Imaging spontaneous currents in superconducting arrays of pi-junctions

TL;DR

This paper visualizes spontaneous supercurrents in superconducting arrays with pi-junctions, revealing temperature-dependent onset of currents and complex behaviors in non-uniform arrays, advancing understanding of quantum phases and potential quantum computing applications.

Contribution

It provides the first direct imaging of spontaneous currents in pi-junction superconducting networks, demonstrating their behavior at the transition temperature and in complex array configurations.

Findings

Spontaneous supercurrents appear at Tpi = 3 K.

Currents are imaged in arrays with mixed pi and non-pi junctions.

Arrays exhibit behaviors relevant to 2D XY-model and quantum computing.

Abstract

Superconductors separated by a thin tunneling barrier exhibit the Josephson effect that allows charge transport at zero voltage, typically with no phase shift between the superconductors in the lowest energy state. Recently, Josephson junctions with ground state phase shifts of pi proposed by theory three decades ago have been demonstrated. In superconducting loops, pi-junctions cause spontaneous circulation of persistent currents in zero magnetic field, analogous to spin-1/2 systems. Here we image the spontaneous zero-field currents in superconducting networks of temperature-controlled pi-junctions with weakly ferromagnetic barriers using a scanning SQUID microscope. We find an onset of spontaneous supercurrents at the 0-pi transition temperature of the junctions Tpi = 3 K. We image the currents in non-uniformly frustrated arrays consisting of cells with even and odd numbers of pi-junctions. Such arrays are attractive model systems for studying the exotic phases of the 2D XY-model and achieving scalable adiabatic quantum computers.