1 This document is the unedited Author’s version of a Submitted Work that was subsequently accepted for publication in Nature Physics. To access the final edited and published work see http://www.nature.com/nphys/journal/vaop/ncurrent/full/nphys3527.html Characterization of collective ground states in single-layer NbSe2 Miguel M. Ugeda* 1,2,3 , Aaron J. Bradley 1 , Yi Zhang 4,5,6 , Seita Onishi 1 , Yi Chen 1 , Wei Ruan 1,7 , Claudia Ojeda-Aristizabal 1,8,9 , Hyejin Ryu 4 , Mark T. Edmonds 1,10 , Hsin-Zon Tsai 1 , Alexander Riss 1,11 , Sung-Kwan Mo 4 , Dunghai Lee 1 , Alex Zettl 1,8,12 , Zahid Hussain 4 , Zhi-Xun Shen 5,13 and Michael F. Crommie* 1,8,12 1 Department of Physics, University of California at Berkeley, Berkeley, California 94720, USA. 2 CIC nanoGUNE, 20018 Donostia-San Sebastian, Spain. 3 Ikerbasque, Basque Foundation for Science, 48011 Bilbao, Spain. 4 Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA. 5 Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA. 6 National Laboratory of Solid State Microstructures, School of Physics, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China. 7 State Key Laboratory of Low Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, China. 8 Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA. 9 Department of Physics & Astronomy, California State University Long Beach, Long Beach, California 90840, USA. 10 School of Physics and Astronomy, Monash University, Clayton,Victoria 3800, Australia. 11 Institute of Applied Physics, Vienna University of Technology, 1040Wien, Austria. 12 Kavli Energy NanoSciences Institute at the University of California Berkeley and the Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA. 13 Geballe Laboratory for Advanced Materials, Departments of Physics and Applied Physics, Stanford University, Stanford, California 94305, USA. * Corresponding authors: mmugeda@berkeley.edu and crommie@berkeley.edu