Superconducting transitions in frustrated Josephson-junction arrays on a dice lattice

Transport measurements were carried out on dice Josephson-junction arrays with the frustration index f = 1/3 and 1/2 which possess, within the limit of the XY model, an accidental degeneracy of the ground states as a consequence of the formation of zero-energy domain walls. The experimental results are in apparent contradiction with the theoretical expectation that frustration effects in the f = 1/3 system are particularly strong enough to suppress a vortex-ordering transition down to near zero temperature. The data for f = 1/2 are more consistent with theoretical evaluations. The agreement between the experiments and the Monte Carlo simulations of a XY model for f = 1/3 suggests that the order-from-disorder mechanism for the removal of an accidental degeneracy may still be effective in the f = 1/3 system.