Superconducting p-loops have a doubly degenerate ground state in zero applied fields,
and interact with each other antiferromagnetically, providing a physical analog for the Ising spin model.
"Ordering and Manipulation of the Magnetic Moments in Large-Scale Arrays of Superconducting π-loops",
Hans Hilgenkamp, Ariando, Henk-Jan Smilde, Dave H.A. Blank, Horst Rogalla, John R. Kirtley and Chang C. Tsuei, Nature 422 (6297), 50 (2003).
"Antiferromagnetic ordering in arrays of superconducting p-rings",
J.R. Kirtley, C.C. Tsuei, Ariando, H.-J. Smilde, and H. Hilgenkamp,
Phys. Rev. B 72 , 214521 (2005).