Anti-ferromagnetic ordering in arrays of superconducting pi-rings

TL;DR

This study investigates anti-ferromagnetic ordering in 1D and 2D arrays of superconducting pi-rings, revealing phase-driven ordering and the absence of long-range order, with implications for understanding flux coupling and disorder effects.

Contribution

It demonstrates phase coupling-driven ordering in pi-ring arrays and models the cooling process, highlighting differences from conventional superconducting rings.

Findings

Strong anti-ferromagnetic order in 1D arrays

Absence of long-range order in 2D arrays

Pi-rings reduce disorder compared to conventional rings

Abstract

We report experiments in which one dimensional (1D) and two dimensional (2D) arrays of YBa2Cu3O7-x-Nb pi-rings are cooled through the superconducting transition temperature of the Nb in various magnetic fields. These pi-rings have degenerate ground states with either clockwise or counter-clockwise spontaneous circulating supercurrents. The final flux state of each ring in the arrays was determined using scanning SQUID microscopy. In the 1D arrays, fabricated as a single junction with facets alternating between alignment parallel to a [100] axis of the YBCO and rotated 90 degrees to that axis, half-fluxon Josephson vortices order strongly into an arrangement with alternating signs of their magnetic flux. We demonstrate that this ordering is driven by phase coupling and model the cooling process with a numerical solution of the Sine-Gordon equation. The 2D ring arrays couple to each other through the magnetic flux generated by the spontaneous supercurrents. Using pi-rings for the 2D flux coupling experiments eliminates one source of disorder seen in similar experiments using conventional superconducting rings, since pi-rings have doubly degenerate ground states in the absence of an applied field. Although anti-ferromagnetic ordering occurs, with larger negative bond orders than previously reported for arrays of conventional rings, long-range order is never observed, even in geometries without geometric frustration. This may be due to dynamical effects. Monte-Carlo simulations of the 2D array cooling process are presented and compared with experiment.