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M. Ugeda, A. Bradley, Y. Zhang, S. Onishi, Y. Chen, W. Ruan, C. Ojeda-Aristizabal, M. Edmonds, H. Tsai, A. Riss, S. Mo, D. Lee, A. Zettl, Z. Hussain, Z. Shen, M. Crommie:
"Characterization of collective ground states in single-layer NbSe₂";
Nature Physics, 12 (2016), 92 - 98.

Layered transition metal dichalcogenides are ideal systems for exploring the e ects of dimensionality on correlated electronic
phases such as charge density wave (CDW) order and superconductivity. In bulk NbSe2 a CDW sets in at TCDWD33 K and
superconductivity sets in at TcD7.2 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 CDWorder 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 D4meV 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.