Nonergodic metallic and insulating phases of Josephson junction chains

Abstract

Significance Conventional equilibrium statistical physics that aims to describe dynamical systems with many degrees of freedom relies crucially on the equipartition postulate: After evolving for a sufficiently long time, the probabilities to find the system in states with the same energy are equal. Time averaging is thus assumed to be equivalent to the averaging over the energy shell—the famous ergodic hypothesis. In this study we show that this hypothesis is not correct for a large class of quantum many-body models that can be implemented in the laboratory. These models are predicted to show a novel type of behavior that we name bad metal, which is neither a many-body insulator nor a conventional conductor.