Abstract
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.
Highlights
Two-dimensional (2D) materials are not expected to be metals at low temperature owing to electron localization[1]
More recent works have revealed the presence of an intermediate quantum metallic state occupying a substantial region of the phase diagram[7,8,9,10], whose nature is intensely debated[11,12,13,14,15,16,17]
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[11,12,13,14]
Summary
Two-dimensional (2D) materials are not expected to be metals at low temperature owing to electron localization[1]. Under a small perpendicular magnetic field, we induce a transition from superconductor to the quantum metal. Within the conventional theoretical framework, increasing sample disorder or magnetic field perpendicular to a strongly disordered film at T = 0 induces a direct transition to an insulating state as the normal state sheet resistance approaches the pair quantum resistance h/(2e)2 = 6.4 k (refs 2,4).
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