Intermediate bosonic metallic state in the superconductor-insulator transition

Whether a metallic ground state exists in a two-dimensional system beyond Anderson localization remains an unresolved question. Here, we study how quantum phase coherence evolves across superconductor-metal-insulator transitions via magneto-conductance quantum oscillations in nanopatterned high-temperature superconducting films. We tune the degree of phase coherence by varying the etching time of our films. Between the superconducting and insulating regimes, we detect a robust intervening anomalous metallic state characterized by saturating resistance and oscillation amplitude at low temperatures. Our measurements suggest that the anomalous metallic state is bosonic and that the saturation of phase coherence plays a prominent role in its formation.