Superconductivity from a melted insulator

TL;DR

This paper reveals that in high-impedance Josephson arrays, temperature can induce a superconducting state from a zero-temperature insulator, challenging traditional views of quantum phase transitions and explaining anomalous metallic behavior.

Contribution

It provides the first quantitative explanation of high-temperature superconductivity in nominally insulating regimes and the onset of metallic resistance saturation, highlighting thermal effects on quantum criticality.

Findings

Temperature shifts the critical region in Josephson arrays.

Superconductivity can emerge at high temperature from a melted insulator.

Thermal fluctuations can stabilize coherence in high-impedance quantum circuits.

Abstract

Quantum phase transitions typically result in a broadened critical or crossover region at nonzero temperature. Josephson arrays are a model of this phenomenon, exhibiting a superconductor-insulator transition at a critical wave impedance, and a well-understood insulating phase. Yet high-impedance arrays used in quantum computing and metrology apparently evade this transition, displaying superconducting behavior deep into the nominally insulating regime. The absence of critical behavior in such devices is not well understood. Here we show that, unlike the typical quantum-critical broadening scenario, in Josephson arrays temperature dramatically shifts the critical region. This shift leads to a regime of superconductivity at high temperature, arising from the melted zero-temperature insulator. Our results quantitatively explain the low-temperature onset of superconductivity in nominally insulating regimes, and the transition to the strongly insulating phase. We further present, to our knowledge, the first understanding of the onset of anomalous-metallic resistance saturation. This work demonstrates a non-trivial interplay between thermal effects and quantum criticality. A practical consequence is that, counterintuitively, the coherence of high-impedance quantum circuits is expected to be stabilized by thermal fluctuations.