Abstract

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.

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Acknowledgements

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

Affiliations

  1. Department of Physics, University of California at Berkeley, Berkeley, California 94720, USA

    • Miguel M. Ugeda
    • , Aaron J. Bradley
    • , Seita Onishi
    • , Yi Chen
    • , Wei Ruan
    • , Claudia Ojeda-Aristizabal
    • , Mark T. Edmonds
    • , Hsin-Zon Tsai
    • , Alexander Riss
    • , Dunghai Lee
    • , Alex Zettl
    •  & Michael F. Crommie
  2. CIC nanoGUNE, 20018 Donostia-San Sebastian, Spain

    • Miguel M. Ugeda
  3. Ikerbasque, Basque Foundation for Science, 48011 Bilbao, Spain

    • Miguel M. Ugeda
  4. Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

    • Yi Zhang
    • , Hyejin Ryu
    • , Sung-Kwan Mo
    •  & Zahid Hussain
  5. Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA

    • Yi Zhang
    •  & Zhi-Xun Shen
  6. National Laboratory of Solid State Microstructures, School of Physics, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China

    • Yi Zhang
  7. State Key Laboratory of Low Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, China

    • Wei Ruan
  8. Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

    • Claudia Ojeda-Aristizabal
    • , Alex Zettl
    •  & Michael F. Crommie
  9. Department of Physics & Astronomy, California State University Long Beach, Long Beach, California 90840, USA

    • Claudia Ojeda-Aristizabal
  10. School of Physics and Astronomy, Monash University, Clayton, Victoria 3800, Australia

    • Mark T. Edmonds
  11. Institute of Applied Physics, Vienna University of Technology, 1040 Wien, Austria

    • Alexander Riss
  12. Kavli Energy NanoSciences Institute at the University of California Berkeley and the Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

    • Alex Zettl
    •  & Michael F. Crommie
  13. Geballe Laboratory for Advanced Materials, Departments of Physics and Applied Physics, Stanford University, Stanford, California 94305, USA

    • Zhi-Xun Shen

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Contributions

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.

Competing interests

The authors declare no competing financial interests.

Corresponding authors

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

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DOI

https://doi.org/10.1038/nphys3527

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