Anomalous metals: From “failed superconductor” to “failed insulator”

Significance A ubiquitous observation of robust metallic ground states arising from “failed superconductors” has attracted much attention in recent years because it presents a fundamental challenge to the standard theory of electron fluids. Meanwhile, observations of analogous metallic phases arising from “failed insulators” are often overlooked in analysis of similar data. Aiming to reconcile observations of both regimes in strongly granular In/InOx two-dimensional films, we propose a unified understanding of these seemingly different anomalous metallic phases by drawing connections to resistive-capacitive-shunted Josephson junction arrays. Effects of quantum phase/charge fluctuations and macroscopic quantum tunneling are invoked to understand the anomalous metallic phase, thus advancing fundamental understandings of metals beyond the standard Fermi liquid theory.