Anomalous metals: from "failed superconductor" to "failed insulator"

Abstract

Resistivity saturation is found on both superconducting and insulating sides of an "avoided" magnetic-field-tuned superconductor-to-insulator transition (H-SIT) in a two-dimensional In/InOx composite, where the anomalous metallic behaviors cut off conductivity or resistivity divergence in the zero-temperature limit. The granular morphology of the material implies a system of Josephson junctions (JJ) with a broad distribution of Josephson coupling EJ and charging energy EC , with a H-SIT determined by the competition between EJ and EC . By virtue of self-duality across the true H-SIT, we invoke macroscopic quantum tunneling effects to explain the temperature-independent resistance where the "failed superconductor" side is a consequence of phase fluctuations and the "failed insulator" side results from charge fluctuations. While true self-duality is lost in the avoided transition, its vestiges are argued to persist, owing to the incipient duality of the percolative nature of the dissipative path in the underlying random JJ system.

Disorder-induced 2D superconductivity in a NbTiN film grown on Si by ultrahigh-vacuum magneton sputtering

“…Various attempts have been made to describe the effect of disorder on superconductivity [22]. Anomalous metallic states have been reported in certain systems [23][24][25][26][27]. In some cases, finite size effects have been linked to quantum fluctuations and observed in certain systems [28].…”