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Characterization of collective ground states in single-layer NbSe2

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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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.

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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.

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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.

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The authors declare no competing financial interests.

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Ugeda, M., Bradley, A., Zhang, Y. et al. Characterization of collective ground states in single-layer NbSe2. Nature Phys 12, 92–97 (2016). https://doi.org/10.1038/nphys3527

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