Characterization of collective ground states in single-layer NbSe2

• Published in: Nature physics 2015-11, Vol.12 (1), p.92-97
• Main Authors: Ugeda, Miguel M., Bradley, Aaron J., Zhang, Yi, Onishi, Seita, Chen, Yi, Ruan, Wei, Ojeda-Aristizabal, Claudia, Ryu, Hyejin, Edmonds, Mark T., Tsai, Hsin-Zon, Riss, Alexander, Mo, Sung-Kwan, Lee, Dunghai, Zettl, Alex, Hussain, Zahid, Shen, Zhi-Xun, Crommie, Michael F.
• Format: Article
• Language: English
• Subjects: 119/118; 639/301/357/1018; 639/301/357/995; 639/766/119/1003; 639/766/119/2795; Atomic; ATOMIC AND MOLECULAR PHYSICS; Boundary layer; Classical and Continuum Physics; Complex Systems; Condensed Matter Physics; Correlation analysis; Low temperature; Mathematical and Computational Physics; Molecular; Optical and Plasma Physics; Physics; Spectroscopy; Superconductivity; Theoretical; Wave power
• ISSN: 1745-2473; 1745-2481
• DOI: 10.1038/nphys3527

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 NbSe 2 a CDW sets in at T CDW = 33 K and superconductivity sets in at T c = 7.2 K. Below T c 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 NbSe 2 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 NbSe 2 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 NbSe 2 , thus providing insight into CDW formation and superconductivity in this model strongly correlated system. What happens to correlated electronic phases—superconductivity and charge density wave ordering—as a material is thinned? Experiments show that both can remain intact in just a single layer of niobium diselenide.