Intermediate bosonic metallic state in the superconductor-insulator transition

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Science  20 Dec 2019:
Vol. 366, Issue 6472, pp. 1505-1509
DOI: 10.1126/science.aax5798

A patterned look into a mysterious phase

A thin superconducting film can become insulating by, for example, exposure to a sufficiently large magnetic field. In between the superconducting and insulating regimes, an intermediate metallic state has been observed whose nature remains unresolved. To study the superconductor–metal insulator transition, C. Yang et al. patterned a film of the high-temperature superconductor yttrium barium copper oxide (YBCO) into a network of triangular superconducting islands connected by bridges (see the Perspective by Phillips). The reactive ion-etching process used for patterning reduced the quality of the film in a controlled manner. By increasing the etching time, the film's transport properties could be tuned from superconducting, through metallic, to insulating. The metallic phase exhibited a bosonic character.

Science, this issue p. 1505; see also p. 1450


Whether a metallic ground state exists in a two-dimensional system beyond Anderson localization remains an unresolved question. We studied how quantum phase coherence evolves across superconductor–metal–insulator transitions through magnetoconductance quantum oscillations in nanopatterned high-temperature superconducting films. We tuned the degree of phase coherence by varying the etching time of our films. Between the superconducting and insulating regimes, we detected 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.

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