Characterization of collective ground states in single-layer NbSe2

M. M. Ugeda, A. J. Bradley, Y. Zhang, S. Onishi, Y. Chen, W. Ruan, C. Ojeda-Aristizabal, H. Ryu, M. T. Edmonds, H.-Z. Tsai, A. Riss, S.-K. Mo, D. Lee, A. Zettl, Z. Hussain, Z.-X. Shen, and M. F. Crommie

Department of Physics, University of California, Berkeley, CA 94720, U. S. A.
CIC nanoGUNE, 20018 Donostia-San Sebastian, Spain
Ikerbasque, Basque Foundation for Science, 48011 Bilbao, Spain
Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, U. S. A.
Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, CA 94025, U. S. A.
National Laboratory of Solid State Microstructures, School of Physics, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China
State Key Laboratory of Low Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, China
Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, U. S. A.
Department of Physics & Astronomy, California State University Long Beach, Long Beach, CA 90840, U. S. A.
School of Physics and Astronomy, Monash University, Clayton, Victoria 3800, Australia
Institut für Angewandte Physik, Technische Universität Wien, 1040 Wien, Austria
Kavli Energy NanoSciences Institute at the University of California Berkeley and the Lawrence Berkeley National Laboratory, Berkeley, California 94720, U. S. A.
Geballe Laboratory for Advanced Materials, Departments of Physics and Applied Physics, Stanford University, Stanford, California 94305, U. S. A.

Nat. Phys. 12 (2016) 92-98

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.

Corresponding author: Miguel M. Ugeda

Users with online access to Nature Physics can load the article from the publisher.