar 

  9. 9

    Peng, J. P. et al. Molecular beam epitaxy growth and scanning tunneling microscopy study of TiSe2 ultrathin films. Phys. Rev. B 91, 121113 (2015).

    ADS  Article  Google Scholar 

  10. 10

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

    ADS  Article  Google Scholar 

  11. 11

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

    ADS  Article  Google Scholar 

  12. 12

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

    ADS  Article  Google Scholar 

  13. 13

    Cao, Y. et al. Quality heterostructures from two dimensional crystals unstable in air by their assembly in inert atmosphere. Nano Lett. 15, 4914–4921 (2015).

    ADS  Article  Google Scholar 

  14. 14

    Varma, C. M. & Simons, A. L. Strong-coupling theory of charge-density-wave transitions. Phys. Rev. Lett. 51, 138–141 (1983).

    ADS  Article  Google Scholar 

  15. 15

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

    ADS  Article  Google Scholar 

  16. 16

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

    ADS  Article  Google Scholar 

  17. 17

    Rahn, D. J. 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).

    ADS  MathSciNet  Article  Google Scholar 

  18. 18

    Soumyanarayanan, A. et al. Quantum phase transition from triangular to stripe charge order in NbSe2 . Proc. Natl Acad. Sci. USA 110, 1623–1627 (2013).

    ADS  Article  Google Scholar 

  19. 19

    Arguello, C. J. et al. Visualizing the charge density wave transition in 2H- NbSe2 in real space. Phys. Rev. B 89, 235115 (2014).

    ADS  Article  Google Scholar 

  20. 20

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

    ADS  Article  Google Scholar 

  21. 21

    Wilson, J. A., Disalvo, F. J. & Mahajan, S. Charge-density waves in metallic, layered, transition-metal dichalcogenides. Phys. Rev. Lett. 32, 882–885 (1974).

    ADS  Article  Google Scholar 

  22. 22

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

    ADS  Article  Google Scholar 

  23. 23

    Shen, D. W. et al. Primary role of the barely occupied states in the charge density wave formation of NbSe2 . Phys. Rev. Lett. 101, 226406 (2008).

    ADS  Article  Google Scholar 

  24. 24

    Borisenko, S. V. et al. Two energy gaps and fermi-surface “Arcs” in NbSe2 . Phys. Rev. Lett. 102, 166402 (2009).

    ADS  Article  Google Scholar 

  25. 25

    Rice, T. M. & Scott, G. K. New mechanism for a charge-density-wave instability. Phys. Rev. Lett. 35, 120–123 (1975).

    ADS  Article  Google Scholar 

  26. 26

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

    ADS  Article  Google Scholar 

  27. 27

    Chen, W. et al. Energy gaps measured by scanning tunneling microscopy. Phys. Rev. B 42, 8890–8906 (1990).

    Article  Google Scholar 

  28. 28

    Hess, H. F., Robinson, R. B. & Waszczak, J. V. STM spectroscopy of vortex cores and the flux lattice. Physica B 169, 422–431 (1991).

    ADS  Article  Google Scholar 

  29. 29

    Harper, J. M. E., Geballe, T. H. & Disalvo, F. J. Heat-capacity of 2H-N NbSe2 at charge-density wave transition. Phys. Lett. A 54, 27–28 (1975).

    ADS  Article  Google Scholar 

  30. 30

    Giambattista, B. et al. Charge-density waves observed at 4.2 K by scanning-tunneling microscopy. Phys. Rev. B 37, 2741–2744 (1988).

    ADS  Article  Google Scholar 

  31. 31

    Ugeda, M. M. et al. Giant bandgap renormalization and excitonic effects in a monolayer transition metal dichalcogenide semiconductor. Nature Mater. 13, 1091–1095 (2014).

    ADS  Article  Google Scholar 

  32. 32

    Wintterlin, J. & Bocquet, M. L. Graphene on metal surfaces. Surf. Sci. 603, 1841–1852 (2009).

    ADS  Article  Google Scholar 

  33. 33

    Johannes, M. D., Mazin, I. I. & Howells, C. A. Fermi-surface nesting and the origin of the charge-density wave in NbSe2 . Phys. Rev. B 73, 205102 (2006).

    ADS  Article  Google Scholar 

  34. 34

    Wang, Q. Y. et al. Large-scale uniform bilayer graphene prepared by vacuum graphitization of 6H-SiC (0001) substrates. J. Phys. Condens. Mater. 25, 095002 (2013).

    ADS  Article  Google Scholar 

  35. 35

    Horcas, I. et al. WSXM: A software forscanning probe microscopy and a tool for nanotechnology. Rev. Sci. Instrum. 78, 013705 (2007).

    ADS  Article  Google Scholar 

Download references


Research supported in part by the Director, Office of Energy Research, Materials Sciences and Engineering Division, of the US Department of Energy (DOE), under grant DE-AC02-05CH11231 supporting the sp2-bonded Materials Program (STM imaging and transport), and by the National Science Foundation under award #DMR-1206512 (STS spectroscopic analysis). Work at the ALS is supported by DOE BES under Contract No. DE-AC02-05CH11231. H.R. acknowledges support from Max Planck Korea/POSTECH Research Initiative of NRF, Korea. M.T.E. is supported by the ARC Laureate Fellowship project (FL120100038). A.R. acknowledges fellowship support by the Austrian Science Fund (FWF): J3026-N16.

Author information




M.M.U. and A.J.B. conceived the work and designed the research strategy. M.M.U., A.J.B., Y.C., W.R. and M.T.E. measured and analysed the STM/STS data. Y.Z., H.R. and S.-K.M. performed the MBE growth and ARPES and LEED characterization of the samples. S.O., C.O.-A., M.M.U. and Y.C. carried out the transport experiments. H.-Z.T. and A.R. helped in the experiments. D.L. participated in the interpretation of the experimental data. Z.H. and Z.-X.S. supervised the MBE and sample characterization. A.Z. supervised the transport measurements. M.F.C. supervised the STM/STS experiments. M.M.U. wrote the paper with help from M.F.C. and A.Z. M.M.U. and M.F.C. coordinated the collaboration. All authors contributed to the scientific discussion and manuscript revisions.

Corresponding authors

Correspondence to Miguel M. Ugeda or Michael F. Crommie.

Ethics declarations

Competing interests

The authors declare no competing financial interests.

Supplementary information

Supplementary information

Supplementary information (PDF 2770 kb)

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Ugeda, M., Bradley, A., Zhang, Y. et al. Characterization of collective ground states in single-layer NbSe2. Nature Phys 12, 92–97 (2016).

Download citation

Further reading


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing