  Two-dimensional (2D) materials are not expected to be metals at low
temperature due to electron localization. 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. However, more recent works have revealed the presence of an
intermediate metallic state occupying a substantial region of the phase diagram
whose nature is intensely debated. Here, we observe such a state in the
disorder-free limit of a crystalline 2D superconductor, produced by mechanical
co-lamination of NbSe$_2$ in inert atmosphere. Under a small perpendicular
magnetic field, we induce a transition from superconductor to the intermediate
metallic state. We find a new power law scaling with field in this phase, which
is consistent with the Bose metal model where metallic behavior arises from
strong phase fluctuations caused by the magnetic field.

Superconductivity