Nature of the quantum metal in a two-dimensional crystalline superconductor

Two-dimensional (2D) materials are not expected to be metals at low temperature owing to electron localization(1). Consistent with this, pioneering studies on thin films reported only superconducting and insulating ground states, with a direct transition between the two as a function of disorder or magnetic field(2-6). However, more recent works have revealed the presence of an intermediate quantum metallic state occupying a substantial region of the phase diagram(7-10), whose nature is intensely debated(11-17). Here, we observe such a state in the disorder-free limit of a crystalline 2D superconductor, produced by mechanical co-lamination of NbSe2 in an inert atmosphere. Under a small perpendicular magnetic field, we induce a transition from superconductor to the quantum metal. We find a unique power-law scaling with field in this phase, which is consistent with the Bose-metal model where metallic behaviour arises from strong phase fluctuations caused by the magnetic field(11-14).NSF MRSEC Program through Columbia in the Center for Precision Assembly of Superstratic and Superatomic Solids [DMR-1420634]; NSF [NEB- 1124894, DMR-1056527]; NSF Cooperative Agreement [DMR 0654118]; State of Florida; Department of Energy; National Basic Research Program of China [2013CB921901, 2014CB239302]; Department of Energy, Division of Basic Energy Sciences [DOE FG02-98ER45706]; Army Research Office [W911NF-14-1-0638]SCI(E)ARTICLEcdean@phys.columbia.edu; pasupathy@phys.clumbia.edu3208-+1