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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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.
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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